Primary sequence and evidence for a physiological function of the flavohemoprotein of Alcaligenes eutrophus

The flavohemoprotein (FHP) encoding gene of the strictly respiratory Gram-negative bacterium Alcaligenes eutrophus was isolated from a megaplasmid library by using FHP-specific antibodies and oligonucleotide probes based on the amino-terminal polypeptide sequence of FHP, determined previously (Zhu, H., and Riggs, A. F. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 5015-5019). The fhp gene codes for a monomeric polypeptide of 403 amino acids (M(r) 44,796) whose structure is highly homologous to the proteins of the two-domain flavohemoglobin family, comprising the hemoproteins from Escherichia coli and Saccharomyces cerevisiae. FHP consists of an amino-terminal-located O2-binding hemoglobin domain and a carboxyl-terminal-located redoxactive domain with potential binding sites for NAD(P)H and FAD. Two potential binding motifs for NARL and FNR upstream of fhp suggest a role of FHP in the anaerobic metabolism of A. eutrophus. Isogenic Fhp-negative mutants showed no significant delay in aerobic or anaerobic growth. Compared with the wild type, however, the mutant did not accumulate nitrous oxide during denitrification with nitrite as electron acceptor. This property was restored by complementation. The result suggests that FHP interacts directly or indirectly with the gas metabolism during denitrification in A. eutrophus.