SPLICING DEFECTIVE-MUTANTS OF THE COXI GENE OF YEAST MITOCHONDRIAL-DNA - INITIAL DEFINITION OF THE MATURASE DOMAIN OF THE GROUP-II INTRON AI2

Six mutations blocking the function of a seven intron form of the mito chondrial gene encoding subunit I of cytochrome c oxidase (COXI) and m apping upstream of exon 3 were isolated and characterized. A cis-domin ant mutant of the group IIA intron 1 defines a helical portion of the C1 substructure of domain 1 as essential for splicing. A trans-recessi ve mutant confirms that the intron 1 reading frame encodes a maturase function. A cis-dominant mutant in exon 2 was found to have no effect on the splicing of intron 1 or 2. A trans-recessive mutant, located in the group IIA intron 2, demonstrates for the first time that intron 2 encodes a maturase. A genetic dissection of the five missense mutatio ns present in the intron 2 reading frame of that strain demonstrates t hat the maturase defect results from one or both of the missense mutat ions in a newly-recognized conserved sequence called domain X.