Mutants of Chlamydomonas reinhardtii deficient in mitochondrial complex I:Characterization of two mutations affecting the nd1 coding sequence

The mitochondrial rotenone-sensitive NADPI:ubiquinone oxidoreductase (compl ex I) comprises more than 30 subunits, the majority of which are encoded by the nucleus. In Chlamydomonas reinhardtii, only five components of complex I are coded for by mitochondrial genes. Three mutants deprived of complex I activity and displaying slow growth in the dark were isolated after mutag enic treatment with acriflavine. A genetical analysis demonstrated that two mutations (dum20 and dum25) affect the mitochondrial genome whereas the th ird mutation (dn26) is of nuclear origin. Recombinational analyses showed t hat dum20 and dum25 are closely linked on the genetic map of the mitochondr ial genome and could affect the nd1 gene. A sequencing analysis confirmed t his conclusion: dum20 is a deletion of one T at codon 243 of nd1; dum25 cor responds to a 6-bp deletion that eliminates two amino acids located in a ve ry conserved hydrophilic segment of the protein.