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.