Transformation of the thermotolerant streptomycete, soil isolate S541, with plasmid cloning vectors of varying size, copy number, and parent replicon (derived from pIJ101, SCP2* and SLP1.2) depressed the biosynthesis of nemadectins (polyketide-derived secondary metabolites possessing anthelmintic activity). However, production of the chemically distinct 21-hydroxyl-oligomycin A, also produced by S541, was either unaffected or increased in plasmid-containing strains. A causal relationship between plasmid carriage and the changes in secondary metabolite yield was confirmed since cured strains were restored to normal production levels and their subsequent retransformation by plasmid DNA was followed by the same effects on nemadectin and oligomycin biosynthesis as before. All the plasmids tested were highly unstable in S541 and it was generally necessary to include an appropriate selective antibiotic (usually thiostrepton) in the growth medium. Thiostrepton was not responsible for the depressive effect, since this was also observed in plasmid-containing strains (i) when grown in antibiotic-free media and (ii) when alternative selective antibiotics such as neomycin were used. In addition, the plasmid-free strain produced both nemadectins and 21-hydroxyl-oligomycin A in the presence of sub-inhibitory levels of thiostrepton. The thiostrepton resistance gene, which was present on many of the plasmids tested, did not mediate the effect since plasmids carrying other selectable markers (pIJ58, neomycin, and pIJ355, viomycin) also depressed nemadectin but not 21-hydroxyl-oligomycin A production. No obvious recombination or integration events between S541 chromosomal DNA and any of the plasmids tested were revealed by DNA-DNA Southern hybridization.


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