engleski

Apparent reversion of deletion mutants for oxytetracycline production in Streptomyces rimosus

The oxytetracycline (OTC) gene cluster lies about 600 kb from the end of the linear chromosome of S. rimosus and mutants that have lost the cluster (Class II mutants) occur at a frequency of up to 1% of spores. These deletion mutants have lost about 800 kb of DNA including the chromosome end and sporulate poorly (Gravius et al., Appl. Environ. Microbiol. 59:2220, 1993). The deletion of the OTC-cluster results in loss of production and sensitivity to OTC because the resistance genes are in the cluster. Surprisingly, it was possible to isolate rare OTC-producing derivatives from the two Class II mutants (MV9 and MV15) that were tested. These derivatives were also resistant to OTC and sporulated well. Southern analysis showed that the OTC-cluster appeared to be present, but PFGE analysis showed that most of the "revertants" differed from the original production strain. However, they were unstable and some derivatives were indistinguishable from the original strain. Careful analysis of the Class II mutants showed that all of them still produce small amounts of OTC. A hypothesis to explain these observations is that the deletion leads to the generation of a molecule carrying the deleted sequences that is not replicated efficiently during mycelial growth and is thus present only at low copy number in the mycelium. If this molecule were selectively segregated into aerial mycelium and spores (e.g. because it carries a gene necessary for sporulation) it would be retained during maintenance of the strain. The "revertants" would be formed by rare reintegration of the molecule into the chromosome restoring a normal copy number.

Streptomyces rimosus; oxytetracycline; deletion mutants; apparent "reversion"

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