Plasmid-dependent co-mutation in Streptomyces coelicolor A3(2).

A spontaneous chromosomalmutation(plc A(-)) in Streptomyces coelicolor A3(2) made the inborn plasmid SCP1 susceptible to curing by UV irradiation. Lack of the SCP1 plasmid (in SCP1(-) strains) prevented the occurrence of the co-mutation process after MNNG treatment, although it left the susceptibility to the lethal effect of the mutagen virtually unaffected. SCP1(-) strains were, however, ultrasensitive to the lethal effect of UV. Curing a plc A(-) strain of its SCPI plasmid made it refractory to co-mutation by MNNG and sensitive to the lethal effect of UV; reinfected by the plasmid, the strain resumed both the co-mutation proficiency and the UV-resistance. The occurrence on the SCP1 plasmid of a gene comparable to the uvrE gene of E. coli (Nevers and Spatz 1975) was assumed.