Abstract
Nitrous acid-induced reversion of the mutant K26 was 20 times or more frequent when compared to the response of K 445 to this mutagen, though the latter did show an increased reversion frequency after treatment with nitrous acid. No evidence was found for extragenic suppressors among 21 phototrophs derived from K26 after nitrous acid treatment. The histidinol dehydrogenase of K26 carries an aspartic acid in place of the alanine in the wild-type (Emerson a) enzyme, and the K 445 enzyme has leucine substituted for the histidine. It is suggested that the more frequent transitions induced by nitrous acid in K26 strains might result in replacement of aspartic acid with other amino acids acceptable in place of alanine for the restoration of multiple enzymic functions of the his- 3 locus whereas the low frequency of reversion of K 445 might reflect a rigorous requirement for histidine.
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