Study of regulators affecting type III and type VI secretion system in Edwardsiella Tarda

Abstract

Edwardsiella tarda is an opportunistic gram-negative bacterial pathogen possessing multifactorial virulence determinants such as abilities to invade epithelial cells, resist phagocytic killing and produce hemolysins and catalases. Type III and type VI gene clusters have been identified which play a pivotal role in the pathogenesis of E. tarda. The PhoP-PhoQ two-component system is commonly used by bacteria to sense environmental factors. Here we show that the PhoP-PhoQ system E. tarda detects changes in environmental temperature and Mg2+ concentration as well as regulates the type III and VI secretion systems through direct activation of esrB. Protein secretion is activated from 23°C to 35°C or at low Mg2+ concentrations, but it is suppressed at or below 20°C, at or above 37°C, or at high Mg2+ concentrations. The effects of temperature and Mg2+ concentration are additive. The PhoQ sensor domain has a low Tm of 37.9°C, and it detects temperatures through a conformational change of its secondary structure. Mutation of specific Pro or Thr residues increased the stability of the PhoQ sensor drastically, altering its temperature-sensing ability. The PhoQ sensor detects Mg2+ concentration through the direct binding of Mg2+ to a cluster of acidic residues (DDDSAD) and through changes that likely affect its tertiary structure. Here, we describe for the first time the use of PhoP-PhoQ as a temperature sensor for bacterial virulence control. We further extended our study on the regulation of E. tarda by another two environmental factors, iron and phosphate, since such environmental cues are also sensed inside host cells. Both high phosphate and high iron decreased the expression and secretion of T3SS and T6SS proteins by modulating the expression of esrC where greater effect was observed in presence of iron in comparison to that of phosphate. We have characterized the iron sensor Fur to be a negative regulator of T3SS and T6SS which functions through esrC. We identified the presence of a high affinity PhoB binding site (pho box) in the promoter region of evpA within the T6SS cluster which allows PhoB to positively regulate the expression of evpC, within type VI gene cluster. The two component regulatory system PhoB/PhoR responds to phosphate changes through esrC since a deletion mutant of phoB renders the bacteria to become ¿phosphate- blind¿ but still responsive to the suppressing effect of Fe2+. PhoU positively controls the expression and secretion of T3SS and T6SS proteins and may be involved in additional regulatory functions. Our study proposes a model where negative cross-talk exists between the high affinity phosphate transporter PstSCAB-phoU and the iron sensor Fur.