Integrative analyses of convergent adaptation in sympatric extremophile fishes.

The evolution of independent lineages along replicated environmental transitions frequently results in convergent adaptation, yet the degree to which convergence is present across multiple levels of biological organization is often unclear. Additionally, inherent biases associated with shared ancestry and variation in selective regimes across geographic replicates often pose challenges for confidently identifying patterns of convergence. We investigated a system in which three species of poeciliid fishes sympatrically occur in a toxic spring rich in hydrogen sulfide (H2S) and an adjacent nonsulfidic stream to examine patterns of adaptive evolution across levels of biological organization. We found convergence in morphological and physiological traits and genome-wide patterns of gene expression among all three species. In addition, there were shared signatures of selection on genes encoding H2S toxicity targets in the mitochondrial genomes of each species. However, analyses of nuclear genomes revealed neither evidence for substantial genomic islands of divergence around genes involved in H2S toxicity and detoxification nor substantial congruence of strongly differentiated regions across population pairs. These non-convergent, heterogeneous patterns of genomic divergence may indicate that sulfide tolerance is highly polygenic, with shared allele frequency shifts present at many loci with small effects along the genome. Alternatively, H2S tolerance may involve substantial genetic redundancy, with non-convergent, lineage-specific variation at multiple loci along the genome underpinning similar changes in phenotypes and gene expression. Overall, we demonstrate variability in the extent of convergence across organizational levels and highlight the challenges of linking patterns of convergence across scales.

TET1 regulates responses to house dust mite by altering chromatin accessibility, DNA methylation, and gene expression in airway epithelial cells.

Background: Previous studies have identified TET1 as a potential key regulator of genes linked to asthma. TET1 has been shown to transcriptionally respond to house dust mite extract, an allergen known to directly cause allergic asthma development, and regulate the expression of genes involved in asthma. How TET1 regulates expression of these genes, however, is unknown. TET1 is a DNA demethylase; therefore, most prior research on TET1-based gene regulation has focused on how TET1 affects methylation. However, TET1 can also interact directly with transcription factors and histone modifiers to regulate gene expression. Understanding how TET1 regulates expression to contribute to allergic responses and asthma development thus requires a comprehensive approach. To this end, we measured mRNA expression, DNA methylation, chromatin accessibility and histone modifications in control and TET1 knockdown human bronchial epithelial cells treated or untreated with house dust mite extract. Results: Throughout our analyses, we detected strong similarities between the effects of TET1 knockdown alone and the effects of HDM treatment alone. One especially striking pattern was that both TET1 knockdown and HDM treatment generally led to decreased chromatin accessibility at largely the same genomic loci. Transcription factor enrichment analyses indicated that altered chromatin accessibility following the loss of TET1 may affect, or be affected by, CTCF and CEBP binding. TET1 loss also led to changes in DNA methylation, but these changes were generally in regions where accessibility was not changing. Conclusions: TET1 regulates gene expression through different mechanisms (DNA methylation and chromatin accessibility) in different parts of the genome in the airway epithelial cells, which mediates inflammatory responses to allergen. Collectively, our data suggest novel molecular mechanisms through which TET1 regulates critical pathways following allergen challenges and contributes to the development of asthma.

Serum plays an important role in reprogramming the seasonal transcriptional profile of brown bear adipocytes.

Understanding how metabolic reprogramming happens in cells will aid the progress in the treatment of a variety of metabolic disorders. Brown bears undergo seasonal shifts in insulin sensitivity, including reversible insulin resistance in hibernation. We performed RNA-sequencing on brown bear adipocytes and proteomics on serum to identify changes possibly responsible for reversible insulin resistance. We observed dramatic transcriptional changes, which depended on both the cell and serum season of origin. Despite large changes in adipocyte gene expression, only changes in eight circulating proteins were identified as related to the seasonal shifts in insulin sensitivity, including some that have not previously been associated with glucose homeostasis. The identified serum proteins may be sufficient for shifting hibernation adipocytes to an active-like state.

Single-Cell RNA-Seq Analysis Reveals Lung Epithelial Cell Type-Specific Responses to HDM and Regulation by Tet1.

Tet1 protects against house dust mite (HDM)-induced lung inflammation in mice and alters the lung methylome and transcriptome. In order to explore the role of Tet1 in individual lung epithelial cell types in HDM-induced inflammation, we established a model of HDM-induced lung inflammation in Tet1 knockout and littermate wild-type mice, then studied EpCAM+ lung epithelial cells using single-cell RNA-seq analysis. We identified eight EpCAM+ lung epithelial cell types, among which AT2 cells were the most abundant. HDM challenge altered the relative abundance of epithelial cell types and resulted in cell type-specific transcriptomic changes. Bulk and cell type-specific analysis also showed that loss of Tet1 led to the altered expression of genes linked to augmented HDM-induced lung inflammation, including alarms, detoxification enzymes, oxidative stress response genes, and tissue repair genes. The transcriptomic regulation was accompanied by alterations in TF activities. Trajectory analysis supports that HDM may enhance the differentiation of AP and BAS cells into AT2 cells, independent of Tet1. Collectively, our data showed that lung epithelial cells had common and unique transcriptomic signatures of allergic lung inflammation. Tet1 deletion altered transcriptomic networks in various lung epithelial cells, which may promote allergen-induced lung inflammation.

Long-term effects of wildfire smoke exposure during early life on the nasal epigenome in rhesus macaques.

BACKGROUND: Wildfire smoke is responsible for around 20% of all particulate emissions in the U.S. and affects millions of people worldwide. Children are especially vulnerable, as ambient air pollution exposure during early childhood is associated with reduced lung function. Most studies, however, have focused on the short-term impacts of wildfire smoke exposures. We aimed to identify long-term baseline epigenetic changes associated with early-life exposure to wildfire smoke. We collected nasal epithelium samples for whole genome bisulfite sequencing (WGBS) from two groups of adult female rhesus macaques: one group born just before the 2008 California wildfire season and exposed to wildfire smoke during early-life (n = 8), and the other group born in 2009 with no wildfire smoke exposure during early-life (n = 14). RNA-sequencing was also performed on a subset of these samples. RESULTS: We identified 3370 differentially methylated regions (DMRs) (difference in methylation ≥ 5%, empirical p < 0.05) and 1 differentially expressed gene (FLOT2) (FDR < 0.05, fold of change ≥ 1.2). The DMRs were annotated to genes significantly enriched for synaptogenesis signaling, protein kinase A signaling, and a variety of immune processes, and some DMRs significantly correlated with gene expression differences. DMRs were also significantly enriched within regions of bivalent chromatin (top odds ratio = 1.46, q-value < 3 × 10-6) that often silence key developmental genes while keeping them poised for activation in pluripotent cells. CONCLUSIONS: These data suggest that early-life exposure to wildfire smoke leads to long-term changes in the methylome over genes impacting the nervous and immune systems. Follow-up studies will be required to test whether these changes influence transcription following an immune/respiratory challenge.

Nasal DNA methylation differentiates severe from non-severe asthma in African-American children.

BACKGROUND: Asthma is highly heterogeneous, and severity evaluation is key to asthma management. DNA methylation (DNAm) contributes to asthma pathogenesis. This study aimed to identify nasal epithelial DNAm differences between severe and nonsevere asthmatic children and evaluate the impact of environmental exposures. METHODS: Thirty-three nonsevere and 22 severe asthmatic African American children were included in an epigenome-wide association study. Genome-wide nasal epithelial DNAm and gene expression were measured. CpG sites associated with asthma severity and environmental exposures and predictive of severe asthma were identified. DNAm was correlated with gene expression. Enrichment for transcription factor (TF) binding sites or histone modifications surrounding DNAm differences were determined. RESULTS: We identified 816 differentially methylated CpG positions (DMPs) and 10 differentially methylated regions (DMRs) associated with asthma severity. Three DMPs exhibited discriminatory ability for severe asthma. Intriguingly, six DMPs were simultaneously associated with asthma, allergic asthma, total IgE, environmental IgE, and FeNO in an independent cohort of children. Twenty-seven DMPs were associated with traffic-related air pollution or secondhand smoke. DNAm at 22 DMPs was altered by diesel particles or allergen in human bronchial epithelial cells. DNAm levels at 39 DMPs were correlated with mRNA expression. Proximal to 816 DMPs, three histone marks and several TFs involved in asthma pathogenesis were enriched. CONCLUSIONS: Significant differences in nasal epithelial DNAm were observed between nonsevere and severe asthma in African American children, a subset of which may be useful to predict disease severity. These CpG sites are subjected to the influences of environmental exposures and may regulate gene expression.

microRNA expression variation as a potential molecular mechanism contributing to adaptation to hydrogen sulphide.

microRNAs (miRNAs) are post-transcriptional regulators of gene expression and can play an important role in modulating organismal development and physiology in response to environmental stress. However, the role of miRNAs in mediating adaptation to diverse environments in natural study systems remains largely unexplored. Here, we characterized miRNAs and their expression in Poecilia mexicana, a species of small fish that inhabits both normal streams and extreme environments in the form of springs rich in toxic hydrogen sulphide (H2 S). We found that P. mexicana has a similar number of miRNA genes as other teleosts. In addition, we identified a large population of mature miRNAs that were differentially expressed between locally adapted populations in contrasting habitats, indicating that miRNAs may contribute to P. mexicana adaptation to sulphidic environments. In silico identification of differentially expressed miRNA-mRNA pairs revealed, in the sulphidic environment, the downregulation of miRNAs predicted to target mRNAs involved in sulphide detoxification and cellular homeostasis, which are pathways essential for life in H2 S-rich springs. In addition, we found that predicted targets of upregulated miRNAs act in the mitochondria (16.6% of predicted annotated targets), which is the main site of H2 S toxicity and detoxification, possibly modulating mitochondrial function. Together, the differential regulation of miRNAs between these natural populations suggests that miRNAs may be involved in H2 S adaptation by promoting functions needed for survival and reducing functions affected by H2 S. This study lays the groundwork for further research to directly demonstrate the role of miRNAs in adaptation to H2 S. Overall, this study provides a critical stepping-stone towards a comprehensive understanding of the regulatory mechanisms underlying the adaptive variation in gene expression in a natural system.

Convergent evolution of conserved mitochondrial pathways underlies repeated adaptation to extreme environments.

Extreme environments test the limits of life; yet, some organisms thrive in harsh conditions. Extremophile lineages inspire questions about how organisms can tolerate physiochemical stressors and whether the repeated colonization of extreme environments is facilitated by predictable and repeatable evolutionary innovations. We identified the mechanistic basis underlying convergent evolution of tolerance to hydrogen sulfide (H2S)-a toxicant that impairs mitochondrial function-across evolutionarily independent lineages of a fish (Poecilia mexicana, Poeciliidae) from H2S-rich springs. Using comparative biochemical and physiological analyses, we found that mitochondrial function is maintained in the presence of H2S in sulfide spring P. mexicana but not ancestral lineages from nonsulfidic habitats due to convergent adaptations in the primary toxicity target and a major detoxification enzyme. Genome-wide local ancestry analyses indicated that convergent evolution of increased H2S tolerance in different populations is likely caused by a combination of selection on standing genetic variation and de novo mutations. On a macroevolutionary scale, H2S tolerance in 10 independent lineages of sulfide spring fishes across multiple genera of Poeciliidae is correlated with the convergent modification and expression changes in genes associated with H2S toxicity and detoxification. Our results demonstrate that the modification of highly conserved physiological pathways associated with essential mitochondrial processes mediates tolerance to physiochemical stress. In addition, the same pathways, genes, and-in some instances-codons are implicated in H2S adaptation in lineages that span 40 million years of evolution.

The Emerging Role of Ten-Eleven Translocation 1 in Epigenetic Responses to Environmental Exposures.

Mounting evidence from epidemiological studies and animal models has linked exposures to environmental factors to changes in epigenetic markers, especially in DNA methylation. These epigenetic changes may lead to dysregulation of molecular processes and functions and mediate the impact of environmental exposures in complex diseases. However, detailed molecular events that result in epigenetic changes following exposures remain unclear. Here, we review the emerging evidence supporting a critical role of ten-eleven translocation 1 (TET1) in mediating these processes. Targeting TET1 and its associated pathways may have therapeutic potential in alleviating negative impacts of environmental exposures, preventing and treating exposure-related diseases.

Method development and validation of dissolution testing for nicotine release from smokeless tobacco products using flow-through cell apparatus and UPLC-PDA.

Developing dissolution testing methods to measure the nicotine release profiles from smokeless tobacco products is valuable for product assessment and product-to-product comparisons. In this work, we developed a robust dissolution method to study the in vitro release of nicotine from smokeless tobacco products using the U.S. Pharmacopeia flow-through cell dissolution apparatus 4 (USP-4). We further developed and validated a sensitive Ultra Performance Liquid Chromatography coupled to Photodiode Array detector (UPLC-PDA) method for the accurate quantitation of the released nicotine into artificial saliva, which is our selected dissolution medium. We have successfully shown the applicability of the validated method by investigating the release profiles of nicotine from various commercial and CORESTA reference smokeless tobacco products [CRP 1.1 (Swedish-style snus pouch), CRP 2.1 (American-style loose moist snuff), CRP 4 (loose-leaf chewing tobacco) and CRP 4.1 (chopped loose-leaf chewing tobacco)]. Nicotine release profiles were analyzed by calculating the difference factor (f1) and similarity factor (f2) by adopting a methodology referenced in the Guidance for Industry from FDA's Center for Drug Evaluation and Research (CDER) and by fitting the release profile curves using a first order kinetic model. Nicotine release was found to be dependent on the form and cut of the smokeless tobacco products, with a slower release observed for snus and loose-leaf, compared to chopped and loose moist snuff smokeless tobacco. This dissolution methodology can be extended to measure and compare release of other constituents from smokeless tobacco products and has the potential for method standardization.

Local ancestry analysis reveals genomic convergence in extremophile fishes.

The molecular basis of convergent phenotypes is often unknown. However, convergence at a genomic level is predicted when there are large population sizes, gene flow among diverging lineages or strong genetic constraints. We used whole-genome resequencing to investigate genomic convergence in fishes ( Poecilia spp.) that have repeatedly colonized hydrogen sulfide (H2S)-rich environments in Mexico. We identified genomic similarities in both single nucleotide polymorphisms (SNPs) and structural variants (SVs) among independently derived sulfide spring populations, with approximately 1.2% of the genome being shared among sulfidic ecotypes. We compared these convergent genomic regions to candidate genes for H2S adaptation identified from transcriptomic analyses and found that a significant proportion of these candidate genes (8%) were also in regions where sulfidic individuals had similar SNPs, while only 1.7% were in regions where sulfidic individuals had similar SVs. Those candidate genes included genes involved in sulfide detoxification, the electron transport chain (the main toxicity target of H2S) and other processes putatively important for adaptation to sulfidic environments. Regional genomic similarity across independent populations exposed to the same source of selection is consistent with selection on standing variation or introgression of adaptive alleles across divergent lineages. However, combined with previous analyses, our data also support that adaptive changes in mitochondrially encoded subunits arose independently via selection on de novo mutations. Pressing questions remain on what conditions ultimately facilitate the independent rise of adaptive alleles at the same loci in separate populations, and thus, the degree to which evolution is repeatable or predictable. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Concordant Changes in Gene Expression and Nucleotides Underlie Independent Adaptation to Hydrogen-Sulfide-Rich Environments.

The colonization of novel environments often involves changes in gene expression, protein coding sequence, or both. Studies of how populations adapt to novel conditions, however, often focus on only one of these two processes, potentially missing out on the relative importance of different parts of the evolutionary process. In this study, our objectives were 1) to better understand the qualitative concordance between conclusions drawn from analyses of differential expression and changes in genic sequence and 2) to quantitatively test whether differentially expressed genes were enriched for sites putatively under positive selection within gene regions. To achieve this, we compared populations of fish (Poecilia mexicana) that have independently adapted to hydrogen-sulfide-rich environments in southern Mexico to adjacent populations residing in nonsulfidic waters. Specifically, we used RNA-sequencing data to compare both gene expression and DNA sequence differences between populations. Analyzing these two different data types led to similar conclusions about which biochemical pathways (sulfide detoxification and cellular respiration) were involved in adaptation to sulfidic environments. Additionally, we found a greater overlap between genes putatively under selection and differentially expressed genes than expected by chance. We conclude that considering both differential expression and changes in DNA sequence led to a more comprehensive understanding of how these populations adapted to extreme environmental conditions. Our results imply that changes in both gene expression and DNA sequence-sometimes at the same loci-may be involved in adaptation.

An evaluation of electronic cigarette formulations and aerosols for harmful and potentially harmful constituents (HPHCs) typically derived from combustion.

U.S. FDA draft guidance recommends reporting quantities of designated harmful and potentially harmful constituents (HPHCs) in e-cigarette e-liquids and aerosols. The HPHC list comprises potential matrix-related compounds, flavors, nicotine, tobacco-related impurities, leachables, thermal degradation products, and combustion-related compounds. E-cigarettes contain trace levels of many of these constituents due to tobacco-derived nicotine and thermal degradation. However, combustion-related HPHCs are not likely to be found due to the relatively low operating temperatures of most e-cigarettes. The purpose of this work was to use highly sensitive, selective, and validated analytical methods to determine if these combustion-related HPHCs (three aromatic amines, five volatile organic compounds, and the polycyclic aromatic hydrocarbon benzo[a]pyrene) are detectable in commercial refill e-liquids, reference e-cigarette e-liquids, and aerosols generated from rechargeable e-cigarettes with disposable cartridges (often referred to as "cig-a-likes"). In addition, the transfer efficiency of these constituents from e-liquid to aerosol was evaluated when these HPHCs were added to the e-liquids prior to aerosol formation. This work demonstrates that combustion-related HPHCs are not present at measurable levels in the commercial and reference e-liquids or e-cigarette aerosols tested. Additionally, when combustion-related HPHCs are added to the e-liquids, they transfer to the aerosol with transfer efficiencies ranging from 49% to 99%.

Genome-scale data reveal that endemic Poecilia populations from small sulphidic springs display no evidence of inbreeding.

Populations with limited ranges can be highly vulnerable to changes in their environment and are, thus, of high conservation concern. Populations that experience human-induced range reductions are often highly inbred and lack genetic diversity, but it is unknown whether this is also the case for populations with naturally small ranges. The fishes Poecilia sulphuraria (listed as critically endangered) and Poecilia thermalis, which are endemic to small hydrogen sulphide-rich springs in southern Mexico, are examples of such populations with inherently small habitats. We used geometric morphometrics and population genetics to quantify phenotypic and genetic variation within and among two populations of P. sulphuraria and one population of P. thermalis. Principal component analyses revealed phenotypic and genetic differences among the populations. Evidence for inbreeding was low compared to populations that have undergone habitat reduction. The genetic data were also used to infer the demographic history of these populations to obtain estimates for effective population sizes and migration rates. Effective population sizes were large given the small habitats of these populations. Our results imply that these three endemic extremophile populations should each be considered separately for conservation purposes. Additionally, this study suggests that populations in naturally small habitats may have lower rates of inbreeding and higher genetic diversity than expected, and therefore may be better equipped to handle environmental perturbations than anticipated. We caution, however, that the inferred lack of inbreeding and the large effective population sizes could potentially be a result of colonization by genetically diverse ancestors.

Complexities of gene expression patterns in natural populations of an extremophile fish (Poecilia mexicana, Poeciliidae).

Variation in gene expression can provide insights into organismal responses to environmental stress and physiological mechanisms mediating adaptation to habitats with contrasting environmental conditions. We performed an RNA-sequencing experiment to quantify gene expression patterns in fish adapted to habitats with different combinations of environmental stressors, including the presence of toxic hydrogen sulphide (H2 S) and the absence of light in caves. We specifically asked how gene expression varies among populations living in different habitats, whether population differences were consistent among organs, and whether there is evidence for shared expression responses in populations exposed to the same stressors. We analysed organ-specific transcriptome-wide data from four ecotypes of Poecilia mexicana (nonsulphidic surface, sulphidic surface, nonsulphidic cave and sulphidic cave). The majority of variation in gene expression was correlated with organ type, and the presence of specific environmental stressors elicited unique expression differences among organs. Shared patterns of gene expression between populations exposed to the same environmental stressors increased with levels of organismal organization (from transcript to gene to physiological pathway). In addition, shared patterns of gene expression were more common between populations from sulphidic than populations from cave habitats, potentially indicating that physiochemical stressors with clear biochemical consequences can constrain the diversity of adaptive solutions that mitigate their adverse effects. Overall, our analyses provided insights into transcriptional variation in a unique system, in which adaptation to H2 S and darkness coincide. Functional annotations of differentially expressed genes provide a springboard for investigating physiological mechanisms putatively underlying adaptation to extreme environments.

The life aquatic: advances in marine vertebrate genomics.

The ocean is hypothesized to be where life on earth originated, and subsequent evolutionary transitions between marine and terrestrial environments have been key events in the origin of contemporary biodiversity. Here, we review how comparative genomic approaches are an increasingly important aspect of understanding evolutionary processes, such as physiological and morphological adaptation to the diverse habitats within the marine environment. In addition, we highlight how population genomics has provided unprecedented resolution for population structuring, speciation and adaptation in marine environments, which can have a low cost of dispersal and few physical barriers to gene flow, and can thus support large populations. Building upon this work, we outline the applications of genomics tools to conservation and their relevance to assessing the wide-ranging impact of fisheries and climate change on marine species.

The Genome of the Self-Fertilizing Mangrove Rivulus Fish, Kryptolebias marmoratus: A Model for Studying Phenotypic Plasticity and Adaptations to Extreme Environments.

The mangrove rivulus (Kryptolebias marmoratus) is one of two preferentially self-fertilizing hermaphroditic vertebrates. This mode of reproduction makes mangrove rivulus an important model for evolutionary and biomedical studies because long periods of self-fertilization result in naturally homozygous genotypes that can produce isogenic lineages without significant limitations associated with inbreeding depression. Over 400 isogenic lineages currently held in laboratories across the globe show considerable among-lineage variation in physiology, behavior, and life history traits that is maintained under common garden conditions. Temperature mediates the development of primary males and also sex change between hermaphrodites and secondary males, which makes the system ideal for the study of sex determination and sexual plasticity. Mangrove rivulus also exhibit remarkable adaptations to living in extreme environments, and the system has great promise to shed light on the evolution of terrestrial locomotion, aerial respiration, and broad tolerances to hypoxia, salinity, temperature, and environmental pollutants. Genome assembly of the mangrove rivulus allows the study of genes and gene families associated with the traits described above. Here we present a de novo assembled reference genome for the mangrove rivulus, with an approximately 900 Mb genome, including 27,328 annotated, predicted, protein-coding genes. Moreover, we are able to place more than 50% of the assembled genome onto a recently published linkage map. The genome provides an important addition to the linkage map and transcriptomic tools recently developed for this species that together provide critical resources for epigenetic, transcriptomic, and proteomic analyses. Moreover, the genome will serve as the foundation for addressing key questions in behavior, physiology, toxicology, and evolutionary biology.

Genome-wide karyomapping accurately identifies the inheritance of single-gene defects in human preimplantation embryos in vitro.

PURPOSE: Our aim was to compare the accuracy of family- or disease-specific targeted haplotyping and direct mutation-detection strategies with the accuracy of genome-wide mapping of the parental origin of each chromosome, or karyomapping, by single-nucleotide polymorphism genotyping of the parents, a close relative of known disease status, and the embryo cell(s) used for preimplantation genetic diagnosis of single-gene defects in a single cell or small numbers of cells biopsied from human embryos following in vitro fertilization. METHODS: Genomic DNA and whole-genome amplification products from embryo samples, which were previously diagnosed by targeted haplotyping, were genotyped for single-nucleotide polymorphisms genome-wide detection and retrospectively analyzed blind by karyomapping. RESULTS: Single-nucleotide polymorphism genotyping and karyomapping were successful in 213/218 (97.7%) samples from 44 preimplantation genetic diagnosis cycles for 25 single-gene defects with various modes of inheritance distributed widely across the genome. Karyomapping was concordant with targeted haplotyping in 208 (97.7%) samples, and the five nonconcordant samples were all in consanguineous regions with limited or inconsistent haplotyping results. CONCLUSION: Genome-wide karyomapping is highly accurate and facilitates analysis of the inheritance of almost any single-gene defect, or any combination of loci, at the single-cell level, greatly expanding the range of conditions for which preimplantation genetic diagnosis can be offered clinically without the need for customized test development.

Co-regulation of nuclear genes encoding plastid ribosomal proteins by light and plastid signals during seedling development in tobacco and Arabidopsis.

Genes encoding plastid ribosomal proteins are distributed between the nuclear and plastid genomes in higher plants, and coordination of their expression is likely to be required for functional plastid protein synthesis. A custom microarray has been used to examine the patterns of accumulation of transcripts from plastid and nuclear genes encoding plastid ribosomal proteins during seedling development in tobacco and Arabidopsis. The transcripts accumulate coordinately during early seedling development and show similar responses to light and to inhibitors, such as norflurazon and lincomycin, affecting plastid signaling. Computational analysis of the promoters of these genes revealed a shared initiator motif and common cis-elements characteristic of photosynthesis genes, specifically the GT-1 element, and the I-box. Analysis of the RPL27 gene of Arabidopsis thaliana indicated that transcription initiates from an initiator-like region. Deletion analysis of the RPL27 promoter in transgenic plants revealed that the identified shared cis-elements were not all required for wild-type expression patterns, and full developmental, light- and plastid-regulation can be conveyed by a region of the promoter from -235 to +1 relative to the transcription start site.