GamR, the LysR-Type Galactose Metabolism Regulator, Regulates hrp Gene Expression via Transcriptional Activation of Two Key hrp Regulators, HrpG and HrpX, in Xanthomonas oryzae pv. oryzae

Abstract

Xanthomonas oryzae pv. oryzae is the causal agent of bacterial leaf blight of rice. For the virulence of the bacterium, the hrp genes, encoding components of the type III secretion system, are indispensable. The expression of hrp genes is regulated by two key hrp regulators, HrpG and HrpX: HrpG regulates hrpX, and HrpX regulates other hrp genes. Several other regulators have been shown to be involved in the regulation of hrp genes. Here, we found that a LysR-type transcriptional regulator that we named GamR, encoded by XOO_2767 of X. oryzae pv. oryzae strain MAFF311018, positively regulated the transcription of both hrpG and hrpX, which are adjacent to each other but have opposite orientations, with an intergenic upstream region in common. In a gel electrophoresis mobility shift assay, GamR bound directly to the middle of the upstream region common to hrpG and hrpX. The loss of either GamR or its binding sites decreased hrpG and hrpX expression. Also, GamR bound to the upstream region of either a galactose metabolism-related gene (XOO_2768) or a galactose metabolism-related operon (XOO_2768 to XOO_2771) located next to gamR itself and positively regulated the genes. The deletion of the regulator gene resulted in less bacterial growth in a synthetic medium with galactose as a sole sugar source. Interestingly, induction of the galactose metabolismrelated gene was dependent on galactose, while that of the hrp regulator genes was galactose independent. Our results indicate that the LysR-type transcriptional regulator that regulates the galactose metabolism-related gene(s) also acts in positive regulation of two key hrp regulators and the following hrp genes in X. oryzae pv. oryzae. IMPORTANCEThe expression of hrp genes encoding components of the type III secretion system is essential for the virulence of many plantpathogenic bacteria, including Xanthomonas oryzae pv. oryzae. It is specifically induced during infection. Research has revealed that in this bacterium, hrp gene expression is controlled by two key hrp regulators, HrpG and HrpX, along with several other regulators in the complex regulatory network, but the details remain unclear. Here, we found that a novel LysR-type transcriptional activator, named GamR, functions as an hrp regulator by directly activating the transcription of both hrpG and hrpX. Interestingly, GamR also regulates a galactose metabolism-related gene (or operon) in a galactose-dependent manner, while the regulation of hrpG and hrpX is independent of the sugar. Our finding of a novel hrp regulator that directly and simultaneously regulates two key hrp regulators provides new insights into an important and complex regulation system of X. oryzae pv. oryzae hrp genes. In several plant-pathogenic bacteria, hrp genes are essential for bacterial pathogenicity (1). The hrp genes generally comprise more than 20 genes and are clustered in the genome of each bacterium. The genes mainly encode components of a type III secretion apparatus, through which the bacteria secrete numerous proteins, s...

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“…a histone‐like nucleoid‐structuring (H‐NS) protein XrvC and a LysR‐type transcriptional regulator GamR directly control the transcription of both hrpG and hrpX in Xanthomonas oryzae pv. oryzae (Liu et al , ; Rashid et al , ), a T3SS component HrcT positively regulates the expression of hrpX via binding to its promoter in X. oryzae pv. oryzicola (Liu et al , ), and the zinc uptake regulator Zur and a GntR family transcriptional regulator HpaR1 indirectly control the expression of hrp genes via HrpX and HrpG, respectively, in Xcc (An et al , ; Huang et al , ).…”

A LysR-Type Transcriptional Regulator LcrX Is Involved in Virulence, Biofilm Formation, Swimming Motility, Siderophore Secretion, and Growth in Sugar Sources in Xanthomonas axonopodis Pv. glycines

“…In Xanthomonas oryzae pv. oryzae (Xoo), a classical LTTR, GamR, is involved in galactose metabolism and positively regulates hrp expression by binding to the promoters of hrpG and hrpX (Rashid et al, 2016). However, the functions of LysR type transcriptional regulators in Xanthomonas spp.…”

Stringent response regulators (p)ppGpp and DksA positively regulate virulence and host adaptation of Xanthomonas citri

“…HrpG positively regulates the expression of HrpX, which in turn binds to a conserved motif (PIP box, plant‐inducible promoter) at promoter regions of some T3SS genes, effector genes and other virulence‐related genes (Guo et al , ; Koebnik et al , ). Several virulence regulators were found to act upstream of HrpG and/or HrpX in Xanthomonas , including post‐transcriptional regulator RsmA/CsrA (Andrade et al , ), Lon protease (Zhou et al , ), sensor kinase HpaS (Li et al , ), LysR‐type transcriptional activator GamR (Rashid et al , ) and small noncoding RNA sX13 (Schmidtke et al ., ). In addition, the virulence of Xanthomonas is also controlled by two‐component systems, quorum sensing (QS) and cyclic di‐GMP (Büttner and Bonas, ).…”