INSTABILITY OF ARTIFICIALLY CIRCULARIZED CHROMOSOMES OF STREPTOMYCES-LIVIDANS

The chromosomes of Streptomyces species are linear molecules, containi ng long terminal inverted repeats and covalently bound terminal protei ns. These chromosomes undergo spontaneous deletions of the terminal se quences at high frequencies and become circularized in several cases e xamined, Artificial circularization of the Streptomyces lividans chrom osome was also achieved by targeted recombination in vivo, in which th e terminal inverted repeats of the chromosome were connected by a kana mycin resistance gene (aphll), Under kanamycin selection, the circular ized chromosomes harboured tandem amplifications of a 20.2 kb sequence that included the aphll gene flan ked by direct repeats and deletions nearby. On re!lease from kanamycin selection, the aphll amplification s and the neighbouring sequences were deleted from the chromosomes, re ndering all the cultures kanamycin sensitive. The chloramphenicol resi stance gene, which was prone to deletion in wild-type S. lividans, bec ame much more stable in the kanamycin-sensitive derivatives, These res ults indicate that the telomeres and/or certain terminal sequences may be involved in the structural instability of Streptomyces chromosomes .