Abstract
In cyanobacteria, the nitrate transporter (NRT) and/or nitrate reductase (NR) are inhibited upon addition of NH4+ to the medium. Cyanobacteria has either an ABC-type NRT ecoded by the nrtABCD genes or an MFS-type transporter ecoded by nrtP, the former of which is sensitive to NH4+ treatment, while the latter is not. Although cyanobacteria has only one type of NR encoded by narB, its activity is sensitive to NH4+ treatment in some strains but not in others. In this study, we used a Synechococcus elongatus PCC7942 mutant (NP1), in which nrtABCD had been replaced with nrtP from Nostoc punctiforme to construct an NR-deficient mutant NP2. The NP2 mutant, which have NH4+-resistant NRT activity, was used as the host for expression of various NR proteins. Examination of the effects of NH4+ on these strains and comparison of the structure of their NR proteins suggested that the NH4+-sensitive NRs commonly have a regulatory loop in the N-terminal [4Fe-4S]-binding domain. Site-specific replacement of one of the conserved residues in the loop (a Pro residue), rendered NR resistant to ammonium treatment, confirming its importance in the regulation of the NR activity.
