Transcripts of Yeast Mitochondrial DNA: Processing of a Split-gene Transcript and Expression of RNA Species during Adaptation and Differentiation Processes

Abstract

Recent progress in our understanding of the organization and expression of the split gene cob (or box) was greatly dependent on the study of about 100 mutations in this gene. These mutations have been assigned to five of the six exon sequences, coding for apocytochrome b, and to the intron sequences I1 to I4 (Fig. 1) (Jacq et al. 1980b; Bechmann et al. 1981; Grosch et al. 1981). All intron mutations (and also some exon mutations) lead to an arrest of cob RNA processing either by altering nucleotide sequences that, at the level of RNA, are essential for splicing or by affecting intron-coded products that are likely to be involved in RNA maturation (Halbreich et al. 1980; Lazowska et al. 1980; Van Ommen et al. 1980; Schmelzer et al. 1981). To study the pathways of processing, we have taken advantage of mutants that accumulate a variety of splicing intermediates. We propose a model that explains some apparently conflicting results and further use it to interpret some findings made in studies on RNA processing in cells grown under different conditions.

PROCESSING OF THE TRANSCRIPT OF THE SPLIT GENE cob
In wild-type yeast cells a series of precursors of the 18S (2.2 kb) mRNA for apocytochrome b are detected (Fig. 2). Analysis of the compositions of their exon and intron sequences provides evidence for a nonrigid order of splicing events during the formation of the 2.2-kb mRNA from a 7.5-kb precursor RNA. Table 1 lists different precursors in the order of their relative amounts detected...