Altered penicillin-binding components in penicillin-resistant mutants of Bacillus subtilis

Penicillin- (cloxacillin-) resistant mutants of Bacillus subtilis were isolated in a stepwise fashion and the five penicillin-binding components (PBCs) in each were examined to determine which of the proteins, if any, corresponds to the penicillin killing site. PBCs II and V were previously eliminated as the likely penicillin targett. In the present work, PBC IV showed no change in sensitivity to cloxacillin in any of the resistant mutants isolated. PBC I did not change until the fifth-step mutant, in which it could not be detected by penicillin binding. Since PBC I did not bind penicillins that are lethal for this mutant, it also cannot be the lethal target. PBC II showed increased resistance to cloxacillin in three discrete steps, i.e., in mutants 1, 4, and 5, accompanied by changes in its electrophoretic mobility. However, the sensitivity of PBC II to penicillin G changed very little. Correspondingly, the cloxacillin-resistant mutants were unaltered in their sensitivity to penicillin G in vivo. Thus, of the five PBCs found in B. subtilis, PBC II is the most likely target for killing by penicillins.