Six mutations blocking the function of a seven intron form of the mitochondrial gene encoding subunit I of cytochrome c oxidase ( COXI ) and mapping upstream of exon 3 were isolated and characterized. A cis -dominant mutant of the group IIA intron 1 defines a helical portion of the C1 substructure of domain 1 as essential for splicing. A trans -recessive 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 IA intron 2, demonstrates for the first time that intron 2 encodes a maturase. A genetic dissection of the five missense mutations present in the intron 2 reading frame of that strain demonstrates that the maturase defect results from one or both of the missense mutations in a newly-recognized conserved sequence called domain X.

Author notes

+ Present addresses: Department of Biological Sciences, Indiana University, South Bend, IN 46634
§ American Cyanamide Corp., Pearl River, NY 10965, USA