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 nitrate 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 grown aerobically on highly reduced carbon sources such as butyrate or caproate, but not when cultured on 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 source. Transcriptional analyses of the nap operon have now identified two initiation sites which were differentially regulated in response to the carbon source, with expression being maximal when cells were grown aerobically with butyrate. Analysis of a P. pantotrophus mutant (M6) deregulated for NAP activity identified a single C-->A transversion in a heptameric inverted-repeat sequence that partially overlapped the proximal promoter. Transcription analysis of this mutant revealed that expression of nap was completely derepressed under all growth conditions examined. Taken together, these findings indicate that nap transcription is negatively regulated during anaerobiosis, such that expression is restricted to aerobic growth, but only when the carbon source is highly reduced.