Characterization of the Dictyostelium homolog of chromatin binding protein DET1 suggests a conserved pathway regulating cell type specification and developmental plasticity.

DET1 (De-etiolated 1) is a chromatin binding protein involved in developmental regulation in both plants and animals. DET1 is largely restricted to multicellular eukaryotes, and here we report the characterization of a DET1 homolog from the social amoeba Dictyostelium discoideum. As in other species, Dictyostelium DET1 is nuclear localized. In contrast to other species, where it is an essential protein, loss of DET1 is nonlethal in Dictyostelium, although viability is significantly reduced. The phenotype of the det1(-) mutant is highly pleiotropic and results in a large degree of heterogeneity in developmental parameters. Loss of DET1 results in delayed and abnormal development with enlarged aggregation territories. Mutant slugs displayed cell type patterning with a bias toward the prestalk pathway. A number of DET1-interacting proteins are conserved in Dictyostelium, and the apparently conserved role of DET1 in regulatory pathways involving the bZIP transcription factors DimB, c-Jun, and HY5 suggests a highly conserved mechanism regulating development in multicellular eukaryotes. While the mechanism by which DET1 functions is unclear, it appears that it has a key role in regulation of developmental plasticity and integration of information on environmental conditions into the developmental program of an organism.

Transposons: a blessing curse.

The genomes of most plant species are dominated by transposable elements (TEs). Once considered as 'junk DNA', TEs are now known to have a major role in driving genome evolution. Over the last decade, it has become apparent that some stress conditions and other environmental stimuli can drive bursts of activity of certain TE families and consequently new TE insertions. These can give rise to altered gene expression patterns and phenotypes, with new TE insertions sometimes causing flanking genes to become transcriptionally responsive to the same stress conditions that activated the TE in the first place. Such connections between TE-mediated increases in diversity and an accelerated rate of genome evolution provide powerful mechanisms for plants to adapt more rapidly to new environmental conditions. This review will focus on environmentally induced transposition, the mechanisms by which it alters gene expression, and the consequences for plant genome evolution and breeding.

DNA methylation in Arabidopsis has a genetic basis and shows evidence of local adaptation.

Epigenome modulation potentially provides a mechanism for organisms to adapt, within and between generations. However, neither the extent to which this occurs, nor the mechanisms involved are known. Here we investigate DNA methylation variation in Swedish Arabidopsis thaliana accessions grown at two different temperatures. Environmental effects were limited to transposons, where CHH methylation was found to increase with temperature. Genome-wide association studies (GWAS) revealed that the extensive CHH methylation variation was strongly associated with genetic variants in both cis and trans, including a major trans-association close to the DNA methyltransferase CMT2. Unlike CHH methylation, CpG gene body methylation (GBM) was not affected by growth temperature, but was instead correlated with the latitude of origin. Accessions from colder regions had higher levels of GBM for a significant fraction of the genome, and this was associated with increased transcription for the genes affected. GWAS revealed that this effect was largely due to trans-acting loci, many of which showed evidence of local adaptation.

Overexpressing tagged proteins in plants using a modified gateway cloning strategy.

In recent years, sequence-specific recombination cloning methods such as the Gateway system have become increasingly popular for (over)expressing tagged proteins in high-throughput investigations in many different organisms, including plants. Because of their versatility and ease of use, these methods have gained favor in low- and medium-throughput investigations as well. However, due to the recombination step, the resulting fusion proteins contain long and often highly charged polylinker sequences that can interfere with their physiological function. Furthermore, in some cases the gene of interest must be cloned twice (once with and once without a stop codon) for N- and C-terminal tagging. Here, we present a hybrid combinatorial cloning strategy that overcomes many of these limitations. In the first step, the gene of interest is cloned into an entry vector containing standardized cloning sites with the desired N- or C-terminal tag and an optimized polylinker sequence. A Gateway recombination reaction is used to transfer the protein-tag fusion from the entry clone to a Gateway destination vector with the desired promoter and selectable marker for the organism of interest. As experimental requirements evolve, constructs for expressing the protein of interest with the desired tag, promoter, and selectable marker or other features can rapidly and easily be created.

A modified Gateway cloning strategy for overexpressing tagged proteins in plants.

BACKGROUND: Recent developments, including the sequencing of a number of plant genomes, have greatly increased the amount of data available to scientists and has enabled high throughput investigations where many genes are investigated simultaneously. To perform these studies, recombinational cloning methods such as the Gateway system have been adapted to plant transformation vectors to facilitate the creation of overexpression, tagging and silencing vectors on a large scale. RESULTS: Here we present a hybrid cloning strategy which combines advantages of both recombinational and traditional cloning and which is particularly amenable to low-to-medium throughput investigations of protein function using techniques of molecular biochemistry and cell biology. The system consists of a series of twelve Gateway Entry cassettes into which a gene of interest can be inserted using traditional cloning methods to generate either N- or C-terminal fusions to epitope tags and fluorescent proteins. The resulting gene-tag fusions can then be recombined into Gateway-based Destination vectors, thus providing a wide choice of resistance marker, promoter and expression system. The advantage of this modified Gateway cloning strategy is that the entire open reading frame encoding the tagged protein of interest is contained within the Entry vectors so that after recombination no additional linker sequences are added between the tag and the protein that could interfere with protein function and expression. We demonstrate the utility of this system for both transient and stable Agrobacterium-mediated plant transformations. CONCLUSION: This modified Gateway cloning strategy is complementary to more conventional Gateway-based systems because it expands the choice of tags and higher orders of combinations, and permits more control over the linker sequence lying between a protein of interest and an epitope tag, which can be particularly important for studies of protein function.

Dimerization of CtIP, a BRCA1- and CtBP-interacting protein, is mediated by an N-terminal coiled-coil motif.

CtIP is a transcriptional co-regulator that binds a number of proteins involved in cell cycle control and cell development, such as CtBP (C terminus-binding protein), BRCA1 (breast cancer-associated protein-1), and LMO4 (LIM-only protein-4). The only recognizable structural motifs within CtIP are two putative coiled-coil domains located near the N and C termini of the protein. We now show that the N-terminal coiled coil (residues 45-160), but not the C-terminal coiled coil, mediates homodimerization of CtIP in mammalian 293T cells. The N-terminal coiled coil did not facilitate binding to LMO4 and BRCA1 proteins in these cells. A protease-resistant domain (residues 27-168) that minimally encompasses the putative N-terminal coiled coil was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. This region is predicted to contain two smaller coiled-coil regions. The CtIP-(45-160) dimerization domain is helical and dimeric, indicating that the domain does form a coiled coil. The two smaller domains, CtIP-(45-92) and CtIP-(93-160), also formed dimers of lower binding affinity, but with less helical content than the longer peptide. The hydrodynamic radius of CtIP-(45-160) is the same as those of CtIP-(45-92) and CtIP-(93-160), implying that CtIP-(45-160) does not form a single long coiled coil, but a more compact structure involving homodimerization of the two smaller coiled coils, which fold back as a four-helix bundle or other compact structure. These results suggest a specific model for CtIP homodimerization via its N terminus and contribute to an improved understanding of how this protein might assemble other factors required for its role as a transcriptional corepressor.