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The C-teminal substrate binding domain of LysR-like nitrogen assimilation control (NAC) protein, contains the type 2 periplasmic binding fold. The NAC is a LysR-type transcription regulator that activates expression of operons such as hut (histidine utilization) and ure (urea utilization), allowing use of non-preferred (poor) nitrogen sources, and represses expression of operons, such as glutamate dehydrogenase (gdh), allowing assimilation of the preferred nitrogen source. The expression of the nac gene is fully dependent on the nitrogen regulatory system (NTR) and the sigma54-containing RNA polymerase (sigma54-RNAP). In response to nitrogen starvation, NTR system activates the expression of nac, and NAC activates the expression of hut, ure, and put (proline utilization). NAC is not involved in the transcription of Sigma70-RNAP operons such as glnA, which directly respond by the NTR system, but activates the transcription of sigma70-RNAP dependent operons such as hut. Hence, NAC allows the coupling of sigma70-RNAP dependent operons to the sigma54-RNAP dependent NTR system. This substrate-binding domain has significant homology to the type 2 periplasmic binding proteins (PBP2), which are responsible for the uptake of a variety of substrates such as phosphate, sulfate, polysaccharides, lysine/arginine/ornithine, and histidine. The PBP2 bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap. After binding their specific ligand with high affinity, they can interact with a cognate membrane transport complex comprised of two integral membrane domains and two cytoplasmically located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",