Effect of Growth Phase of Host Bacteria on Synthesis of R Factor‐Mediated Penicillinase

Abstract

The R factor is a plasmid which is parasitic on many gram-negative bacteria and confers drug resistance on its host organism. It is also suspected that the stability of the R factor in the host and the expression of the genes on the R factor are affected by physiological conditions of the host bacteria. During an investigation on the mechanism of penicillinase (E.C. 3.5.2.6, PCase) synthesis mediated by two R factors, RGN14 and RGN823, we observed an interesting phenomenon that PCase and chloramphenicol acetyltransferase (CATase) syntheses of the R factors were significantly affected by the growth age of Proteus mirabilis as the host bacterium. There also was a striking contrast between the PCase and CATase formations in the host bacteria though the enzymes were governed by genetic determinants amp and cml on the same plasmid. In this communication, we report the effect of the growth phase of P. mirabilis 25 and Escherichia coli ML 1410 as the host organisms on the expression of the genes governing the two kinds of enzymes.R factors, RGN14 and RGN823, were capable of confering resistance to ampicillin, chloramphenicol, tetracycline, streptomycin, and sulfonamide on the host organisms, and were originally isolated from E. coli [2] and Klebsiella pneumoniae [5], respectively. RGN14 and RGN823 mediate the syntheses of type Ia and type Ib PCases respectively. Type Ib PCase is similar in enzymological and immunochemical properties to type Ia PCase [9], and our recent study suggested that RGN823 can produce a larger number of PCase molecules than RGN14 (our unpublished. data). PCase activity was determined iodometrically according to the method of Perret [6], and CATase activity was assayed by the method of Shaw and Brodsky [10].PCase and CATase activities in the E. coli