MtrAB activates ectoine production and triggers sporulation in response to osmotic stress in Streptomyces venezuelae

The MtrAB two-component system is a master regulator of antibiotic biosynthesis in Streptomyces species. MtrA is also required for sporulation under certain growth conditions, which means that on some growth media, ∆mtrA mutant colonies do not produce aerial hyphae or spores. These mutants are referred to as (conditionally) bald because they lack the hairy appearance of wild-type colonies. Here, we report that Streptomyces venezuelae NRRL B-65442 ∆mtrA is bald on R2YE agar but sporulates normally on MYM (Maltose Yeast Extract Medium) agar, and we demonstrate that this is caused by the presence of 10.2% sucrose in R2YE agar. Consistent with this, we found that adding 10.2% sucrose to MYM agar also inhibits sporulation of the ∆mtrA mutant. Proteomics combined with DNA binding studies revealed that MtrA directly activates the expression of the key developmental regulator genes bldM and whiI on R2YE but not MYM agar. BldM and WhiI work together to activate genes required for aerial hyphae production and sporulation. Crucially, over-expression of bldM-whiI in the ∆mtrA mutant restored normal sporulation in the presence of 10.2% sucrose. We hypothesized that MtrAB must sense and respond to osmotic stress, and consistent with this, we found that ∆mtrA and ∆mtrB mutants are bald on MYM agar containing 0.5 M NaCl. MtrA also directly activates biosynthesis of the compatible solute and osmoprotectant ectoine on growth media containing 10.2% sucrose. We propose a model in which high concentrations of sucrose or salt induce hyperosmotic stress in Streptomyces species, and this activates MtrAB. The response regulator MtrA then directly activates expression of the ectABCD operon to switch on ectoine biosynthesis and expression of bldM and whiI to trigger entry into sporulation.