Mutants of E. coli, completely devoid of nitrite reductase activity with glucose or formate as donor were studied. Biochemical analysis indicates that they are simultaneously affected in nitrate reductase, nitrite reductase, fumarate reductase and hydrogenase activities as well as in cytochrome C552 biosynthesis. The use of an antiserum specific for nitrate reductase shows that the nitrate reductase protein is probably missing. A single mutation is responsible for this phenotype: the gene affected, nir R, is located close to tyr R i.e. at 29 min on the chromosomal map.

A strain of Escherichia coli with a mutation in the ana gene was shown to lack acetaldehyde dehydrogenase and alcohol dehydrogenase. The requirement of this strain for an external oxidant to grow anaerobically on glucose shows that the reduction of acetyl-CoA is the principal means of reoxidation of NADH produced during glycolysis in E. coli. Further mutants derived from the ana strain were shown to be affected in the enzymes involved in the fermentation of pyruvate (pyruvate formate-lyase, phosphotransacetylase, acetate kinase). A gene controlling acetate kinase (ackB) activity has been located at 39 min on the chromosomal map. Evidence is presented that anaerobic nitrite reduction with pyruvate involves at least the dehydrogenase subunit of the pyruvate dehydrogenase complex.

(Kccei\ctl hij\i.n1hi.r 7. l S 7 S .I~IIIU~II~~ IS. lV7cJ) Some physiological characteristics of the formate nitrite reductase activity were studied on whole cells.1. Highest level of activity was found in cells grown in anaerobiosis on glycerol in the presence of nitrite. Maximal activity was obtained at pH 8 and at 45°C. Copper ions and cyanide inhibit the activity whereas 0.1 mM azide is without effect.2. No reduction of nitrite was detected in the presence of oxygen, even at very low concentrations. Nitrate has a double effect: it competes with nitrite for electrons supplied by formate, and has an inhibitory action on nitrite reductase activity.3. The stoichiometry between formate and nitrite is 3 : 1 ; the formate nitrite reduction is accompanied by C02 evolution with the same stoichiometry.The reduction of nitrite by cell-free extracts of Escherichia coli is mainly performed through the NADH-specific nitrite reductase [l]. The enzyme is soluble and has recently been purified [2]. At least another enzyme, the NADPH-sulfite reductase [3] participates in nitrite reduction ; nevertheless, this enzyme, which displays a gratuitous nitrite reductase activity, accounts for only 5 % of the total reduction of this acceptor.Until recently, the reduction of nitrite by unbroken cells of E. coli was reported to be accomplished using glucose as electron donor. However, during the study of mutants affected on the reduction of nitrite some strains were found to exhibit a peculiar behaviour : these mutants' simultaneously affected in nitrite reductase and pyruvate-formate lyase activities [4], are phenotypically reverted for nitrite reduction by the addition of formate during the growth.This observation lead us to conclude that formate is an effective electron donor for nitrite reduction by whole cells of E. coli K12. As formate is also known to be a good electron donor for nitrate reduction it was of interest to compare the properties of the formate nitrite and the formate nitrate reductase systems.

No abstract