The heat shock response in E. coli is positively regulated by σ32, a product of the rpoH, which has been reviewed by Arsene et al. (2000). Several of the genes involved in the heat shock response, including rpoH, groESL, and grpE-dnaKJ, have been characterized in X. campestris (Huang et al., 1998; Weng et al., 2001; Chang et al., 2005). Crosstalk between oxidative stress and heat shock responses has been investigated intensively in the eukaryotic organisms, in which the activity of catalase, a peroxide-degrading enzyme, contributes to protection against heat stress of fungal cells (Noventa-Jordao et al., 1999). Genome-wide analysis in several bacteria
has revealed overlapping and cross-induction of heat shock gene expression by hydrogen peroxide (H2O2) (Stohl et al., 2005; Zeller et al., 2005) and induction of oxidative stress-protective genes by heat stress (Guckenberger et al., 2002; find more Gunasekera et al., 2008; Luders et al., 2009). see more These observations indicate the important roles of bacterial heat stress and oxidative stress responses. Xanthomonas campestris
has evolved multiple systems to protect itself from oxidative stress. These well-orchestrated systems require the coordination of several transcriptional regulators, one of which is OxyR, the global regulator of peroxide stress response genes (Mongkolsuk et al., 1998). The known members of the OxyR regulon in X. campestris pv. campestris are katA, katG, and ahpC, encoding KatA monofunctional catalase, KatG catalase–peroxidase, and alkyl hydroperoxide reductase, respectively (Jittawuttipoka et al., 2009). The roles of KatG and KatA in providing protection against H2O2 toxicity in X. campestris pv. campestris have been elucidated. KatG plays a primary role in the Interleukin-3 receptor protection
of X. campestris pv. campestris from low levels of H2O2 toxicity, whereas KatA serves a principal function against high concentrations of H2O2 (Jittawuttipoka et al., 2009). Observation in a Gram-positive bacterium Staphylococcus aureus has demonstrated that a catalase-deficient strain is more susceptible to heat injury than its parental wild type (Martin & Chaven, 1987). The current study demonstrates that katG and katA, as well as oxyR, are essential for bacterial survival under heat stress. All X. campestris pv. campestris strains were grown aerobically in Silva–Buddenhagen (SB) medium (0.5% sucrose, 0.5% yeast extract, 0.5% peptone, and 0.1% glutamic acid; pH 7.0) at 28 °C. Overnight cultures were inoculated into a fresh SB medium to yield an OD600 nm of 0.1 . Exponential-phase cells (OD600 nm of 0.5, after 4 h growth) were used in all the experiments. The pBBR1-MCS (Kovach et al., 1995), a medium-copy-number plasmid with the lacZ promoter, was used to complement X.campestris pv. campestris mutant strains. Aliquots of exponential-phase cultures (0.5 mL in each 1.5-mL microcentrifuge tube) were placed in a water bath at 45 °C.