Plasmid loss and changes within the chromosomal DNA of Streptomyces reticuli

The sporulating wild-type strain of Streptomyces reticuli, which produces a melanin pigment and the macrolide leucomycin, contains plasmid DNA of 48 to 49 megadaltons. Plasmidless variants had an altered secondary metabolism and a changed antibiotic resistance pattern. By using a new colony hybridization technique developed for streptomycetes, it could be shown that plasmidless variants could be transformed with the wild-type plasmid DNA, which, however, is quickly lost from regenerated mycelium. In contrast to the wild-type strain, the plasmidless variants contain amplified nucleotide sequences within the chromosomal DNA. The number and size of these sequences vary with the strain tested. Hybridization studies revealed that the reiterated sequences are neither amplified ribosomal nor plasmid genes, but are present in small concentrations within the wild-type chromosome. Some of them share extensive homologies with each other and are located at different positions within the chromosome. It is assumed that alterations in secondary metabolism are due to changes within both the chromosomal and the extrachromosomal DNAs of S. reticuli.