SUMMARY: The last four enzymes in the pathway of L-histidine biosynthesis have been assayed in various histidineless mutants of . Three classes of mutants (B, C and D) have been shown to be associated with lack of imidazoleglycerol phosphate dehydrase, imidazoleacetol phosphate transaminase, and histidinol dehydrogenase, respectively. These studies, in conjunction with the genetic work of Hartman, Loper & Šerman (1960), indicate that the sequence of the genes on the chromosome corresponds to the sequence of the enzymes in the pathway of the biosynthesis. Certain mutants, which were shown genetically to behave as multisite mutants, have been shown to be missing these three enzymes. These multisite mutants are also missing a fourth biosynthetic enzyme, histidinol phosphate phosphatase, for which no single site mutants are available. It has been found that the level of activity of the series of enzymes of histidine biosynthesis can be raised about 15-fold over the wild-type level by growing a histidine-requiring mutant on formylhistidine as a source of histidine.


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