Copy number mutants of the broad-host-range Streptomyces plasmid pMG200.

pMG200, isolated from the bacteriocin-releasing strain Streptomyces chrysomallus, was further physically mapped. Variants of S. chrysomallus were isolated which inhibited the parental strain. Two types of plasmids, pMG210 and pMG220, were isolated from these variants, with copy numbers of 10-30 and 300, respectively, compared with 1-3 for pMG200. pMG210 is apparently physically identical to pMG200 but presumably differs at a level not detected by simple restriction mapping; pMG220 is deleted for 1.6 kb. Genes for thiostrepton and viomycin resistance were subcloned from pIJ364 on to pMG200 and a fragment containing the gene for nourseothricin resistance was subcloned on to pMG220. In this way nonessential sites were identified.