In Silico Comparative Analysis of Type VI Secretion Systems in Pseudomonas putida LS46

Abstract

Six secretion systems (Type I–Type VI) have been reported in Gram-negative bacteria. The Type VI secretion (T6SS) system was identified for the first time in two human pathogens, and its components assemble a system for secretion of the hemolysin-coregulated protein (Hcp1) and the valine glycine repeat protein (VgrG). Putative genes encoding T6SS are present not only in bacteria identified as human or plant pathogens but also in symbiotic nitrogen-fixing bacteria and nonpathogenic soil bacteria. Pseudomonas putida LS46 was isolated from wastewater and was identified as a producer of medium chain length polyhydroxyalkanoates (mcl-PHAs), natural polyester polymers synthesized by bacteria as energy storage molecules. Using the COG ID of the T6SS from P. aerugonosa PAO1 as a probe, three putative T6SSs were identified in P. putida LS46. Two of the T6SSs (S1 and S2) were identical in their organization, and their gene products showed a high degree of amino acid sequence homology. The third T6SS (S3) had a different gene organization, and some of the T6SS genes present in S1/S2 were absent in S3. Many P. putida strains have highly conserved T6SS genes, which have low amino acid sequence identity to T6SS proteins of Vibrio cholerae, Aeromonas hydrophila, Rhizobium leguminosarum, and Burkholderia mallei. Accumulation of PHA in bacteria helps to withstand starvation and survive under hostile environmental conditions. We speculate the GacA/GacS system regulates Quorum Sensing, which in turn regulates T6SS. T6SS effector molecules could be involved in signal transduction within the population for induction of PHA synthesis and accumulation.