Reiterated sequences of the Streptomyces griseus plasmid pSG1

pSG1 is a 16.9-kb element that is comaintained with its chromosomally integrated form in Streptomyces griseus YP5344. Three forms of amplifiable units of DNA (AUDs) were found on pSG1 in S. griseus YP5344 derivatives. All three share a 3.1-kb fragment, but each AUD is flanked by different sequences. Fragments bracketing the AUDs on pSG1 and fragments of junctions between the AUDs from amplifiable DNA sequences (ADS) containing pSG1 derivatives were cloned and sequenced. The edges of the AUDs are not homologous, nor are they flanked by direct repeats. One AUD is bracketed by a short (8-nt) inverted repeat. This AUD was reiterated in six independent isolates, suggesting preferred sites for the reiteration mechanism. Cells harboring plasmids with ADS also harbor circular molecules consisting of the same AUD. These findings suggest that reiteration is initiated by an illegitimate recombination event that yields an AUD duplication on the plasmid or an AUD DNA circle. Either of these structures may serve as an intermediate in the reiteration process. Circular AUDs may also be formed by intramolecular recombination between the repeats during the process of deamplification.