Abstract
For assimilation of nitrate, marine cyanobacteria of the Synechococcus group have an MFS-type nitrate/nitrite transporter. Most marine cyanobacteria of the Prochlorococcus group do not assimilate nitrate or nitrite, but some retain nitrite reductase (NiR). Some marine Synechococcus species and the NiR-containing Prochlorococcus strains have focA, a gene encoding a putative nitrite transporter. Expression of focA from Synechococcus sp. PCC7002 conferred nitrite uptake activity on the double mutant (NA4) of the freshwater cyanobacterium Synechococcus elongatus that lacks the ABC-type nitrate/nitrite- and cyanate/nitrite transporters. However, the focA genes from other two marine cyanobacteria did not confer nitrite uptake activity, supposing that the FocA transporters of marine cyanobacteria have different structure and function in different species. The latter FocA transporters have conserved hydrophilic region at the C-terminus and expression of truncated forms of the latter focA genes lacking the 3'-terminal part conferred nitrite uptake activity on NA4, suggesting that the C-terminal regions of FocA inhibit nitrite uptake activity of the transporters.
