Peptidase mutants of Salmonella typhimurium

Six peptidase activities have been distinguished electrophoretically in cell extracts of Salmonella typhimurium with the aid of a histochemical stain. The activities can also be partially separated by chromatography on diethylaminoethyl-cellulose. These peptidases show overlapping substrate specificities. Mutants (pepN) of the parent strain leu-485 lacking one of these enzymes (peptidase N) were obtained by screening for colonies that do not hydrolyze the chromogenic substrate l-alanyl-beta-naphthylamide. The absence of this broad-specificity peptidase in leu-485 pepN(-) mutants allowed the selection of mutants unable to use l-leucyl-l-alaninamide as a leucine source. These mutants (leu-485 pepN(-)pepA(-)) lack a broad-specificity peptidase (peptidase A) similar to aminopeptidase I previously described in Escherichia coli. Mutants (pepD) lacking a dipeptidase (peptidase D) have been isolated from a leu-485 pepN(-)pepA(-) parent by penicillin selection for mutants unable to use l-leucyl-l-glycine as a leucine source. Mutants (pepB) lacking a fourth peptidase (peptidase B) have been isolated from a leu-485 pepN(-)pepA(-)pepD(-) strain by penicillin selection for failure to utilize l-leucyl-l-leucine as a source of leucine. Single recombinants were obtained by transduction for each of the peptidases missing in a leu-485 pepN(-)pepA(-)pepD(-)pepB(-) strain. The growth response of these recombinants to leucine peptides shows that all of these peptidases can function in the catabolism of peptides and that they display overlapping substrate specificities in vivo.