Evidence for translated intervening sequences in the mitochondrial genome of Saccharomyces cerevisiae

Abstract

In yeast, the mitochondrial genes for subunit I of cytochrome oxidase (oxi3) and for apocytochrome b (cob) are known to be split. In some strains, the latter contains five intervening sequences, three of which coincide with clusters of mutational sites referred to in their order of transcription as the loci box3, 10, and 7, respectively. Mutations at the first of these result in the accumulation of novel, large polypeptides (apparent Mr congruent to 43,000) believed to originate from a fusion of sequences found in the NH2-terminal segment of apocytochrome b to others encoded in the intervening sequence itself. We now provide evidence for close similarities of at least a part of translated intron sequences between (a) mutants in box7 in "long" form and "short" form strains (which lack the first three introns including the one for the box3 locus); (b) mutants in a subset of box7 mutants and those in box3, and (c) between intron sequences in box7 and a sequence presumably encoded in oxi3. These structural homologies have been analyzed and shown to be referable to sequence homologies in two proteins, one derived from the second intron (box3) in cob and the other from oxi3. The accumulation in certain cob mutants of proteins and of a transcript containing a sequence specified by oxi3 provides additional strong evidence for the previously suggested regulation of oxi3 by the penultimate, box7-containing intron of cob.