Expression and characterization of the Escherichia coli fdo locus and a possible physiological role for aerobic formate dehydrogenase.

In the presence of nitrate, the major anaerobic respiratory pathway includes formate dehydrogenase (FDH-N) and nitrate reductase (NAR-A), which catalyze formate oxidation coupled to nitrate reduction. Two aerobically expressed isoenzymes, FDH-Z and NAR-Z, have been recently characterized. Enzymatic analysis of plasmid subclones carrying min 88 of the Escherichia coli chromosome was consistent with the location of the fdo locus encoding FDH-Z between the fdhD and fdhE genes which are necessary for the formation of both formate dehydrogenases. The fdo locus produced three proteins (107, 34, and 22 kDa) with sizes similar to those of the subunits of the purified FDH-N. In support to their structural role, these polypeptides were recognized by antibodies specific to FDH-N. Expression of a chromosomal fdo-uidA operon fusion was induced threefold by aerobic growth and about twofold by anaerobic growth in the presence of nitrate. However, it was independent of the two global regulatory proteins FNR and ArcA, which control genes for anaerobic and aerobic functions, respectively, and of the nitrate response regulator protein NARL. In contrast, a mutation affecting either the nucleoid-associated H-NS protein or the CRP protein abolished the aerobic expression. A possible role for FDH-Z during the transition from aerobic to anaerobic conditions was examined. Synthesis of FDH-Z was maximal at the end of the aerobic growth and remained stable after a shift to anaerobiosis, whereas FDH-N production developed only under anaerobiosis. Furthermore, in an fnr strain deprived of both FDH-N and NAR-A activities, aerobically expressed FDH-Z and NAR-Z enzymes were shown to reduce nitrate at the expense of formate under anaerobic conditions, suggesting that this pathway would allow the cell to respond quickly to anaerobiosis.