Cytoplasmic contractile injection systems mediate cell death in Streptomyces

Abstract

AbstractContractile injection systems (CIS) are bacteriophage tail-like structures that mediate bacterial cell–cell interactions. While CIS are highly abundant across diverse bacterial phyla, representative gene clusters in Gram-positive organisms remain poorly studied. Here we characterize a CIS in the Gram-positive multicellular model organismStreptomyces coelicolorand show that, in contrast to most other CIS,S. coelicolorCIS (CISSc) mediate cell death in response to stress and impact cellular development. CISScare expressed in the cytoplasm of vegetative hyphae and are not released into the medium. Our cryo-electron microscopy structure enabled the engineering of non-contractile and fluorescently tagged CISScassemblies. Cryo-electron tomography showed that CISSccontraction is linked to reduced cellular integrity. Fluorescence light microscopy furthermore revealed that functional CISScmediate cell death upon encountering different types of stress. The absence of functional CISSchad an impact on hyphal differentiation and secondary metabolite production. Finally, we identified three putative effector proteins, which when absent, phenocopied other CISScmutants. Our results provide new functional insights into CIS in Gram-positive organisms and a framework for studying novel intracellular roles, including regulated cell death and life-cycle progression in multicellular bacteria.