At least four genetically distinct classes of mutations give rise to triazolealanine resistance in Salmonella typhimurium. Each class studied affects regulation of the histidine operon.

Mutants of one class, hisO, 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, hisS, is linked in transduction tests to the guaA and strB 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, hisR, are linked in transduction tests to the ilv and metE 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 purF and aroD loci.