Intertwining nutrient-sensory networks and the control of antibiotic production in Streptomyces.

Actinobacteria are producers of a plethora of natural products of agricultural, biotechnological and clinical importance. In an era where mankind has to deal with rapidly spreading antimicrobial resistance, streptomycetes are of particular importance as producers of half of all antibiotics used in the clinic. Genome sequencing efforts revealed that their capacity as antibiotic producers has been underestimated, in particular as many biosynthetic pathways are silent under standard laboratory conditions. Here we review the global regulatory networks that control antibiotic production in streptomycetes, with emphasis on carbon- and aminosugar-related nutrient sensory pathways. Recent research has revealed intriguing connections between these regulons, and overlap and antagonism between the activities of among others the global regulatory proteins AtrA, DasR and Rok7B7 as well as GlnR (nitrogen control) and PhoP (phosphate control), are discussed. Finally, we provide ideas as to how these novel insights might help us to find ways to activate the transcription of silent biosynthetic gene clusters.

Multiple allosteric effectors control the affinity of DasR for its target sites.

The global transcriptional regulator DasR connects N-acetylglucosamine (GlcNAc) utilization to the onset of morphological and chemical differentiation in the model actinomycete Streptomyces coelicolor. Previous work revealed that glucosamine-6-phosphate (GlcN-6P) acts as an allosteric effector which disables binding by DasR to its operator sites (called dre, for DasR responsive element) and allows derepression of DasR-controlled/GlcNAc-dependent genes. To unveil the mechanism by which DasR controls S. coelicolor development, we performed a series of electromobility shift assays with histidine-tagged DasR protein, which suggested that N-acetylglucosamine-6-phosphate (GlcNAc-6P) could also inhibit the formation of DasR-dre complexes and perhaps even more efficiently than GlcN-6P. The possibility that GlcNAc-6P is indeed an efficient allosteric effector of DasR was further confirmed by the high and constitutive activity of the DasR-repressed nagKA promoter in the nagA mutant, which lacks GlcNAc-6P deaminase activity and therefore accumulates GlcNAc-6P. In addition, we also observed that high concentrations of organic or inorganic phosphate enhanced binding of DasR to its recognition site, suggesting that the metabolic status of the cell could determine the selectivity of DasR in vivo, and hence its effect on the expression of its regulon.

Genome-wide analysis of in vivo binding of the master regulator DasR in Streptomyces coelicolor identifies novel non-canonical targets.

Streptomycetes produce a wealth of natural products, including over half of all known antibiotics. It was previously demonstrated that N-acetylglucosamine and secondary metabolism are closely entwined in streptomycetes. Here we show that DNA recognition by the N-acetylglucosamine-responsive regulator DasR is growth-phase dependent, and that DasR can bind to sites in the S. coelicolor genome that have no obvious resemblance to previously identified DasR-responsive elements. Thus, the regulon of DasR extends well beyond what was previously predicted and includes a large number of genes with functions far removed from N-acetylglucosamine metabolism, such as genes for small RNAs and DNA transposases. Conversely, the DasR regulon during vegetative growth largely correlates to the presence of canonical DasR-responsive elements. The changes in DasR binding in vivo following N-acetylglucosamine induction were studied in detail and a possible molecular mechanism by which the influence of DasR is extended is discussed. Discussion of DasR binding was further informed by a parallel transcriptome analysis of the respective cultures. Evidence is provided that DasR binds directly to the promoters of all genes encoding pathway-specific regulators of antibiotic production in S. coelicolor, thereby providing an exquisitely simple link between nutritional control and secondary metabolism.