NITRIC-OXIDE, NITRITE, AND FNR REGULATION OF HMP (FLAVOHEMOGLOBIN) GENE-EXPRESSION IN ESCHERICHIA-COLI K-12

Escherichia coli possesses a soluble flavohemoglobin, with an unknown function, encoded by the hmp gene, A monolysogen containing an hmp-lac Z operon fusion was constructed to determine how the hmp promoter is r egulated in response to heme ligands (O-2, NO) or the presence of anae robically utilized electron accepters (nitrate, nitrite), Expression o f the Phi(hmp-lacZ)1 fusion was similar during aerobic growth in minim al medium containing glucose, glycerol, maltose, or sorbitol as a carb on source, Mutations in cya (encoding adenylate cyclase) or changes in medium pH between 5 and 9 were without effect on aerobic expression, Levels of aerobic and anaerobic expression in glucose-containing minim al media were similar; both were unaffected by an arcA mutation. Anaer obic, but not aerobic, expression of Phi(hmp-lacZ)1 was stimulated thr ee- to four-fold by an fnr mutation; an apparent Fnr-binding site is p resent in the hmp promoter, Iron depletion of rich broth medium by the chelator 2'2'-dipyridyl (0.1 mM) enhanced hmp expression 40-fold unde r anaerobic conditions, tentatively attributed to effects on Fnr, At a higher chelator concentration (0.4 mM), hmp expression was also stimu lated aerobically. Anaerobic expression was stimulated 6-fold by the p resence of nitrate and 25-fold by the presence of nitrite, Induction b y nitrate or nitrite was unaffected by narL and/or narP mutations, dem onstrating regulation of Amp by these ions via mechanisms alternative to those implicated in the regulation of other respiratory genes. Nitr ic oxide (10 to 20 mu M) stimulated aerobic Phi(hmp-lacZ)I activity by up to 19-fold; soxS and soxR mutations only slightly reduced the NO e ffect. We conclude that hmp expression is negatively regulated by Fnr under anaerobic conditions and that additional regulatory mechanisms a re involved in the responses to oxygen, nitrogen compounds, and iron a vailability. Hmp is implicated in reactions with small nitrogen compou nds.