MUTATIONS AFFECTING THE MITOCHONDRIAL GENES ENCODING THE CYTOCHROME-OXIDASE SUBUNIT-I AND APOCYTOCHROME-B OF CHLAMYDOMONAS-REINHARDTII

Mitochondrial mutants of the green alga Chlamydomonas reinhardtii that are inactivated in the cytochrome pathway of respiration have previou sly been isolated. Despite the fact that the alternative oxidase pathw ay is still active the mutants have lost the capacity to grow heterotr ophically (dark + acetate) and display reduced growth under mixotrophi c conditions (light + acetate). In crosses between wild-type and mutan t cells, the meiotic progeny only inherit the character transmitted by the mt(-) parent, which indicates that the mutations are located in t he 15.8 kb linear mitochondrial genome. Two new mutants (dum-18 and du m-19) have now been isolated and characterized genetically, biochemica lly and at the molecular level. In addition, two previously isolated m utants (dum-11 and dum-15) were characterized in more detail. dum-11 c ontains two types of deleted mitochondrial DNA molecules: 15.1 kb mono mers lacking the subterminal part of the genome, downstream of codon 1 47 of the apocytochrome b (COB) gene, and dimers resulting from head-t o-head fusion of asymmetrically deleted monomers (15.1 and 9.5 kb DNA molecules, respectively). As in the wild type, the three other mutants contain only 15.8 kb mitochondrial DNA molecules. dum-15 is mutated a t codon 140 of the COB gene, a serine (TCT) being changed into a tyros ine (TAC). dum-18 and dum-19 both inactivate cytochrome c oxidase, as a result of frameshift mutations (addition or deletion of 1 bp) at cod ons 145 and 152, respectively, of the COX1 gene encoding subunit I of cytochrome c oxidase. In a total of ten respiratory deficient mitochon drial mutants characterized thus far, only mutations located in COB or COX1 have been isolated. The possibility that the inactivation of the other mitochondrial genes is lethal for the cells is discussed.