Regulation of Bacteriodes fragilis katB mRNA by oxidative stress and carbon limitation

Regulation of the katB catalase gene in the anaerobic bacterium Bacteroides fragilis was studied. Northern blot hybridization analyses revealed that katB was transcribed as an approximately 1.6-kb monocistronic mRNA. The levels of katB mRNA increased > 15-fold when anaerobic, mid-logarithmic-phase cultures were exposed to O2, O2 with paraquat, or hydrogen peroxide. Under anaerobic conditions, the low levels of katB mRNA increased in a growth-dependent manner, reaching maximum expression at late logarithmic or early stationary phase, followed by a decrease in stationary phase. Under anaerobic conditions, the expression of katB mRNA was strongly repressed by glucose and to a lesser extent by xylose. However, glucose repression was completely abolished upon exposure to oxygen. The nonfermentable carbon sources fumarate, succinate, acetate, and pyruvate did not significantly affect expression. Phosphate, nitrogen, and hemin limitation did not affect the expression of katB mRNA, suggesting that the nutritional control of katB expression is restricted to carbon and energy sources and not other forms of nutrient limitation. Primer extension analysis revealed that during both oxidative stress and carbon or energy limitation, katB utilized the same promoter region but transcription initiation occurred at two different nucleotides separated by 3 or 4 bases. Interestingly, a 6-bp inverted repeat sequence present in the katB regulatory region was also observed upstream of the B. fragilis superoxide dismutase gene sod. It is possible that this is a recognition site for a DNA binding protein involved in the regulation of oxidative stress genes in this organism.