Steps in processing of the mitochondrial cytochrome oxidase subunit I pre-mRNA affected by a nuclear mutation in yeast.

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Abstract
In S. cerevisiae, the mitochondrial gene encoding the subunit I of cytochrome c oxidase (oxi-3 gene) is interrupted by intervening sequences. A nuclear mutation [referred to as mss51 in Faye and Simon, 1983] that specifically affects the processing of oxi-3 pre-mRNA is further characterized. DNA probes covering each oxi-3 exon-intron boundary were individually hybridized to wild-type and mutant mitochondrial RNA. A technique relying on the S1 nuclease resistance or sensitivity of the RNA-DNA hybrids thereof showed which site needs the MSS51 gene product to be cleaved. The mutation in the MSS51 gene gave rise to a complex pattern of splicing; the 3rd intron was excised efficiently but the first 2 introns remained bracketed by their flanking exons. The 4th and 5th introns were only partially split from their common exon and remained fused to their upstream and downstream flanking exon, respectively. Several plausible roles for the MSS51 gene product are discussed.