Natural merodiploidy involving duplicated rpoB alleles affects secondary metabolism in a producer actinomycete

Actinomadura sp. ATCC 39727 produces the glycopeptide antibiotic A40926, structurally similar to teicoplanin. Production of A40926 is governed by the stringent response at the transcriptional level. In fact, addition of an amino acid pool prevented the transcription of dbv cluster genes involved in the A40926 biosynthesis and the antibiotic production in chemically defined media, and a thiostrepton-resistant relaxed mutant was severely impaired in its ability to produce the antibiotic. The derivative strain rif19, highly resistant to rifampicin (minimal inhibitory concentration, MIC > 200 mug ml(-1)), was isolated from the wild type strain that exhibited low resistance to rifampicin (MIC < 25 mug ml(-1)). In this strain A40926 production started earlier than in the wild type, and reached higher final levels. Moreover, the antibiotic production was not subjected to the stringent control. Molecular analysis led to the identification of two distinct rpoB alleles, rpoB(S) and rpoB(R), in both the wild type and the rif19. rpoB(R) harboured the H426N missense which is responsible for rifampicin-resistance in bacteria, in addition to other nucleotide substitutions affecting the primary structure of the RNA polymerase beta-chain. Transcript analysis revealed that rpoB(R) was expressed at a very low level in the wild type strain during the pseudo-exponential growth phase, and that the amount of rpoB(R) mRNA increased during the transition to the stationary phase. In contrast, expression of rpoB(R) was constitutive in the rif19. The results of mRNA half-life analysis did not support the hypothesis that post-transcriptional events are responsible for the different rpoB expression patterns in the two strains, suggesting a role of transcriptional mechanisms.