Deamplification and deletion of amplified DNA in Streptomyces lividans and S. fradiae

Streptomyces lividans arginine auxotrophs which show amplification of a 5.7-kb DNA sequence, arose at a very high frequency, varying from 10% to 25% of Cmls spores. The amplifiable DNA sequence was shown to be stable over many generations. However, treatment of Cmls arg mutants with subinhibitory concentrations of antibiotics such as spectinomycin, streptomycin, chloramphenicol, thiostrepton and kanamycin, either during sporulation or during vegetative growth of mycelia, led to the deletion of the entire amplified DNA sequence, including the left and right junction sequences. Depending upon the method of antibiotic treatment a reduction in the copy number of the amplified DNA was also observed. This reduction in copy number apparently occurred without drastically affecting the basic structure of the amplifiable unit of DNA. This phenomenon appears to be universal since deamplification and deletion were observed also in S. fradiae. Further, spontaneous arg mutants arose at much lower frequency from spectinomycin-pretreated Cmls cells compared to untreated cells. These arg mutants isolated in the presence of spectinomycin did not show amplification of the 5.7-kb sequence. Southern blot analysis using the 5.7-kb probe showed that the entire DNA sequence homologous to the amplifiable DNA sequence had been deleted.