A nuclear mutation in maize blocks the processing and translation of several chloroplast mRNAs and provides evidence for the differential translation of alternative mRNA forms.
A mutant designated crp1 (chloroplast RNA processing 1) was identified in a screen for transposon-induced maize mutants with defects in chloroplast gene expression. crp1 is a recessive, nuclear mutation that causes the loss of the cytochrome f/b6 complex and a reduction in photosystem I. The molecular basis for these protein losses is unique relative to previously described mutants with defects in organelle gene expression; it involves defects in the metabolism of two organellar mRNAs and in the translation of two organellar proteins. Mutants lack the monocistronic forms of the petB and petD mRNAs (encoding cytochrome f/b6 subunits), but contain normal levels of their polycistronic precursors. Pulse-labeling experiments revealed normal synthesis of the petB gene product, but a large decrease in synthesis of the petD gene product. These results suggest that petD sequences are more efficiently translated in a monocistronic than in a polycistronic context, thereby providing evidence that the elaborate RNA processing typical of chloroplast transcripts can play a role in controlling gene expression. Structural predictions suggest that the petD start codon lies in a stable hairpin in the polycistronic RNA, but remains unpaired in the monocistronic transcript. Thus, processing to a monocistronic form may increase translational efficiency by releasing the translation initiation region from inhibitory interactions with upstream RNA sequences. Synthesis of a third cytochrome f/b6 subunit, encoded by the petA gene, was undetectable in crp1, although its mRNA appeared unaltered. Two mechanisms are consistent with the simultaneous loss of both petA and petD protein synthesis: the translation of the petA and petD mRNAs might be coupled via a mechanism independent of crp1, or the crp1 gene may function to coordinate the expression of the two genes, which encode subunits of the same complex.
Barkan
A
A
Institute of Molecular Biology, University of Oregon, Eugene 97403.
Walker
M
M
Nolasco
M
M
Johnson
D
D
D017398:Alternative Splicing; D001483:Base Sequence; D001665:Binding Sites; D002736:Chloroplasts / Q000378:metabolism; D003573:Cytochrome b Group / Q000096:biosynthesis / Q000235:genetics; D045346:Cytochrome b6f Complex; D004247:DNA; D017343:Genes, Plant / Q000235:genetics; D005808:Genes, Recessive; D008969:Molecular Sequence Data; D009154:Mutation; D045322:Photosynthetic Reaction Center Complex Proteins / Q000096:biosynthesis / Q000235:genetics; D045331:Photosystem I Protein Complex; D010940:Plant Proteins / Q000096:biosynthesis / Q000235:genetics; D011132:Polyribosomes / Q000378:metabolism; D014176:Protein Biosynthesis; D012322:RNA Precursors / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000378:metabolism*; D014158:Transcription, Genetic; D003313:Zea mays / Q000235:genetics*
D003573:Cytochrome b Group; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D012322:RNA Precursors; D012333:RNA, Messenger; D004247:DNA; D045346:Cytochrome b6f Complex