Control of periplasmic nitrate reductase gene expression (napEDABC) from Paracoccus pantotrophus in response to oxygen and carbon substrates

The napEDABC operon of Paracoccus pantotrophus encodes a periplasmic nitrat e reductase (NAP), together with electron-transfer components and proteins required for the synthesis of a fully functional enzyme. Previously, it had been shown that high NAP activity was observed when P. pantotrophus was gr own aerobically on highly reduced carbon sources such as butyrate or caproa te, but not when cultured an more oxidized substrates such as succinate or malate. The enzyme is not present to any extent when the organism is grown anaerobically under denitrifying conditions, regardless of the carbon sourc e. Transcriptional analyses of the nap operon have now identified two initi ation sites which were differentially regulated in response to the carbon s ource, with expression being maximal when cells were grown aerobically with butyrate. Analysis of a P. pantotrophus mutant (M6) deregulated for NAP ac tivity identified a single C-->A transversion in a heptameric inverted-repe at sequence that partially overlapped the proximal promoter. Transcription analysis of this mutant revealed that expression of nap was completely dere pressed under all growth conditions examined. Taken together these findings indicate that nap transcription is negatively regulated during anaerobiosi s, such that expression is restricted to aerobic growth, but only when the carbon source is highly reduced.