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

From Streptomyces 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.