Possible Plasmid Nature of the Determinant for Production of the Antibiotic Nisin in Some Strains of Streptococcus lactis

During recent investigations into the genetics of lactic streptococci performed in our laboratory, some observations were made which indicated that gene(s) determining the production of the polypeptide antibiotic nisin (Nis) in some strains of Streptococcus lactis may be located on a plasmid (Kozak, Rajchert-Trzpil & Dobrzanski, 1973a, 1974). The evidence included (i) the frequent and spontaneous occurrence of stable nisin-negative (Nis-) clones in populations of some nisin-producing (Nis+) strains at frequencies of 0.03 to 0.82% and (ii) an increase in the frequency of stable Nisclones as a result of growth of some Nis+ strains in the presence of proflavin (Pro), ethidium bromide (EB) or at the elevated temperature of 40 "C. A 5.4to g5-fold increase occurred following growth with Pro or EB and a 1.4to 20-fold increase as a result of growth at 40 "C. We tested for the presence of plasmid DNA in certain of these S. lactis nisin-producing strains and in Nisclones isolated from them after treatment with Pro and EB (Kozak et al. 1973a9 1974)-