DYNAMICS OF DENITRIFICATION ACTIVITY OF PARACOCCUS-DENITRIFICANS IN CONTINUOUS-CULTURE DURING AEROBIC-ANAEROBIC CHANGES

Induction and repression of denitrification activity were studied in a continuous culture of Paracoccus denitrificans during changes from ae robic to anaerobic growth conditions and vice versa, The denitrificati on activity of the cells was monitored by measuring the formation of d enitrification products (nitrite, nitric oxide, nitrous oxide, and din itrogen), individual mRNA levels for the nitrate, nitrite, and nitrous oxide reductases, and the concentration of the nitrite reductase enzy me with polyclonal antibodies against the cd(1)-type nitrite reductase , On a change from aerobic to anaerobic respiration, the culture enter ed an unstable transition phase during which the denitrification pathw ay became induced. The onset of this phase was formed by a 15- to 45-f old increase of the mRNA levels for the individual denitrification enz ymes, All mRNAs accumulated during a short period, after which their o verall concentration declined to reach a stable value slightly higher than that observed under aerobic steady-state conditions, Interestingl y, the first mRNAs to be formed were those for nitrate and nitrous oxi de reductase, The nitrite reductase mRNA appeared significantly later, suggesting different modes of regulation for the three genes, Unlike the mRNA levels, the level of the nitrite reductase protein increased slowly during the anaerobic period, reaching a stable value about 30 h after the switch. All denitrification intermediates could be observed transiently, but when the new anaerobic steady state was reached, din itrogen was the main product. When the anaerobic cultures were switche d back to aerobic respiration, denitrification of the cells stopped at once, although sufficient nitrite reductase was still present, We cou ld observe that the mRNA levels for the individual denitrification enz ymes decreased slightly to their aerobic, uninduced levels. The nitrit e reductase protein was not actively degraded during the aerobic perio d.