Regulation of Nitrite Reductase Activity under CO2 Limitation in the Cyanobacterium Synechococcus sp. PCC7942

Abstract

During photoautotrophic growth under C0,-limited conditions, cells of Synechococcus sp. PCC7942 excreted into the medium about 30% of the nitrite produced by reduction of nitrate. No nitrite was excreted under C0,-sufficient conditions. After transfer of high-C0,-grown cells to C0,-limited conditions, nitrite reductase activity started to decline within 0.5 h and decreased to 50% of the initial leve1 in 3 h, whereas nitrate reductase activity was virtually unchanged. Nitrite started to accumulate i n the medium about 3 h after the transfer of the cells to COJimited conditions and reached a concentration of >0.4 mM at 17 h. These findings suggested that the nitrite excretion was due to an imbalance of the activities of nitrite reductase and nitrate reductase. Since ammonium, the product of nitrite reduction, was not detected in the medium, it was concluded that the step of nitrite reduction limits the rate of nitrate assimilation under C0,-limited conditions. The extent of decrease i n nitrite reductase activity under C0,-limited conditions was much larger than that caused by rifampicin (an inhibitor of RNA synthesis) treatment under high-CO, conditions. Addition of CO, , i n the form of sodium bicarbonate, to the C0,-limited culture increased the nitrite reductase activity, but rifampicin inhibited this increase. These findings suggested the presence of a mechanism that irreversibly inactivates nitrite reductase under C0,-limited conditions.

Involvement of the C-terminal Domain of an ATP-binding Subunit in the Regulation of the ABC-type Nitrate/Nitrite Transporter of the Cyanobacterium Synechococcus sp. Strain PCC 7942

“…When appropriate, kanamycin was added to the media at 25 g/ml. Expression of the nitrate assimilation genes was induced by transfer of ammonium-grown cells to the nitrate-or nitritecontaining medium as described previously (9).…”

“…2, lanes 1 and 2). The size of the hybridization signal ranged from 0.25 to Ͼ7 kilobases representing rapid degradation of the nirA operon transcript (9). The NC2 and NC3 mutants also accumulated the transcripts hybridizing with the nrtD-specific probe when transferred to nitrate-containing medium (Fig.…”

“…To determine specifically nitrate transport activity, intracellular nitrate accumulation in the light was measured in the NR-deficient mutants NR1 (24) and NC4, which were constructed from the wild-type strain and NC2, respectively. As observed previously in a mutant of strain PCC 7942 with no appreciable NR activity (15), NR1 accumulated about 1.5 mM of nitrate intracellularly at 20 M external nitrate concentration in the absence of ammonium but did not accumulate nitrate in the presence of ammonium (Fig.…”

“…strain PCC 7942, the genes encoding the nitrate transport system (nrtA, nrtB, nrtC, and nrtD) (3)(4)(5), NR (narB) (6,7), and NiR (nirA) (8,9) form an nirA-nrtABCD-narB operon (the nirA operon), and the transcription from the operon is inhibited by the addition of ammonium to the cyanobacterial cultures (9). Ammonium inhibits transcription through its fixation into Gln, but Gln is not the direct regulator of transcription (9,10). We have proposed that cyanate, a metabolite of Gln via carbamoylphosphate, acts as the metabolic signal for the ammonium-promoted repression of the nirA operon (11).…”

Nitrite utilization by Chaetoceros muelleri under elevated CO2 concentration

“…The resulting ammonium is incorporated into carbon skeletons of the cell. It has been shown that nitrite, the product of nitrate reduction, was excreted into the medium under CO 2 -limited conditions (Azuara and Aparicio 1983;Krä-mer et al 1988;Suzuki et al 1995). Nitrite, the key factor concerned with the water quality in aquaculture, is quite toxic to zooplankton, shrimp, fish, etc.…”