Cytochrome b of cob revertants in yeast. 1. Isolation and characterization of revertants derived from cob exon mutants of Saccharomyces cerevisiae.

About 300 revertants were derived from 44 cob- mutants, mapping in the structure coding regions (exon 1, 3, 4, 5, or 6) of the mitochondrial apocytochrome b gene in Saccharomyces cerevisiae, strain 777-3A. Most of the revertants could not be distinguished from the wild-type by means of physiological properties. Twenty-two revertants different in phenotype are described here in more detail. The suppressor mutations (supa) that compensate the primary cob- mutations (i.e., restore growth on glycerol) are mitochondrially inherited. They were localized in the same cob exon regions as the respective primary mutations, except for one revertant with a primary mutation in exon 6 and a suppressor, 4.2 map units distant, which may be located either in intron 5 or downstream in exon 6. Of 21 suppressors 17 are closely coupled to the primary mutation with recombination frequencies of less than or equal to 0.1%-0.3%. An estimate predicts that in more than 80% of these revertants only one amino acid is altered at that point of the polypeptide corresponding to the cob- site in the gene. The most interesting revertant phenotypes are: reduced growth rate on glycerol. The respective cob-/supa mutations are scattered over the whole cob region and cannot be correlated exclusively with special gene regions. decreased cytochrome b content. The most extreme reductions (28% and 30% of wild-type level) were observed to be due to mutations located in the 5' proximal part of exon 1. The highest percentage of revertants with decreased cytochrome b content was predominantly found mapping in exon 3.(ABSTRACT TRUNCATED AT 250 WORDS)