Salmonella typhimurium mutations affecting utilization of L-leucine beta-naphthylamide

L-leucine-beta-naphthylamide (LNA) will support growth of a leucine auxotroph of Salmonella typhimurium. Utilization of this compound depends on the presence in the cells of active peptidase N. Selection for improved growth on a suboptimal concentration of LNA yields mutants some of which contain elevated levels of peptidase N. The properties of these strains indicate that they carry tandem genetic duplication of the pepN locus: they show rec-dependent genetic instability; they contain an approximately doubled level of the pepN gene product; neighboring chromosomal loci are also duplicated; and, the mutants occur with a greatly diminished frequency in rec- strains. When selection for improved growth on LNA is applied to a rec- strain, the mutants obtained do not contain duplications. These strains appear to contain lesions in the pepN gene that lead to the production of a peptidase N with altered substrate specificity.