Bactericidal type IV secretion system homeostasis in Xanthomonas citri.

William Cenens,Germán G Sgro,Maxuel O Andrade,Cristina E Alvarez-Martinez,Edgar Llontop,Chuck S Farah,Nian Wang

doi:10.1371/journal.ppat.1008561

Abstract

Several Xanthomonas species have a type IV secretion system (T4SS) that injects a cocktail of antibacterial proteins into neighbouring Gram-negative bacteria, often leading to rapid lysis upon cell contact. This capability represents an obvious fitness benefit since it can eliminate competition while the liberated contents of the lysed bacteria could provide an increase in the local availability of nutrients. However, the production of this Mega Dalton-sized molecular machine, with over a hundred subunits, also imposes a significant metabolic cost. Here we show that the chromosomal virB operon, which encodes the structural genes of this T4SS in X. citri, is regulated by the conserved global regulator CsrA. Relieving CsrA repression from the virB operon produced a greater number of T4SSs in the cell envelope and an increased efficiency in contact-dependent lysis of target cells. However, this was also accompanied by a physiological cost leading to reduced fitness when in co-culture with wild-type X. citri. We show that T4SS production is constitutive despite being downregulated by CsrA. Cells subjected to a wide range of rich and poor growth conditions maintain a constant density of T4SSs in the cell envelope and concomitant interbacterial competitiveness. These results show that CsrA provides a constant though partial repression on the virB operon, independent of the tested growth conditions, in this way controlling T4SS-related costs while at the same time maintaining X. citri's aggressive posture when confronted by competitors.

Highlights

Type IV secretion systems (T4SS) are large multiprotein nanomachines spanning both the inner and outer membrane of many Gram-negative bacterial species [1]
Xanthomonas citri is a member of a family of phytopathogenic bacteria that can cause substantial losses in crops
At different stages of the infection cycle, these cells will encounter other bacterial species with whom they will have to compete for space and nutrients

Summary

Introduction

Type IV secretion systems (T4SS) are large multiprotein nanomachines spanning both the inner and outer membrane of many Gram-negative bacterial species [1]. Other examples include the Brucella suis T4SS produced inside acidic phagocytic vacuoles of macrophages [16] and the Ehrlichia ruminantium T4SS whose genes are induced during iron starvation [17] This strict environmentally-dependent regulation is common in other secretion systems; for example, the Vibrio cholera type VI secretion system (T6SS) is induced during high cell densities on chitinous surfaces [18], the Xanthomonas citri T6SS is induced in the presence of amoeba [19], the Shigella flexneri type III secretion system (T3SS) is tightly regulated by oxygen levels [20] and T3SS expression in Xanthomonas species is induced upon interaction with their plant hosts [21,22]

Methods

Results

Discussion

Conclusion