Existence of three regulatory regions each containing a highly conserved motif in the promoter of plastid-encoded RNA polymerase gene (rpoB)

The transcription machineries of plant mitochondria and chloroplasts: Composition, function, and regulation

Although genomes of mitochondria and plastids are very small compared to those of their bacterial ancestors, the transcription machineries of these organelles are of surprising complexity. With respect to the number of different RNA polymerases per organelle, the extremes are represented on one hand by chloroplasts of eudicots which use one bacterial-type RNA polymerase and two phage-type RNA polymerases to transcribe their genes, and on the other hand by Physcomitrella possessing three mitochondrial RNA polymerases of the phage type. Transcription of genes/operons is often driven by multiple promoters in both organelles. This review describes the principle components of the transcription machineries (RNA polymerases, transcription factors, promoters) and the division of labor between the different RNA polymerases. While regulation of transcription in mitochondria seems to be only of limited importance, the plastid genes of higher plants respond to exogenous and endogenous cues rather individually by altering their transcriptional activities.

Analysis of the tip-to-base gradient of CaM in pollen tube pulsant growth using in vivo CaM-GFP system

Ca(2+)-CaM signaling is involved in pollen tube development. However, the distribution and function of CaM and the downstream components of Ca(2+)-CaM signal in pollen tube development still need more exploration. Here we obtained the CaM-GFP fusion protein transgenic line of Nicotiana tobacum SRI, which allowed us to monitor CaM distribution pattern in vivo and provided a useful tool to observe CaM response to various exogenous stimulations and afforded solid evidences of the essential functions of CaM in pollen tube growth. CaM-GFP fusion gene was constructed under the control of Lat52-7 pollen-specific promoter and transformed into Nicotiana tobacum SRI. High level of CaM-GFP fluorescence was detected at the germinal pores and the tip-to-base gradient of fluorescence was observed in developing pollen tubes. The distribution of CaM at apical dome had close relationship with the pulsant growth mode of pollen tubes: when CaM aggregated at the apical dome, pollen tubes stepped into growth state; When CaM showed non-polarized distribution, pollen tubes stopped growing. In addition, after affording exogenous Ca(2+), calmidazolium (antagonism of CaM) or Brefeldin A (an inhibitor of membrane trafficking), CaM turned to a uniform distribution at the apical dome and pollen tube growth was held back. Taken together, our results showed that CaM played a vital role in pollen tube elongation and growth rate, and the oscillation of tip-to-base gradient of CaM was required for the normal pulsant growth of pollen tube.

Optimization of recombinant protein expression in the chloroplasts of green algae

Through advances in molecular and genetic techniques, protein expression in the chloroplasts of green algae has been optimized for high-level expression. Recombinant proteins expressed in algae have the potential to provide novel and safe treatment of disease and infection where current, high-cost drugs are the only option, or worse, where therapeutic drugs are not available due to their prohibitively high-cost to manufacture. Optimization of recombinant protein expression in Chlamydomonas reinhardtii chloroplasts has been accomplished by employing chloroplast codon bias and combinatorial examination of promoter and UTR combinations. In addition, as displayed by the expression of an anti-herpes antibody, the C. reinhardtii chloroplast is capable of correctly folding and assembling complex mammalian proteins. These data establish algal chloroplasts as a system for the production of complex human therapeutic proteins in soluble and active form, and at significantly reduced time and cost compared to existing production systems. Production of recombinant proteins in algal chloroplasts may enable further development of safe, efficacious and cost-effective protein therapeutics.

High diversity of plastidial promoters in Arabidopsis thaliana

Arabidopsis thaliana is well established as a model plant in modern plant biology. However, remarkably few details are known about plastidial promoters in Arabidopsis. Here, we report on the identification and analyses of sequences at transcription start sites of selected genes. The genes encoded by the plastome of higher plants are transcribed by a plastid-encoded (PEP) and a nuclear-encoded RNA plastid polymerase (NEP). To discriminate between NEP and PEP promoters we compared the 5'-ends of transcripts from chlorophyll-deficient Arabidopsis plants, which were grown on prokaryotic translation inhibitor spectinomycin to inhibit biosynthesis of PEP, with those of untreated plants. Using 5'-RACE combined with enzymatic treatment of RNAs to recognize primary and secondary 5'-ends, we unambiguously identified transcription initiation sites of the Arabidopsis accD, atpB, atpI, rpoB, rps4, rps15, and ycf1 genes. Comparison of plastidial promoters from tobacco and Arabidopsis revealed a high diversity, which may also apply to other plants. Furthermore, the diversity in individual promoter usage in different plants suggests that there are species-specific solutions for attaining control over gene expression in plastids.

Possible involvement of the 5'-flanking region and the 5'UTR of plastid accD gene in NEP-dependent transcription

In many developmentally and functionally important higher plant plastid genes, expression depends on a specific nuclear-encoded RNA polymerase (NEP). Molecular mechanisms for NEP-mediated gene expression are poorly understood. We have improved a transient expression assay based on biolistics and the dual-luciferase reporter technique, which facilitated investigations into the regulation of plastid genes in vivo. We scrutinized the 5'-flanking region and the 5'-untranslated region (5'UTR) of accD, a plastid gene encoding a subunit of the prokaryotic-type acetyl-CoA carboxylase which is transcribed exclusively by NEP. The results indicated that two AT-rich sequences, one of them containing two overlapping YRTA-like motifs, were essential for accD expression in vivo. The results also revealed that the length of the 5'UTR rather than a particular sequence element was a determinant for the level of accD expression. Because transcripts accumulated in proportion to reporter enzyme activity and protein levels, and transcript degradation rates were independent of the nature of the 5'UTR, it was unlikely that the 5'UTR acts as a translational enhancer or a stabilizer of the transcripts. Therefore, the length of 5'UTR might be a factor contributing to the efficiency of NEP-dependent transcription in plastids.

The plastid chromosome of Atropa belladonna and its comparison with that of Nicotiana tabacum: the role of RNA editing in generating divergence in the process of plant speciation

The nuclear and plastid genomes of the plant cell form a coevolving unit which in interspecific combinations can lead to genetic incompatibility of compartments even between closely related taxa. This phenomenon has been observed for instance in Atropa-Nicotiana cybrids. We have sequenced the plastid chromosome of Atropa belladonna (deadly nightshade), a circular DNA molecule of 156,688 bp, and compared it with the corresponding published sequence of its relative Nicotiana tabacum (tobacco) to understand how divergence at the level of this genome can contribute to nuclear-plastid incompatibilities and to speciation. It appears that (1) regulatory elements, i.e., promoters as well as translational and replicational signal elements, are well conserved between the two species; (2) genes--including introns--are even more highly conserved, with differences residing predominantly in regions of low functional importance; and (3) RNA editotypes differ between the two species, which makes this process an intriguing candidate for causing rapid reproductive isolation of populations.

Organellar RNA polymerases of higher plants

The nuclear genome of the model plant Arabidopsis thaliana contains a small gene family consisting of three genes encoding RNA polymerases of the single-subunit bacteriophage type. There is evidence that similar gene families also exist in other plants. Two of these RNA polymerases are putative mitochondrial enzymes, whereas the third one may represent the nuclear-encoded RNA polymerase (NEP) active in plastids. In addition, plastid genes are transcribed from another, entirely different multisubunit eubacterial-type RNA polymerase, the core subunits of which are encoded by plastid genes [plastid-encoded RNA polymerase (PEP)]. This core enzyme is complemented by one of several nuclear-encoded sigma-like factors. The development of photosynthetically active chloroplasts requires both PEP and NEP. Most NEP promoters show certain similarities to mitochondrial promoters in that they include the sequence motif 5'-YRTA-3' near the transcription initiation site. PEP promoters are similar to bacterial promoters of the -10/-35 sigma 70 type.