Summary: Anaerobic, but not aerobic, cultures of K-12 catalysed the rapid nitrosation of the model substrate 2,3-diaminonaphthalene when incubated with nitrite. Formate and lactate were effective electron donors for the nitrosation reaction, which was inhibited by nitrate. Optimal growth conditions for the expression of nitrosation activity by various strains and mutants were determined. Highest activities were found with bacteria that had been grown anaerobically in a minimal medium rather than in Lennox broth, with glycerol and fumarate rather than glucose as the main carbon and energy source, and in the presence of a low concentration of nitrate. Bacteria harvested in the early exponential phase were more active than those harvested in later stages of growth. Well-characterized mutants defective in the synthesis of one or more anaerobically induced electron transfer chains were screened for nitrosation activity under these optimal growth conditions: only the respiratory nitrate reductase encoded by the operon was implicated as a major contributor to nitrosation activity. Due to the limited sensitivity of the assays currently available, a minor contribution from the two alternative nitrate reductases or even other molybdoproteins could not be excluded. The role of formate in nitrosation was complex and was clearly not limited simply to that of an electron donor in the bacterial reduction of nitrite to nitric oxide: at least two further, chemical roles were inferred. This extensive study of more than 400 independent cultures of K-12 and its derivatives resolved some, but not all, of the apparently conflicting data in the literature concerning nitrosation catalysed by enteric bacteria.


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