Functional analysis of the Streptomyces ambofaciens element pSAM2

pSAM2 is an 11-kb element integrated in the Streptomyces ambofaciens ATCC23877 genome and found additionally as a free replicon present at several copies per chromosome in strain JI3212, the derivative of ATCC23877 isolated after uv irradiation. In spite of its small size, this element specifies numerous functions including maintenance, site-specific integration, self-transmissibility, pock formation, and mobilization of chromosomal markers. After transfer of the free form of pSAM2 to Streptomyces lividans, the free and the integrated forms coexist. A functional map of pSAM2 was deduced from phenotypes exhibited in S. lividans by numerous deletion or insertion derivatives. In addition to the previously characterized regions sufficient for site-specific integration we have shown that separate regions are involved in either plasmid maintenance as a free molecule, plasmid transfer, and pock formation. Transfer of pSAM2 could depend on its ability to be maintained in a free form, since plasmids deficient in this function are transferred at very low frequency. Deletions of some regions of the plasmid are lethal for the plasmid or the host, but if some other regions are deleted simultaneously, transformants can be obtained.