A nucleus-encoded suppressor defines a new factor which can promote petD mRNA stability in the chloroplast of Chlamydomonas reinhardtii.

Mutations in the Chlamydomonas reinhardtii nuclear gene MCD1 specifically destabilize the chloroplast petD mRNA, which encodes subunit IV of the cytochrome b6/f complex. The MCD1 gene product is thought to interact with the mRNA 5' end to protect it from degradation by a 5' --> 3' exoribonuclease and may also have a role in translation initiation. Here we report the isolation and characterization of a semidominant, allele-specific, nucleus-encoded suppressor of the mcd1-2 mutation. The suppressor mutation, which defines a new locus MCD2, allows accumulation of 10% of the wild-type level of petD mRNA and as much as 50% of the wild-type subunit IV level. Taken together, these results suggest the suppressor mutation restores photosynthetic growth by stabilizing petD mRNA. In addition, it may promote increased translational efficiency, an inference supported by direct measurements of the subunit IV synthesis rate. Thus, both MCD1 and MCD2 may participate in both chloroplast RNA stability and translation initiation.

Small cis-acting sequences that specify secondary structures in a chloroplast mRNA are essential for RNA stability and translation.

Nucleus-encoded proteins interact with cis-acting elements in chloroplast transcripts to promote RNA stability and translation. We have analyzed the structure and function of three such elements within the Chlamydomonas petD 5' untranslated region; petD encodes subunit IV of the cytochrome b(6)/f complex. These elements were delineated by linker-scanning mutagenesis, and RNA secondary structures were investigated by mapping nuclease-sensitive sites in vitro and by in vivo dimethyl sulfate RNA modification. Element I spans a maximum of 8 nucleotides (nt) at the 5' end of the mRNA; it is essential for RNA stability and plays a role in translation. This element appears to form a small stem-loop that may interact with a previously described nucleus-encoded factor to block 5'-->3' exoribonucleolytic degradation. Elements II and III, located in the center and near the 3' end of the 5' untranslated region, respectively, are essential for translation, but mutations in these elements do not affect mRNA stability. Element II is a maximum of 16 nt in length, does not form an obvious secondary structure, and appears to bind proteins that protect it from dimethyl sulfate modification. Element III spans a maximum of 14 nt and appears to form a stem-loop in vivo, based on dimethyl sulfate modification and the sequences of intragenic suppressors of element III mutations. Furthermore, mutations in element II result in changes in the RNA structure near element III, consistent with a long-range interaction that may promote translation.

5' to 3' exoribonucleolytic activity is a normal component of chloroplast mRNA decay pathways.

Molecular genetic studies have shown that determinants of chloroplast mRNA stability lie in both the 5' and 3' untranslated regions. While it is well-known that chloroplast mRNAs are unstable in the absence of certain nucleus-encoded factors, little is known of the decay mechanisms for chloroplast mRNA in wild-type cells. Here we used a poly(G)18 sequence, which impedes both 5'-->3' and 3'-->5' exoribonucleolytic RNA decay in vivo, to study the degradation pathway of petD mRNA in wild-type and mcd1 mutant chloroplasts of Chlamydomonas; the mcd1 mutant lacks a nucleus-encoded factor required for petD mRNA accumulation. Upon inserting poly(G) at positions -20, +25, +165 or +25/+165 relative to the mature petD 5' end, mRNAs accumulate with 5' ends corresponding to the poly(G) sequence, in addition to the normal RNA with its 5' end at +1. We interpret these results as evidence for continuous degradation of petD mRNA in wild-type cells by a 5'-->3' exoribonucleolytic activity. In the case of the -20 insertion, the accumulating RNA can be interpreted as a processing intermediate, suggesting that 5' end maturation may also involve this activity. When examined in the mcd1 mutant background, petD mRNAs with the poly(G) 5' ends, but not normal +1 ends, accumulated. However, no expression of SUIV, the petD gene product, was detected. Insertion of poly(G) at +165 in wild-type cells did not demonstrably affect SUIV accumulation, suggesting that ribosomal scanning does not occur upstream of this position. However, since neither poly(G) -20 nor +165 RNA could be translated in mcd1 cells, this raises the possibility that the MCD1 product is essential for translation.

Inversions in the Chlamydomonas chloroplast genome suppress a petD 5' untranslated region deletion by creating functional chimeric mRNAs.

FUD6 is a non-photosynthetic Chlamydomonas mutant that lacks the cytochrome b6/f complex, due to a 236 bp deletion that removes the promoter and part of the 5' untranslated region (UTR) of the chloroplast petD gene, which encodes subunit IV of the complex. Two photosynthetic revertants of FUD6 that synthesized wild-type levels of subunit IV were found to contain related inversions of the chloroplast genome that resulted from recombination between small inverted repeats. These inversions created a functional chimeric petD gene that includes the promoter and part of the 5' UTR of the newly identified ycf9-psbM transciption unit, fused to the petD 5' UTR upstream of the FUD6 deletion. Accumulation of the ycf9-psbM dicistronic transcript was disrupted in the revertants, but monocistronic psbM mRNA accumulated normally. The FUD6 revertants demonstrate the ability of the Chlamydomonas chloroplast genome to undergo a large inversion without a deleterious effect on chloroplast function, reminiscent of events that have led to the evolutionary divergence of chloroplast genomes.

Abundance and half-life of the distinct oat phytochrome A3 and A4 mRNAs.

Gene-preferential oligonucleotide probes were used to determined the relative abundance and half-lives of distinct oat phytochrome A (PHYA) mRNAs. Oat PHYA mRNAs are highly conserved in the 5'-untranslated region and the coding region, but the 3'-untranslated region has an overall lower sequence conservation and was the source of gene-preferential probes. PHYA3 mRNA was estimated to be ca. 61% of the oat PHYA mRNA pool present in poly(A)+ RNA from dark-grown seedlings. The half-lives for PHYA3 and PHYA4 mRNAs were both estimated to be ca. 30 min, and a similar short half-life was estimated for the average PHYA mRNA. Sequence comparisons of PHYA mRNAs from four grass species identified conserved sequences within the 5'- and 3'-untranslated regions that might be important for PHYA mRNA degradation.

Oat phytochrome A mRNA degradation appears to occur via two distinct pathways.

We have identified possible mechanisms for the degradation of oat phytochrome A (PHYA) mRNA. The majority of PHYA mRNA molecules appeared to be degraded prior to removal of the poly(A) tail, a pathway that differs from that reported for the degradation of other eukaryotic mRNAs. Polyadenylated PHYA mRNA contained a pattern of putative degradation products that is consistent with a 5'-->3' exoribonuclease, although the participation of a stochastic endoribonuclease cannot be excluded. The poly(A) tail of PHYA mRNA was heterogeneous in size and ranged from approximately 14 to 220 nucleotides. Early PHYA mRNA degradation events did not appear to involve site-specific endoribonucleases. Approximately 25% of the apparently full-length PHYA mRNA was poly(A) deficient. Oat H4 histone, beta-tubulin, and actin mRNA populations had lower amounts of apparently full-length mRNAs that were poly(A) deficient. Degradation of the poly(A)-deficient PHYA mRNA, a second pathway, appeared to be initiated by a 3'-->5' exoribonucleolytic removal of the poly(A) tail followed by both 5'-->3' and 3'-->5' exoribonuclease activities. Polysome-associated RNA contained putative PHYA mRNA degradation products and was a mixture of polyadenylated and deadenylated PHYA messages, suggesting that the two distinct degradation pathways are polysome associated.

ß-glucuronidase gene expression and mRNA stability in oat protoplasts.

Protoplasts derived from oat (Avena sativa L.) suspension culture cells (7 days after subculturing) were electroporated with plasmid DNA containing the Escherichia coli uidA gene encoding the ß-glucuronidase reporter enzyme. Consistently high enzyme activity was observed with electroporation conditions of 500 µF and 1125 volts/cm. Enzyme activity and mRNA accumulation time courses were determined. The maximum enzyme activity was detected at 24 hours after electroporation, while the maximum mRNA level was detected at 12 hours after electroporation. ß-glucuronidase mRNA was in vitro synthesized with and without a 5' methylated cap and then electroporated into protoplasts. Only capped mRNA produced significant enzyme activity. By electroporating radiolabeled, in vitro synthesized mRNA, the ß-glucuronidase mRNA half-life was estimated to be ∼35 minutes in oat protoplasts.

RNase protection assays and RNA gel blots: a direct comparison of sensitivity.

RNase protection assays are commonly thought to be a more sensitive means of detecting and quantitating specific mRNAs than are RNA gel blots (Northern blots). We have directly compared the sensitivity of these two approaches by assaying for known amounts of in vitro synthesized beta-glucuronidase mRNA. With the probes and protocols employed here, the ability to detect a specific mRNA was similar whether RNase protection or RNA gel blot analyses were performed.