Regulation of Type VI Secretion System during Burkholderia pseudomallei Infection

Abstract

Type III and type VI secretion systems (T3SSs and T6SSs, respectively) are critical virulence determinants in several Gram-negative pathogens. In Burkholderia pseudomallei, the T3SS-3 and T6SS-1 clusters have been implicated in bacterial virulence in mammalian hosts. We recently discovered a regulatory cascade that coordinately controls the expression of T3SS-3 and T6SS-1. BsaN is a central regulator located within T3SS-3 for the expression of T3SS-3 effectors and regulators for T6SS-1 such as VirA-VirG (VirAG) and BprC. Whereas T6SS-1 gene expression was completely dependent on BprC when bacteria were grown in medium, the expression inside host cells was dependent on the two-component sensor-regulator VirAG, with the exception of the tssAB operon, which was dependent primarily on BprC. VirAG and BprC initiate different transcriptional start sites within T6SS-1, and VirAG is able to activate the hcp1 promoter directly. We also provided novel evidence that virAG, bprC, and tssAB are critical for T6SS-1 function in macrophages. Furthermore, virAG and bprC regulator mutants were avirulent in mice, demonstrating the absolute dependence of T6SS-1 expression on these regulators in vivo.

BPSS1504 , a Cluster 1 Type VI Secretion Gene, Is Involved in Intracellular Survival and Virulence of Burkholderia pseudomallei

“…We next evaluated whether BPSS1504 is expressed during infection of host cells. It was recently shown that the expression of various T6SS1 genes increased after internalization of B. pseudomallei in RAW macrophages and peaked around 5 to 6 h after infection (19,33). As shown in Fig.…”

“…3B). Thus, BPSS1504 does not seem to have any role in active invasion but codetermines the ability to replicate inside host cells, as was shown for other T6SS1 genes, such as hcp1, or in the absence of the T6SS regulator genes virAG (10,19; this study). BPSS1504 contributes to the induction of cell death in macrophages.…”

“…T6SS proteins, such as Hcp1, are known to be involved in the induction of MNGC formation of macrophages after B. pseudomallei and Burkholderia mallei infection (10,19,34). We therefore assessed and quantified the ability of the Bp⌬BPSS1504 mutant to induce MNGC in RAW macrophages.…”

“…VirAG represents a two-component sensor-regulator system that is encoded within the T6SS1 cluster, upregulating other T6SS1 genes, such as hcp1, after internalization of bacteria in host cells (19,21). It was also demonstrated previously that iron and zinc could negatively regulate T6SS1 gene expression (21).…”

“…At present, there is vivid activity in unraveling the quite complex regulatory cascades of T6SS gene expression and the remarkable cross talk between T3SS and T6SS proteins in B. pseudomallei BprC, encoded within cluster 3 of the T3SS in B. pseudomallei, was found to regulate T6SS1 gene expression (19,20), and the T3SS regulator BsaN was shown to be involved in the transcriptional activation of the T6SS1 regulator virA-virG (virAG) (20). VirAG represents a two-component sensor-regulator system that is encoded within the T6SS1 cluster, upregulating other T6SS1 genes, such as hcp1, after internalization of bacteria in host cells (19,21).…”