Characterization of β-lactamase-deficient (bla) mutants of the R plasmid R1 in Escherichia coli K-12 and comparison with similar mutants of RP1

Thirty-eight mutants of R1, an R plasmid specifying the type IIIa (TEM) β-lactamase, were isolated; these mutants are partially or totally unable to synthesize the type IIIa β-lactamase. The loss of β-lactamase activity was associated with a reduction in the level of penicillin resistance conferred by the mutants upon their host strain. At least two of the mutants synthesized a β-lactamase with altered substrate specificity. These properties are compared with those of two β-lactamase-deficient mutants of plasmid RP1. The results suggest that, for both R plasmids, penicillin resistance is entirely attributable to the presence of β-lactamase activity. The properties of two R1 derivatives, pUB251 and pUB252, which have phenotypes similar to that of RP1, support this conclusion.