Investigating Mycobacteriophage-Host Protein Interactions

Mycobacteriophages are viruses that infect bacterial cells of the genus Mycobacterium. They possess a multitude of unfamiliar or novel genes – genes encoding protein sequences that do not resemble any previously studied proteins – and thus encode products with functions not readily predicted. We hypothesized that some of those genes encode products that interfere with the normal metabolism of the host cell, possibly through specific phage-host protein-protein interactions, and thus have a role in enabling phage infection. Further, we predicted that those gene products when expressed alone in host cells would still be toxic and impair cell growth. We have investigated unfamiliar genes in two genetically distinct mycobacteriophages, Pumpkin and Vix, and have identified 4 single genes (Pumpkin_115, Pumpkin_119, Pumpkin_142, Vix_80) and several small genomic regions (Pumpkin gene segment 130-133 and Vix gene segments 65-66, 68-72, 87-88) that are cytotoxic to M. smegmatis. We are taking a multi-prong approach to further identify the specific functions associated with these genes and their products and to determine their roles in the infection process: 1) we have identified mutants of M. smegmatis that are resistant to the expression of those genes, 2) we are using E. coli- expressed phage genes to screen for interacting host proteins, 3) we are collecting microscopy data that could identity phage interruption of normal cellular function, and 4) we are in the process of deleting these genes from the phage genome to determine the effect on infection. 24-hour expression of individual cytotoxic phage genes in M. smegmatis resulted in a significant increase in mean host cell length and some subtle effects on cell shape. Ongoing analysis of the mutants has identified a common mechanism of resistance to distinct phage gene expression, while protein-protein interaction studies have not yet identified a potential host target involved in translation