Molybdenum cofactor negative mutants of Escherichia coli use citrate anaerobically

Anaerobically, Escherichia coli cannot grow using either glycerol or citrate as sole carbon and energy source. However, it has been reported that a mixture of glycerol and citrate will support growth. We have found that wild-type strains of E. coli K-12 do not grow on glycerol plus citrate anaerobically. However, growth eventually occurs due to the frequent appearance of mutants. We found that such Cit+ mutants were defective in anaerobic respiration with nitrate or trimethylamine-N-oxide and were chlorate resistant (i.e. molybdenum cofactor deficient). Conversely, well characterized mutants in any of chlA, B, D, E, G and N were also able to use citrate anaerobically. No anaerobic growth differences between wild type and chl mutants were observed either with fermentable sugars or with glycerol plus fumarate or glycerol plus tartrate. Citrate lyase was induced anaerobically by citrate and repressed by glucose in both wild type strains and chl mutants. Furthermore, levels of citrate lyase, fumarate reductase, malate dehydrogenase, fumarase and alcohol dehydrogenase were similar in both types of strains under anaerobic conditions. It is conceivable that a functioning molybdenum cofactor prevents use of citrate by keeping citrate lyase in the inactive form.