A mutational hot-spot in the hisM gene of the histidine transport operon in Salmonella typhimurium is due to deletion of repeated sequences and results in an altered specificity of transport.

Duplicated sequences within hisM, a gene coding for a membrane-bound component of histidine transport, result in frequent deletions which, being in frame, allow production of an altered protein with apparent changed specificity of transport. While the wild-type transport system does not transport L-histidinol but does transport L-histidine and several of its analogs, the hisM deletion mutants do not transport the latter compounds but do transport L-histidinol. These results are interpreted as supporting the hypothesis (Ames and Higgins 1983) that transport through periplasmic systems involves binding of the substrate by the cytoplasmic membrane-bound components.