Frequent loss and restoration of antibiotic production by Streptomyces lasaliensis

Antibiotic nonproducing variants of Streptomyces lasaliensis NRRL 3382R, which makes the polyether antibiotic lasalocid A (Las) and the quinoxaline antibiotic echinomycin (Ech), arose at a frequency of 3-11% after treatment with three different mutagens or regeneration of protoplasts compared with a spontaneous frequency of less than 0.1%. Cosynthesis of lasalocid A was not observed upon testing a large number of Las- mutants in different pair-wise combinations, nor did these mutants accumulate probable intermediates of lasalocid A biosynthesis. These results suggest that loss of the las genes or their expression is induced at a high frequency by mutagenic treatments. In fusions of protoplasts of a strain with the las+ ech+ spo+ nic-1 rif-3 markers with strains bearing the Las- LasS Ech- Bld- (or spo+) str-1 markers, Las+ Ech+ Spo+ StrR progeny were produced at a 61-89% frequency compared with a 1-9% frequency of StrR antibiotic producing progeny with the nic-1 or rif-3 genotypes. The more frequent restoration of antibiotic production than prototrophy or rifampicin sensitivity indicates that these antibiotic characters did not behave as normal chromosomal markers. Therefore the genetic instability might be due to the involvement of a plasmid in antibiotic production. The apparent lack of infectious transfer of the Las+ character to Las- parents in conjugal matings between the few strains tested and no correlation between the presence of a large plasmid, pKSL, and lasalocid A production in several strains of S. lasaliensis do not favor the latter hypothesis, but they do not conclusively disprove it. Consequently, we suggest that a plasmid or another mobile genetic element is controlling antibiotic production in S. lasaliensis.