In vivo analysis of plastid psbA, rbcL and rpl32 UTR elements by chloroplast transformation: tobacco plastid gene expression is controlled by modulation of transcript levels and translation efficiency

5' and 3' untranslated regions (UTRs) of plastid RNAs act as regulatory elements for post-transcriptional control of gene expression. Polyethylene glycol-mediated plastid transformation with UTR-GUS reporter gene fusions was used to study the function of the psbA, rbcL and rpl32 UTRs in vivo. All gene fusions were expressed from the same promoter, i.e. the promoter of the 16S-rRNA gene, such that variations in RNA and protein levels would be due to the involved UTR elements alone. Transgenic tobacco lines containing different combinations of UTRs showed fivefold variation in the uidA-mRNA level (RNA stability) and approximately 100-fold differences in GUS activity, a measure of translation activity. The rbcL 5'-UTR conferred greater mRNA stability than the psbA 5'-UTR on uidA transcripts. In contrast, the psbA 5'-UTR enhanced translation of GUS to a much greater extent compared to the rbcL 5'-UTR. The psbA 5'-UTR also mediated light-induced activation of translation which was not observed with other constructs. Deletion mutagenesis of an unanalysed terminal sequence element of the psbA 5'-UTR resulted in a twofold drop in uidA-mRNA level and a fourfold decrease in translation efficiency. Exchange of 3'-UTRs results in up to fivefold changes of mRNA levels and does not significantly influence translation efficiency. The mechanical impacts of these results on plastid translation regulation are discussed.

Nicotinic receptor partial agonists alter catecholamine homeostasis and response to nicotine in PC12 cells.

Repeated stress is a major public health concern where many stress responses are mediated by neuronal nicotinic acetylcholine receptors. In the present study we evaluated the effects of the nicotinic receptor partial agonists, cytisine and its derivative 3-(pyridin-3'-yl)-cytisine (3-pyr-Cyt) on two main biological outputs associated with activation of nAChR-release of neurotransmitters and increase in catecholamine biosynthesis to replenish the releasable pool. We compared these substances to the maximal response triggered by nicotine (full agonist) in PC12 cells. Cytisine, 3-pyr-Cyt or nicotine induced time-, dose- and Ca(2+)-dependent significant release of norepinephrine (NE) into the culture media. These effects were completely inhibited by mecamylamine but not by α-bungarotoxin, and only partially affected by α-conotoxin AulB, consistent with the involvement of α3ß4 receptors. Co-application of cytisine (or 3-pyr-Cyt) and nicotine resulted in attenuated nicotine-induced NE release. Cytisine or 3-pyr-Cyt alone induced a modest rise in tyrosine hydroxylase (TH) mRNA levels (index of the cell's catecholamine biosynthetic capacity). We conclude that both, cytisine and 3-pyr-Cyt (i) display typical partial agonist properties at naturally existing ganglionic nAChR (α3ß4 and α7 nAChR) with regard to catecholamine homeostasis (i.e. NE release and re-synthesis) and (ii) modulated the effect of nicotine during combined treatment.

The stem-loop region of the tobacco psbA 5'UTR is an important determinant of mRNA stability and translation efficiency.

Regulation of chloroplast gene expression involves networked and concerted interactions of nucleus-encoded factors with their target sites on untranslated regions (UTRs) of chloroplast transcripts. So far, only a few cis-acting elements within such 5'UTR sequences have been identified as functional determinants of mRNA stability and efficient translation in Chlamydomonas in vivo. In this study, we have used chloroplast transformation and site-directed mutagenesis to analyse the functions of the 5'UTRs of tobacco psbA and rbcL fused to the coding region of the reporter gene uidA. Various mutant versions of the psbA leader, as well as rbcL/psbA hybrid leader elements, were investigated. Our results showed a 1.5- to 3-fold decrease in uidA mRNA levels and a 1.5- to 6-fold reduction in uidA translation efficiency in all psbA 5'UTR stem-loop mutants generated by sequence deletions and base alterations. This indicates that the correct primary sequence and secondary structure of the psbA 5'UTR stem-loop are required for mRNA stabilisation and translation. The 5'-terminal segment of the rbcL 5'UTR did not enhance the stability or translational activity of chimeric uidA mRNA under the standard light-dark regime of 16 h light and 8 h dark. Stabilising effects were, however, observed when the cells were kept continuously in the dark. Possible reasons for the influence of the 5'UTR of the tobacco psbA on mRNA stability and translation efficiency are discussed.

Polyester synthesis in transplastomic tobacco (Nicotiana tabacum L.): significant contents of polyhydroxybutyrate are associated with growth reduction.

The pathway for synthesis of polyhydroxybutyrate (PHB), a polyester produced by three bacterial enzymes, was transferred to the tobacco plastid genome by the biolistic transformation method. The polycistronic phb operon encoding this biosynthetic pathway was cloned into plastome transformation vectors. Following selection and regeneration, the content and structure of plant-produced hydroxybutyrate was analysed by gas chromatography. Significant PHB synthesis was limited to the early stages of in vitro culture. Within the transformants, PHB synthesis levels were highly variable. In the early regeneration stage, single regenerates reached up to 1.7% PHB in dry weight. At least 70% of plant-produced hydroxybutyric acid was proven to be polymer with a molecular mass of up to 2,500 kDa. PHB synthesis levels of the transplastomic lines were decreasing when grown autotrophically but their phb transcription levels remained stable. Transcription of the three genes is divided into two transcripts with phbB being transcribed separately from phbC and phbA. In mature plants even low amounts of PHB were associated with male sterility. Fertility was only observed in a mutant carrying a defective phb operon. These results prove successful expression of the entire PHB pathway in plastids, concomitant, however, with growth deficiency and male sterility.

Integration of foreign sequences into the tobacco plastome via polyethylene glycol-mediated protoplast transformation.

A new vector, pFaadAII, for transformation of plastids of Nicotiana tabacum L. has been developed. It harbours a chimeric gene consisting of the aadA coding region from Escherichia coli, the 16S rDNA promoter from tobacco combined with a synthetic ribosome-binding site, a 500-bp fragment containing the 3' untranslated transcript region (UTR) of the Chlamydomonas rbcL gene and 3.75-kb (5') and 0.95-kb (3') tobacco plastome sequences allowing for targeting the foreign sequences to the intergenic region between the rpl32 and trnL genes of the tobacco plastome. The vector thus targets foreign sequences to the small single-copy region of the plastome, which has so far not been modified by transformation. Leaf protoplasts of Nicotiana tabacum L. were treated with polyethylene glycol (PEG) in the presence of the vector. The protocol for PEG treatment aiming at plastome transformation was optimized. Cell lines were cultured in the presence of spectinomycin and streptomycin using a novel and efficient protoplast culture and selection system. Regenerants were characterized by polymerase chain reaction (PCR) analysis, Southern hybridization and reciprocal crossing. The transformation procedure is described in detail and parameters influencing its efficiency are presented. Special effort is placed on analyzing suitable selection conditions. Only a proportion of the cell lines with a resistant phenotype could be confirmed by molecular analysis and/or reciprocal crossings to represent plastome transformants. Integration of the plastome specific aadA cassette into the nuclear genome accounted for a fraction of the resistant cell lines. Still, as many as 20-40 plastome transformants can be expected from the treatment of 10(6) protoplasts. Therefore, the improved protocol for PEG-mediated plastome transformation in combination with the new aadA-vector supplies a simple, reproducible and cost-efficient alternative to the biolistic procedure.

Analysis of barley chloroplast psbD light-responsive promoter elements in transplastomic tobacco.

The plastid gene psbD encodes D2, a photosystem II reaction center chlorophyll-binding protein. psbD is transcribed from a conserved chloroplast promoter that is activated by blue, white, or UV-A light. In this study, various forms of the barley (Hordeum vulgare L.) chloroplast psbD-LRP were fused to the uidA reporter gene and introduced into the tobacco (Nicotiana tabacum L.) plastid genome through homologous recombination. Primer extension analysis of transcripts from the psbD-LRP-uidA construct showed that the barley psbD-LRP was activated in tobacco by blue or white light. Transcription from this construct was also regulated by circadian cycling indicating that the barley psbD-LRP could respond to light modulated regulatory pathways in tobacco. Mutation of the psbD-LRP prokaryotic -10 promoter element reduced transcription to very low levels in all light regimes. In contrast, mutation of a prokaryotic -35 promoter element had no effect on transcription from the psbD-LRP. Deletion or mutation of an upstream activating element, the AAG-box (-36 to -64), also reduced transcription from the construct to very low levels. In contrast, deletion of the upstream PGT-box (-71 to -100) did not alter promoter activation by blue light, or responsiveness to circadian cycling. These in vivo studies confirm the importance of the psbD-LRP -10 promoter element and AAG-box in light regulation and demonstrate that these elements are sufficient to mediate circadian cycling of the barley psbD promoter.