Abstract
Two-component regulatory system (TCS), a major type of cellular signal transduction system, is widely used by bacteria to adapt to different conditions and to colonize certain ecological niches in response to environmental stimuli. TCSs are of distinct functional diversity, genetic diversity, and species specificity (pathovar specificity, even strain specificity) across bacterial groups. Although TCSs have been demonstrated to be crucial to the virulence of Xanthomonas, only a few researches have been reported about the studies of TCSs in Xanthomonas oryzae pathovar oryzicola (hereafter Xoc), the pathogen of rice bacterial streak disease. In the genome of Xoc strain GX01, it has been annotated 110 TCSs genes encoding 54 response regulators (RRs), 36 orthodox histidine kinase (HKs) and 20 hybrid histidine kinase (HyHKs). To evaluate the involvement of TCSs in the stress adaptation and virulence of Xoc, we mutated 50 annotated RR genes in Xoc GX01 by homologous vector integration mutagenesis and assessed their phenotypes in given conditions and tested their virulence on host rice. 17 RR genes were identified to be likely involved in virulence of Xoc, of which 10 RR genes are novel virulence genes in Xanthomonas, including three novel virulence genes for bacteria. Of the novel candidate virulence genes, some of which may be involved in the general stress adaptation, exopolysaccharide production, extracellular protease secretion and swarming motility of Xoc. Our results will facilitate further studies on revealing the biological functions of TCS genes in this phytopathogenic bacterium.
Highlights
Two-component regulatory system (TCSs), known as two-component signal transduction systems, are widely used by bacteria to adapt to different conditions and to colonize specific ecological niches in response to environmental signals (Stock et al, 2000)
A detailed genome surveys showed that Xanthomonas spp. are of significant genetic diversities in TCS genes (Supplementary Table S1), including the strain-specific genes that might be from gene gain, loss or duplication, and the slight allelic variations that might be generated from gene fusion, fission or point mutations
Two-component regulatory system, consisting of histidine kinase (HKs) and RR, is a predominant system in bacteria to sense and adapt to changing environment, and RR is the executor of this signal transduction system (Capra and Laub, 2012)
Summary
Two-component regulatory system (TCSs), known as two-component signal transduction systems, are widely used by bacteria to adapt to different conditions and to colonize specific ecological niches in response to environmental signals (Stock et al, 2000). A typical TCS consists of a sensor with membrane-bound histidine kinase (HK) that senses a specific environmental clues and an associated response regulator (RR) with certain output domains that mediate the differential expression of target genes or the cellular level of the signal molecules, through a phosphorelay. Most recently, Wang et al (2017) reported that PcrK, a HyHK type receptor from Xcc strain 8004, can sense the plant cytokinin and regulate its autokinase activity, and PcrK-PcrR regulates oxidative stress response and bacterial virulence under cytokinin stimulation. Knockout of ravR orthologs in Xcc ATCC 33913 and Xcc XC1 led to attenuation in virulence, suggesting that strain-specificity exists in the regulatory function of some TCSs (Qian et al, 2008b; He et al, 2009). The above studies suggest that the bacterial TCSs are more complex than anticipated
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