Transcriptional regulation of Streptomyces coelicolor antibiotic-specific regulatory genes

Antibiotics produced by streptomycetes are of great medical value and may play an important role in soil ecology. The regulatory mechanisms governing production of these antibiotics are not well understood. In the model organism Streptomyces coelicolor, all four antibiotics produced are affected by mutations in a putative two-component signal transduction system encoded by absA1/absA2 and by mutations in a putative RNAase encoded by absB. S1 nuclease protection assays were used to assess whether these loci control synthesis of the antibiotics actinorhodin and undecylprodigiosin by regulating transcript abundance from biosynthetic and regulatory genes specific for each antibiotic. Strains that were antibiotic-minus due to mutations in absA or absB were examined. In the absA mutant strain, transcripts for the actinorhodin biosynthetic genes actVI-ORF1 and actI, and for the pathway-specific regulatory gene actII-ORF4 were substantially lower in abundance than in the parent strain. The level of transcript for the undecylprodigiosin pathway-specific regulatory gene redD was similarly reduced in this mutant. Additionally, a strain that exhibits precocious hyperproduction of antibiotics (Pha phenotype) due to disruption of the absA locus contained elevated levels of the actVI-ORF1, actII-ORF4 and redD transcripts. In the absB mutant strain, actVI-ORF1, actI, actII-ORF4, and redD transcript levels were also substantially lower than in the parent strain. The simplest explanation for these results is that the absA and the absB regulatory pathways include transcriptional control of actII-ORF4 and redD. Studies of transcriptional fusions between the actII-ORF4 promoter and the xylE reporter gene confirmed the effects of absB on actII-ORF4 transcription. However, a mutation in absA had no apparent effect on the expression of this fusion, suggesting that absA and absB have distinct regulatory mechanisms. Of seven bld mutants tested, none appeared to significantly affect transcription from the actII-ORF4:: xylE fusion. In a separate study, transcriptional reporter gene fusions between the actII-ORF4 promoter and the lux reporter gene were used in attempts to detect antibiotic synthesis in natural soil. Expression of the fusion was detected during growth in sterile, nutrient-amended soil microcosms; it was concluded, however, that the technique used is unlikely to possess the sensitivity for detection under more natural conditions.