engleski

Reversion of apparent deletion mutants in Streptomyces species

Streptomyces species have linear chromosomes of about 8 Mb in size. Deletion events can result in loss of both chromosome ends and circularisation of the chromosome. During studies of some deletion mutants of S. lividans 66 faint hybridisation bands corresponding to deleted sequences were observed. This suggested that a low copy number of the "deleted" sequences were maintained during the population through several cycles of single spore isolation. The sequences were also retained through a protoplast regeneration step. Revertants were isolated using rescue of non-replicating plasmids carrying a copy-number-dependent marker gene by homologous recombination into the "deleted" region. These strains were characterised using pulsed-field gel electrophoresis. A similar phenomenon was also observed in the oxytetracycline (OTC) producer S. rimosus R6. "Deletion" mutants that have lost the OTC cluster still produce small amounts of OTC and can revert to produce normal levels of OTC. A model will be presented to explain the data which assumes that a low copy level of the "deleted" sequences is maintained in the mycelium and the chromosomal end sequences are preferentially incorporated into spores.

Streptomyces; deletion mutants; apparent

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