Abstract
Contractile injection systems (CISs) are prokaryotic phage tail-like nanostructures loading effector proteins that mediate various biological processes. Although CIS functions have been diversified through evolution and hold the great potential as protein delivery systems, the functional characterisation of CISs and their effectors is currently limited to a few CIS lineages. Here, we show that the CISs of Streptomyces davawensis belong to a unique group of bacterial CISs distributed across distant phyla and facilitate sporogenic differentiation of this bacterium. CIS loss results in decreases in extracellular DNA release, biomass accumulation, and spore formation in S. davawensis. CISs load an effector, which is a remote homolog of phage tapemeasure proteins, and its C-terminal domain has endonuclease activity responsible for the CIS-associated phenotypes. Our findings illustrate that CISs can contribute to the reproduction of bacteria through the action of the effector and suggest an evolutionary link between CIS effectors and viral cargos.