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The active nitrate transport system of the cyanobacterium Synechococcus sp. PCC7942 is encoded by the four genes nrtA, nrtB, nrtC and nrtD. It is essential for the growth of the cyanobacterium at physiological concentrations of nitrate and has been shown to be involved in the active transport of nitrite as well. The deduced amino acid sequences of the NrtB, NrtC and NrtD proteins indicate that the transporter is a member of the ABC (ATP-binding cassette) superfamily of active transporters. Among the prokaryotic ABC transporters, the cyanobacterial nitrate/nitrite transporter is unique in having a membrane-bound protein NrtA and an NrtA-like extra domain linked to one of the ATP-binding subunits (C-terminal domain of NrtC). Molecular biological, biochemical and physiological studies suggest that NrtA is the substrate-binding protein required for the transport of nitrate/nitrite and that the C-terminal domain of NrtC has a regulatory role. Comparison of the structures of nitrate transporters from eukaryotic and prokaryotic, photosynthetic and non-photosynthetic organisms indicate that the nrt nitrate/nitrite transporter represents a prokaryotic nitrate transporter distinct from the nitrate transporters of eukaryotes.
Tatsuo Omata (Wed,) studied this question.