Mutual relation of three pock-forming plasmids resident in Streptomyces noursei.

Abstract

From Streptonryces noursei B3, two plasmids -designated pSCY5 and pSCY6 -were isolated in addition to the known plasmid pSCY3. Strains carrying one or more of these plasmids generated pocks on a lawn of the plasmid-free strain, S. noursei KL3. The pocks elicited by pSCY3 and pSCY5 belonged to the A type; i.e., showed a clear inhibition zone, while those produced by pSCY6 were of the B type; i.e., exhibited a turbid inhibition zone. The strains carrying either pSCY3 or pSCY5, were also capable of forming pocks on a lawn of S. noursei 6T-11 harboring pSCY6, and vice versa. However, pock formation was not observed between strains harboring pSCY3 and strains carrying pSCY5. The endonuclease cleavage maps of these plasmids revealed that pSCY3 differed clearly from that of pSCY6, whereas pSCY5 was found to be a hybrid plasmid consisting of the entire pSCY3 plasmid and an 8.4 Md or longer fragment originating from pSCY6. The pocks elicited by pSCY5 were much smaller than those produced by pSCY3. Transformation experiments showed that pSCY6 elicited pocks in Streptomyces lividans as well as in S. noursei, whereas the pSCY6 transformants of S. lividans failed to produce pocks on a lawn of plasmid-free S. noursei.Numerous plasmids have recently been isolated from streptomycetes. Their functions have been examined from the point of view of antibiotic biosynthesis, regulation of antibiotic production, resistance to antibiotics by the producing organisms, relation with pleiotropic phenotypes, etc.1~5). However, their role is still poorly understood, except in the case of the plasmid carrying the genetic determinants for methylenomycin biosynthesis and resistance, sex plasmids and self-transmissible plasmids capable of producing pocks5~9). Pock formation, initially designated as lethal zygosis, was described by BIBB et al.7) in the case of SCP2 and SCP2* plasmids of Streptomyces coelicolor A3 (2)8) and this phenomenon is thought to be characteristic of several streptomycete plasmids10~12) Subsequently, KIESER et al.13) assigned the probable pock-determinant region to a part of piJ101 which was named the "spread" region . In addition to the detailed biological studies of plasmids, BIBB et al.14) showed that SLP1 plasmids in Streptonryces lividans were formed by excision of DNA segments from the chromosome of S. coelicolor during mating.On the other hand, SERMONTI et al.15) predicted that S. coelicolor might contain a "jumping" gene capable of transposing onto plasmid SCPI. In spite of these findings, there remains much to be studied about the biological and physical properties of streptomycete plasmids.S. noursei 2217-G 1 is a producer of cycloheximide and nystatin, and carries several types of plasmids.HAYAKAWA et al.16) reported the isolation of plasmids pSCY1 and pSCY2 from this streptomycete, but their functions were not elucidated. TAKEDA et al.11) of our laboratory, described pSCY2, pSCY3 and pSCY4 in S. noursei variant strain B3 and constructed the endonuclease cleavage map of pSCY3 which had been transformed into Esche...