The mutational alteration of the primary structure of yeast iso-1-cytochrome c.

A benzidine-HzOz staining procedure and genetic tests were used for the detection of cytochrome c-deficient mutants of the cyl, cy2, cy3, cy4, and cy6 loci. The chromosomal gene, CY1, was shown to encode the primary structure of iso-l-cytochrome c from bakers’ yeast. Sixteen cyl mutants were isolated after examining over 1 million colonies. These cyl mutants could be categorized as follows: (a) those which were completely deficient in iso-1-cytochrome c and failed to grow on lactate medium; (b) those which were partially deficient in iso-1-cytochrome c but failed to grow on lactate medium; and (c) those which were partially deficient in iso-1-cytochrome c and grew slightly on lactate medium. Both the concentrations of cytochrome c and the extent of growth on lactate medium of some, but not all, of the cyl mutants were increased after incubation at lower temperatures. Up to three intragenic revertants were isolated from each of 15 cyl mutants and iso-1-cytochromes c were prepared. Peptide mapping and amino acid analysis of 34 samples revealed that all of the revertants from eight of the cyl mutants apparently contained normal iso-1-cytochrome c, while a revertant which contained an altered protein was obtained from each of the other seven cyl mutants. The alterations in revertants from six of these latter cyl mutants appear to be single amino acid substitutions which are located at six different residue positions. The alteration in a re-