INVOLVEMENT OF THE C-TERMINAL DOMAIN OF AN ATP-BINDING SUBUNIT IN THEREGULATION OF THE ABC-TYPE NITRATE NITRITE TRANSPORTER OF THE CYANOBACTERIUM SYNECHOCOCCUS SP. STRAIN PCC-7942/

In Synechococcus sp. strain PCC 7942, an ATP-binding cassette transpor ter encoded by the genes nrtA, nrtB, nrtC, and nrtD mediates active tr ansport of nitrate and nitrite, which is inhibited by ammonium, a pref erred source of nitrogen for the cyanobacterium. One of the ATP-bindin g subunits of the transporter, NrtC, has a distinct C-terminal domain of 380 amino acid residues. A mutant NC2, constructed by removal of th is domain using genetic engineering techniques, assimilated low concen trations of nitrate and nitrite and accumulated nitrate intracellularl y, showing that the domain is not essential for the transporter activi ties, Assimilation of low concentrations of nitrite was only partially inhibited by ammonium in NC2 but was completely inhibited in the wild -type cells, Cells of NC2 and its derivative (nitrate reductase-less s train NC4) carrying the truncated NrtC but not the cells with the wild -type NrtC accumulated nitrate intracellularly in the presence of ammo nium in medium, These findings indicated that the C-terminal domain of NrtC is involved in the ammonium-promoted inhibition of the nitrate/n itrite transporter, In the presence of ammonium, NC2 could not assimil ate nitrate despite its ability to accumulate nitrate intracellularly, which suggested that reduction of intracellular nitrate by nitrate re ductase is also subject to inhibition by ammonium.