Abstract
Pseudomonas aeruginosa is a common nosocomial pathogen causing acute infections in immunocompromised patients and chronic infections in cystic fibrosis patients. Gram-negative bacteria, especially pathogens, have developed diversified secretion systems to deliver effector molecules through their complex double membrane structure. P. aeruginosa contains three gene clusters named T6SS-I, -II, and -III that can encode a type VI secretion system. The valine-glycine repeat protein G (VgrG), structurally similar with the complex puncturing device of T4 bacteriophage, is one of the proteins secreted by type VI secretion systems P. aeruginosa PAO1 contains 10 vgrG-like genes. While PA0091, PA0095, and PA2685 have been shown to be the VgrGs of T6SS-I, the VgrG associated with T6SS-II is less clear. Based on comparative genomic analysis, either PA1511, PA0262, or PA5266 could be the VgrG associated with T6SS-II. Protein domain analysis revealed that PA1511 and PA0262 contain a functionally unknown extended segment in the C-terminal region. Further cytotoxicity assay of PA1511 and PA0262 by transfecting the genes into cultured HCT-8 cells failed to detect any significant effects. This study then constructed the vgrG deletion strains and examined their biological properties. Among commonly assayed phenotypes, no significant effect could be observed resulting from the vgrG deletions. In contrast, cytotoxicity and virulence to Chinese cabbage leaves are reduced in the ΔPA1511 and ΔPA5266 mutants. Finally, this study demonstrated that PA5266 and Hcp2 are secreted when PAO1 was grown in M8 minimal medium. Although the exact VgrGs secreted by T6SS-II and their functions remain to be determined, this study has established a strong basis for future analysis of the three VgrGs.