EXPRESSION AND CHARACTERIZATION OF THE ESCHERICHIA-COLI FDO LOCUS ANDA POSSIBLE PHYSIOLOGICAL-ROLE FOR AEROBIC FORMATE DEHYDROGENASE

In the presence of nitrate, the major anaerobic respiratory pathway in cludes formate dehydrogenase (FDH-N) and nitrate reductase (NAR-A), wh ich catalyze formate oxidation coupled to nitrate reduction, Two aerob ically expressed isoenzymes, FDH-Z and NAR-Z, have been recently chara cterized, Enzymatic analysis of plasmid subclones carrying min 88 of t he Escherichia coli chromosome was consistent with the location of the fdo locus encoding FDH-Z between the fdhD and fdhE genes which are ne cessary for the formation of both formate dehydrogenases. The fdo locu s 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 struc tural role, these polypeptides were recognized by antibodies specific to FDH-N. Expression of a chromosomal fdo-uidA operon fusion was induc ed threefold by aerobic growth and about twofold by anaerobic growth i n the presence of nitrate, However, it was independent of the two glob al regulatory proteins FNR and ArcA, which control genes for anaerobic and aerobic functions, respectively, and of the nitrate response regu lator protein NARL, In contrast, a mutation affecting either the nucle oid-associated H-NS protein or the CRP protein abolished the aerobic e xpression, A possible role for FDH-Z during the transition from aerobi c 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 anaerobio sis, Furthermore, in an fnr strain deprived of both FDH-N and NAR-A ac tivities, aerobically expressed FDH-Z and NAR-Z enzymes were shown to reduce nitrate at the expense of formate under anaerobic conditions, s uggesting that this pathway would allow the cell to respond quickly to anaerobiosis.