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.
Institute of Molecular Biology, University of Oregon, Eugene 97403.
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Barkan
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Walker
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Nolasco
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Johnson
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