Studies on a Widely Distributed Phage Tail-Derived Toxin Injection System

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The viruses that infect bacteria (bacteriophages or phages) are the most abundant biological entities on earth. Most phages possess an icosahedral head containing their double-stranded DNA genome and a “tail” attached to one vertex of the head. The tail is responsible for attaching to the host bacterium and delivering the genome to the interior of the cell. As such, phage tails are very complex nanomachines with the ability to specifically bind the outer surface of bacteria, penetrate the cell wall and membranes, and inject macromolecules (DNA and proteins) into the cytoplasm. The tremendous effectiveness of phage tails as macromolecular injection devices has led to their being co-opted by bacteria for a variety of purposes, as exemplified by the Type VI secretion system, which is a widespread phage tail-derived virulence determinant in diverse Gram-negative organisms. ***The subject of this proposal is another contractile phage tail-derived injection system known as the Photorhabdus Virulence Cassette (PVC). Few PVCs have been characterized despite their sporadic occurrence in almost all bacterial clades and in Archaea. In addition to proteins similar to phage tail proteins, PVC operons also encode proteins commonly associated with secretion systems, such as a AAA ATPases and toxins. Two different PVCs have been shown to kill insect cells and another stimulates morphogenesis of a marine tube worm. The functions of many hundreds of other diverse PVCs are unknown. ***Most bacterial species within the Streptomyces genus encode PVCs, but no studies have ever been undertaken on them. We have found that the PVC operon is expressed in a cell cycle specific manner within one Streptomyces species and that the operon encodes a phage tail-like entity. We have identified distinct phenotypes in strains bearing mutations in PVC operons. Our future work is aimed at uncovering the functions of PVCs in Streptomyces, determining the biochemical properties and functions of individual PVC components, and gaining insight into the general roles in nature of these mysterious structures. ***Streptomyces produce a huge diversity of antibiotics and other useful pharmaceuticals. Our preliminary data indicate that PVCs play a role in Streptomyces antibiotic production, and further insight into this function may be significant to the antibiotic field. Additionally, PVCs represent a unique, soluble protein injecting nanomachine with the potential to inject proteins into a variety of eukaryotic cells. Determining how PVCs target cells would allow us to direct PVCs to any desired cell type. Similarly, deciphering the mechanism by which PVCs inject proteins presents the potential to inject any desired protein into a targeted cell. Thus, the knowledge gained through our work will create opportunities to engineer PVCs for a variety of novel biotechnological purposes that could be relevant in agriculture, industry, and human health.