RNA processing in yeast mitochondria: characterization of mit(-) mutants disturbed in the synthesis of subunit I of cytochrome c oxidase.

Mit(-) mutants disturbed in the synthesis of cytochrome c oxidase subunit I lack the mRNA for this protein and accumulate longer RNAs still containing intron sequences. We have analyzed the patterns of transcripts occurring in several such mutants in an attempt to define a pathway of processing events and to demarcate intron-sequences involved in RNA splicing. We find that processing does not follow a strictly ordered pathway and, in contrast to the situation for the cytochrome b gene, that a block in the processing of an intron does not necessarily lead to a block in the processing of introns downstream. Although in some cases, this may result from overlapping specificities of intronic-URF encoded RNA maturases, an internal start of translation on precursor RNAs seems more likely.M5-16, a mutant deleted for a large part of the central portion of the subunit I gene exhibits delayed processing and a highly simplified pattern of intermediates. The lengths of these indicate that maturation of the mRNA for subunit I involves processing, as well as splicing.