Histidine regulatory mutants in Salmonella typhimurium. 11. Histidine regulatory mutants having altered histidyl-tRNA synthetase

At least four genetically distinct classes of mutations give rise to triazolealanine resistance in SaZmoneZZa typhimurium. Each class studied affects regulation of the histidine operon. Mutants of one class, GO, are located at one end of the histidine operon. This class may be similar to operator constitutive mutants studied in other systems. At least two genetic sites are present in this region. A second class, hid’, is linked in transduction tests to the guaA and s&B loci. This class contains a low specific activity of histidyl-tRNA synthetase and may represent mutations in the structural gene for this enzyme (Roth & Ames, 1966). Mutations of a third class, hi.sR, are linked in transduction tests to the ilv and m&E loci (Roth & Hartman, 1965) and affect the levels of effective transfer RNA(his) (Silbert, Fink & Ames, 1966). A fourth class of mutants, hisT, is linked to the pzcrB ’ and aroD loci. 1