INHIBITION OF DENITRIFICATION ACTIVITY BUT NOT OF MESSENGER-RNA INDUCTION IN PARACOCCUS-DENITRIFICANS BY NITRITE AT A SUBOPTIMAL PH

The influence of pH on the denitrification activity of a continuous cu lture of Paracoccus denitrificans was studied in relation to the prese nce of nitrite. After a transition from aerobic to anaerobic condition s at the suboptimal pH of 6.8, P. denitrificans was not able to build up a functional denitrification pathway. Nitrite accumulated in the me dium as the predominant denitrification product. Although the nitrite reductase gene was induced properly, the enzyme could not be detected at sufficient amounts in the culture. These observations indicate that either translation was somehow inhibited, or once synthesized nitrite reductase was inactivated, possibly by the high concentrations of nit rous acid (HNO2). Interestingly, when a P. denitrificans culture which was grown to steady-state under anaerobic conditions was then exposed to suboptimal pHs, cells exhibited a reduced overall denitrification activity, but neither nitrite nor any other denitrification intermedia te accumulated.