Abstract
The 8.8-kb amplifiable unit of DNA of Streptomyces achromogenes subsp. rubradiris, AUD-Sar 1, which carries 0.8-kb terminal direct repeats and a spectinomycin resistance determinant, can mediate high-level amplification of an AUD-Sar 1-derived 8.0-kb DNA sequence not only in S. achromogenes but also in the heterologous host Streptomyces lividans. This was seen upon introduction of AUD-Sar 1 into chloramphenicol-sensitive strains of S. lividans via the temperature-sensitive (39 degrees C) plasmid pMT660, which contains the thiostrepton resistance gene tsr. Following the cultivation of transformants at 39 degrees C on media containing spectinomycin, a number of strains which were unable to grow on thiostrepton and which carried the amplified 8.0-kb DNA sequence as arrays of 200 to 300 copies of tandem 8.0-kb repeats were found. Chloramphenicol-resistant strains of S. lividans did not yield amplified sequences under similar conditions. Studies with plasmids carrying inserted antibiotic resistance genes at two sites of AUD-Sar 1 yielded coamplified sequences which contain the inserted DNA. Transformation with a plasmid carrying a 1.0-kb deletion in AUD-Sar 1 followed by growth under similar conditions yielded a 7.0-kb repeated DNA sequence. Southern analysis revealed the absence of vector sequences located on the right side of AUD-Sar 1 in the input plasmids in all examined DNA samples of amplified strains. In contrast, a majority of the samples revealed the presence at unit copy level of AUD-Sar 1 left-adjacent sequences which are part of the input plasmids and in several samples the presence of certain vector sequences located near them. The results suggest input plasmid integration into the S. lividans chromosome prior to the generation of the amplified sequences and the deletion of AUD-Sar 1 adjacent sequences.
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