Elucidating factors involved in Streptomyces venezuelae exploratory growth

Abstract

The antibiotic-producing bacterium Streptomyces venezuelae has two distinct modes of growth: a multi-staged sporulating (classical) growth cycle, and a recently discovered ‘exploratory’ growth mode, where colonies rapidly expand across solid surfaces. Unusually, classical growth and exploratory growth appear to involve distinct cell wall synthesis strategies. Cell wall synthesis in Streptomyces occurs at the cell poles and is directed by the DivIVA protein. This is unlike many other bacteria where cell-synthesis occurs in a dispersed manner along the lateral (side) walls and is mediated by the MreB protein. Intriguingly, Streptomyces encodes both DivIVA and MreB, although MreB had only been known to function at the late stages of the sporulating cycle, promoting maturation of the dormant spores. We have discovered that during exploration, MreB functions alongside DivIVA, promoting lateral wall synthesis, with loss of mreB leading to defective exploration and cell lysis. We isolated suppressors of the mreB exploration defect. Sequencing of these mutant strains and subsequent genetic analyses led to the discovery that loss-of-function mutations in an uncharacterized gene (quiK) effectively suppressed the exploration defects associated with loss of mreB. quiK encodes a putative transcriptional regulator with predicted DNA- and ligand-binding domains. Deleting quiK within an mreB mutant background restored wild type exploration characteristics, while in a wild type background, loss of quiK resulted in significant exploration enhancement. Interestingly this quiK mutant also shows delayed progression through the classical life cycle. We are now working to understand how QuiK impacts exploration, and how its loss suppresses the mreB exploration defects. Our results are suggesting that quiK mutants have altered cell phenotypes and colony architecture, and that QuiK interacts with another protein (QuiR) of similar function to enact its effects. RNA sequencing of ΔquiK and ΔquiR mutants, compared with wild type, have revealed altered expression of multiple toxin-encoding genes, including a recently characterized contractile injection system (CIS) in Streptomyces that promotes programmed cell death. These connections are being investigated to shed light on the factors and regulatory networks influencing this unusual bacterial growth mechanism.