Transcriptional and translational adaptation to aerobic nitrate anabolism in the denitrifier Paracoccus denitrificans

Transcriptional adaptation to nitrate-dependent anabolism by PD1222 was studied. A total of 74 genes were induced in cells grown with nitrate as N-source compared with ammonium, including and genes. The and genes were cotranscribed, although was more strongly induced by nitrate than The genes constituted a transcriptional unit, which is preceded by a non-coding region containing hairpin structures involved in transcription termination. The and transcripts were detected at similar levels with nitrate or glutamate as N-source, but transcript was undetectable in ammonium-grown cells. The nitrite reductase NasG subunit was detected by two-dimensional polyacrylamide gel electrophoresis in cytoplasmic fractions from nitrate-grown cells, but it was not observed when either ammonium or glutamate was used as the N-source. The mutant lacked both transcript and nicotinamide adenine dinucleotide (NADH)-dependent nitrate reductase activity. On the contrary, the mutant showed similar levels of the transcript to the wild-type strain and displayed NasG protein and NADH-nitrate reductase activity with all N-sources tested, except with ammonium. Ammonium repression of was dependent on the Ntr system. The and genes were expressed at low levels regardless of the nitrogen source supporting growth. Mutational analysis of the genes indicated that while genes are required for nitrate assimilation, genes can only partially restore growth on nitrate in the absence of genes. The existence of a regulation mechanism for nitrate assimilation in , by which nitrate induction operates at both transcriptional and translational levels, is proposed.