OXYGEN-REGULATED GENE-EXPRESSION IN FACULTATIVELY ANAEROBIC-BACTERIA

In facultatively anaerobic bacteria such as Escherichia coli, oxygen and other electron accepters fundamentally influence catabolic and anabolic pathways. E. coli is able to grow aerobically by respiration and in the absence of O-2 by anaerobic respiration with nitrate, nitrite, fumarate, dimethylsulfoxide and trimethylamine N-oxide as accepters or by fermentation. The expression of the various catabolic pathways occurs according to a hierarchy with 3 or 4 levels. Aerobic respiration at the highest level is followed by nitrate respiration (level 2), anaerobic respiration with the other accepters (level 3) and fermentation. In other bacteria, different regulatory cascades with other underlying principles can be observed. Regulation of anabolism in response to O-2 availability is important, too. It is caused by different requirements of cofactors or coenzymes in aerobic and anaerobic metabolism and by the requirement for different On-independent biosynthetic routes under anoxia. The regulation mainly occurs at the transcriptional level. In E. coli, 4 global regulatory systems are known to be essential for the aerobic/anaerobic switch and the described hierarchy. A two-component sensor/regulator system comprising ArcB (sensor) and ArcA (transcriptional regulator) is responsible for regulation of aerobic metabolism. The FNR protein is a transcriptional sensor-regulator protein which regulates anaerobic respiratory genes in response to O-2 availability. The gene activator Fh1A regulates fermentative formate and hydrogen metabolism with formate as the inductor. ArcA/B and FNR directly respond to O-2, Fh1A indirectly by decreased levels of formate in the presence of O-2. Regulation of nitrate/nitrite catabolism is effected by two 2-component sensor/regulator systems NarX(Q)/NarL(P) in response to nitrate/nitrite. Co-operation of the different regulatory systems at the target promoters which are in part under dual (or manifold) transcriptional control causes the expression according to the hierarchy. The sensing of the environmental signals by the sensor proteins or domains is not well understood so far. FNR, which acts presumably as a cytoplasmic 'one component' sensor-regulator, is suggested to sense directly cytoplasmic O-2-levels corresponding to the environmental O-2-levels.