Abstract
Nitrate is transported into the cell by the ABC nitrate-nitrite transporter (NRT) in Synechococcus sp. PCC7942. The NA3 mutant lacking NRT is defective in uptake of low concentrations (<2 mM) of nitrate in the medium and requires high concentrations (>30 mM) of nitrate for sustained growth. Prolonged incubation of the mutant in a medium containing 2 mM nitrate led to isolation of a pseudorevertant capable of uptake of low concentrations of nitrate. Three genes, encoding a sulfate transporter-like protein, a response regulator with no DNA-binding domain, and a hybrid histidine kinase, were required for nitrate transport by the pseudorevertant. In the pseudorevertant, a nonsense mutation in the hybrid histidine kinase gene led to expression of a truncated histidine kinase, lacking the two receiver domains, which was essential for expression of the latent nitrate transport activity. The results suggested involvement of phosphotransfer reaction in the regulation of the novel transporter.
