Abstract
For assimilation of nitrate, freshwater cyanobacteria have an ABC-type nitrate/nitrite transporter, whereas marine cyanobacteria of the Synechococcus group have an MFS-type nitrate/nitrite transporter NrtP. Although most marine cyanobacteria of the Prochlorococcus group do not assimilate nitrate or nitrite, some retain nitrite reductase (NiR). Interestingly, the marine Synechococcus species and the NiR-containing Prochlorococcus strains have focA, a gene encoding a transporter similar to the green algal nitrite transporter and the bacterial formate transporter. To determine the function of focA, we expressed the gene from three cyanobacterial strains in a mutant of the freshwater cyanobacterium Synechococcus elongatus that is completely defective in nitrite transport activity due to inactivation of the ABC-type cyanate/nitrite- and nitrate/nitrite transporters. Expression of focA from Synechococcus sp. PCC7002 was found to allow the mutant to take up nitrite with a Km value of 8 μM and to grow on low concentrations of nitrite. The nitrite uptake activity was not inhibited by nitrate, cyanate, nor formate, suggesting that the FocA protein specifically transports nitrite.
