Ten thousand events for each sample were collected using facsdiva™ software and the data were stored and calculated after mathematical modeling using modfit lt™ software version 3.0 (Verity Software House, Topsham, ME). Cells treated with 100 μM H2O2 for 2 min were used as positive controls. Cell lysate preparation was performed as described previously (Chauvatcharin et al., 2005). Briefly, bacterial cells in 20 mL cultures were harvested and washed once with 50 mM sodium phosphate buffer pH 7.0 (PB). Cell pellets were resuspended in PB containing 1.0 mM
phenylmethylsulfonyl fluoride, a protease inhibitor, and lysed by intermittent sonication. Cleared lysates, separated by centrifugation at 10 000 g for 10 min, were used for the catalase activity assay (Beers & Sizer, 1952) and total protein determination (Bradford, 1976). One unit of catalase was defined as the amount of enzyme Selleckchem EPZ015666 capable of catalyzing the turnover of 1 μmol substrate min−1 under an assay condition. In order to test whether catalases were required for heat shock tolerance in X. campestris
pv. campestris, a series of mutants lacking catalases, that is, katA, katG, and katA-katG mutants (Jittawuttipoka et al., 2009), were assessed for their ability to survive the heat treatment by exposing the exponential-phase cultures of the mutant strains to a high temperature of 45 °C for 10 and 15 min. The results are illustrated in Fig. 1. Inactivation of katA reduced Megestrol Acetate the bacterial viability by 100-fold, while the katG mutant showed roughly a 10-fold selleck screening library reduction in the survival after the heat treatment at 45 °C for either 10 or 15 min of treatment compared with a parental strain.
The katA-katG double mutant was over 1000-fold more sensitive to the heat treatment than a parental strain. In X. campestris pv. campestris, KatA is the major catalase responsible for 80% of the total catalase activity in the exponential-phase cells, while the remaining 20% of the activity could be accounted for by KatG (Jittawuttipoka et al., 2009). When the total catalase activity in the kat mutant strains was taken into consideration, a correlation between the ability to survive the heat treatment and the total catalase activity emerged (Table 1). Among the X. campestris pv. campestris kat mutants, the katG mutant had the highest total catalase activity (4.7 ± 0.5 U mg−1 protein) and also the highest heat-treatment survival rate among the kat mutants. The katA mutant had intermediate levels for both the survival of heat treatment and the total catalase activity (Table 1). The katA katG double mutant, whose catalase activity was not detectable, also showed the lowest heat-treatment survival (Fig. 1 and Table 1). The ectopic expression of katG from pKatG (pBBR1MCS containing a full-length katG) (Jittawuttipoka et al., 2009) could complement the reduced heat resistance of the katG mutant as well as the katG katA double mutant (Fig. 1).