The genetic control of histidine biosynthesis in was examined by an analysis of 66 histidine-dependent mutants by using stable and abortive transduction. These mutants, which previously had been differentiated into classes according to their biochemical characteristics, all occupied sites within a single linkage group, referred to as the histidine region. This region has been separated into six gene loci on the basis of complementation studies and the biochemical characteristics of the mutants. The order of genes within the histidine region, , was determined from the results of reciprocal transductions to wild type and donor type. The order of mutant sites was also linearly related to the ability of these mutants to form histidine-independent transductants when infected with phage prepared on the parent strain. This observation, and the inequalities in reciprocal transduction frequencies obtained in inter-mutant transductions, support the hypothesis that all donor fragments which participate in transductions involving the histidine region are identical, and that the histidine region is located extremely close to one terminus of the donor fragment. Intergenic complementation occurred among mutants of all gene loci; in addition, three complementation units within the gene and two complementation units within the gene were detected.


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