The respiratory deficient yeast nuclear mutant MK3 is defective in the synthesis of the mature transcripts of the mitochondrial COB and OX13 genes, which code for apocytochrome b and subunit I of cytochrome c oxidase, resp. Introns 3 and 4 of the COB transcript (bI3 and bI4) and intron 4 (aI4) of the OXI3 transcript can not be excised (Pillar et al. 1983a, b). When combined with mitochondrial genomes lacking introns bI1, bI2 and bI3, or lacking intron bI3 alone the mutant is respiratory competent. Thus, the non-excision of bI4 and aI4 turns out to be an indirect effect of the mutation. From a wild type yeast genebank a plasmid has been isolated with a 3.3 kb DNA insert, which complements the mutant. Subcloning experiments assigned the functional gene to a 1.6 kb HaeIII-Sau3A fragment. Hybridization experiments showed, that it is (i) a single copy gene, (ii) also present in strain D273-10B, containing the "short form" mitochondrial genome (lacking the COB introns bI1-bI3), and (iii) located on chromosome IX. The nuclear gene defective in mutant MK3, was named MRS1 (Mitochondrial RNA Splicing). The involvement of this nuclear gene in the excision of a single group I mitochondrial intron (bI3) of the COB transcript is discussed.
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