Aspartate-specific peptidases in Salmonella typhimurium: mutants deficient in peptidase E.

The only dipeptide found to serve as a leucine source for a Salmonella strain lacking peptidases N, A, B, D, P, and Q was alpha-L-aspartyl-L-leucine. A peptidase (peptidase E) that specifically hydrolyzes Asp-X peptides was identified and partially purified from cell extracts. The enzyme (molecular weight, 35,000) is inactive toward dipeptides with N-terminal asparagine or glutamic acid. Mutants (pepE) lacking this enzyme were isolated by screening extracts for loss of the activity. Genetic mapping placed the pepE locus at 91.5 map units and established the gene order metA pepE zja-861::Tn5 malB. Duplications of the pepE locus showed a gene dosage effect on levels of peptidase E, suggesting that pepE is the structural gene for this enzyme. Mutations in pepE resulted in the loss of the ability to grow on Asp-Pro as a proline source but did not affect utilization of other dipeptides with N-terminal aspartic acid. Loss of peptidase E did not cause a detectable impairment in protein degradation. Two other peptidases present in cell extracts of mutants lacking peptidases N, A, B, D, P, Q, and E also hydrolyze many Asp-X dipeptides.