Metabolic changes and genetic regulation of Streptomyces exploration

Abstract

Streptomyces bacteria have two distinct growth modes on solid medium. When growing via the classical developmental life cycle, a colony transitions between a metabolically active vegetative mycelium and dormant, physically resistant spores without substantial colonization of the surrounding environment. Conversely, during exploratory growth, a Streptomyces colony rapidly expands outwards as a vegetative mycelium to colonize a substantial area of the growth substrate. Exploration can occur when Streptomyces are grown alone or in the presence of various yeasts; successful exploratory growth in both conditions depends on adequate iron acquisition by the streptomycete. We characterized the transcriptional changes associated with exploration for S. venezuelae and observed upregulation of respiration and oxidative stress responses. Transcriptional data led to our discovery that adding glycerol to exploration-promoting conditions accelerated the rate of exploration. In characterizing the transcriptional and metabolic changes accompanying these new conditions, we observed induced production of an antibiotic (chloramphenicol) and a siderophore (foroxymithine). Loss of foroxymithine reduced the rate of exploration during coculture with yeast, highlighting a role for foroxymithine during competition. In studying how S. venezuelae metabolizes glycerol, we observed that adding glycerol to classical development-promoting medium resulted in colonies that explored from the edge and sporulated from the centre, suggesting that the two growth modes are not mutually exclusive. We demonstrated that the phenotype depended on regulators of classical development that function to repress sporulation and on the cell envelope stress-responsive extracytoplsmic function (ECF) sigma factor, SigE. The network that regulates exploration is incomplete. In this work, we described how the nucleotide second messengers cAMP and c-di-GMP contribute to exploration. We provide evidence that cAMP signalling through Crp activates transcription of cdgB, encoding a diguanylate cyclase. c-di-GMP produced by CdgB appears to negatively regulate the function of the classical developmental regulator WhiG; inappropriate activation of WhiG disrupts normal exploratory growth.