Physical and genetical characterisation of a second sex factor, SCP2, for Streptomyces coelicolor A3(2)

Summary

Covalently closed circular (ccc) DNA of uniform monomer size (c. 18×10⁶ daltons) and restriction endonuclease cleavage pattern was isolated from strains of S. coelicolor A3(2) of differing constitution in respect of the SCP1 sex factor: SCP1⁺, SCP1′, SCP1^- and NF (integrated SCP1). No such ccc DNA was found in strains of S. lividans 66 or S. parvulus ATCC 12434 whether or not they contained SCP1. These results confirmed that the 18×10⁶ dalton plasmid is not, and does not include, SCP1, which has not so far been isolated by any of a variety of methods.

Genetic data served to identify a second sex factor, SCP2, postulated to be present in SCP2⁺ state in the starting strains and to be capable of mutation to a variant form, SCP2^*, with enhanced sex factor activity. From SCP2^* strains, SCP2^- cultures were isolated, at an average spontaneous frequency of about 0.8%. Crosses of pairs of SCP1^- SCP2^- strains were almost, but not completely, sterile; thus SCP1 and SCP2 probably contribute nearly all the fertility naturally occurring in the A3(2) strain. The two sex factors share the property of exerting an effect that may be comparable with “lethal zygosis” caused by F in E. coli: it is shown by SCP1-carrying strains against SCP1^-, or SCP2^* (but not SCP2⁺) strains against SCP2^- and is revealed as a narrow zone of growth inhibition surrounding the plasmid-carrying culture on a background of the appropriate plasmid-negative strain.

Genetically defined SCP^2- strains lacked the ccc DNA found in SCP2⁺ and SCP2^* strains. Thus this DNA apparently represents the SCP2 sex factor. A preliminary restriction endonuclease cleavage map of SCP2 was constructed, with single sites for EcoRI and HindIII, four sites for SalPI (=PstI) and more than 20 sites for SalGI (SalI).