Chloroplast gene expression in nuclear, photosynthetic mutants of maize

To assess possible roles of nuclear genes in chloroplast biogenesis, five photosynthetic mutants of maize with nuclear lesions were characterized with respect to their chloroplast proteins and chloroplast RNAs. Each mutant is deficient in a characteristic set of polypeptides. The high sensitivity and resolution of immunoblot analysis revealed that in no case are polypeptides completely missing. Mutants deficient in the cytochrome f/b6 or photosystem I complex show a coordinate reduction of all of the core subunits of these complexes such that the correct stoichiometries are maintained. RNA transcripts encoding most of the affected plastid-encoded proteins were analyzed on Northern blots. In general, chloroplast RNAs in mutant seedlings were not altered in size or abundance, even when the corresponding protein was barely detectable. Therefore, in most cases the nuclear lesions act within the chloroplast at a post-transcriptional level to prevent expression of chloroplast-encoded polypeptides. One mutant, hcf-38, does show striking alterations in the abundance and size of many chloroplast transcripts. The hcf-38 locus might therefore encode a product that functions in chloroplast transcription or RNA processing.