OXYGEN-REGULATED GENE-EXPRESSION IN FACULTATIVELY ANAEROBIC-BACTERIA

In facultatively anaerobic bacteria such as Escherichia coli, oxygen a nd other electron accepters fundamentally influence catabolic and anab olic 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 occ urs according to a hierarchy with 3 or 4 levels. Aerobic respiration a t the highest level is followed by nitrate respiration (level 2), anae robic respiration with the other accepters (level 3) and fermentation. In other bacteria, different regulatory cascades with other underlyin g principles can be observed. Regulation of anabolism in response to O -2 availability is important, too. It is caused by different requireme nts of cofactors or coenzymes in aerobic and anaerobic metabolism and by the requirement for different On-independent biosynthetic routes un der 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-compo nent sensor/regulator system comprising ArcB (sensor) and ArcA (transc riptional regulator) is responsible for regulation of aerobic metaboli sm. The FNR protein is a transcriptional sensor-regulator protein whic h regulates anaerobic respiratory genes in response to O-2 availabilit y. The gene activator Fh1A regulates fermentative formate and hydrogen metabolism with formate as the inductor. ArcA/B and FNR directly resp ond to O-2, Fh1A indirectly by decreased levels of formate in the pres ence of O-2. Regulation of nitrate/nitrite catabolism is effected by t wo 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) transc riptional control causes the expression according to the hierarchy. Th e sensing of the environmental signals by the sensor proteins or domai ns is not well understood so far. FNR, which acts presumably as a cyto plasmic 'one component' sensor-regulator, is suggested to sense direct ly cytoplasmic O-2-levels corresponding to the environmental O-2-level s.