Abstract
The nrtP gene from cyanobacteria encodes an MFS-type nitrate permease that is similar to NRT2 of algae and plants. The gene is found in marine strains of cyanobacteria, with an exception in the fresh-water strain Nostoc punctiforme ATCC29133. NrtP of N. punctiforme (NpNrtP) was previously reported to be a nitrate/nitrite permease with higher affinity for nitrate than for nitrite by heterologous expression in a Synechococcus elongatus mutant lacking the ABC-type nitrate/nitrite transporter. However, the S. elongatus mutant was recently found to have a cyanate/nitrite bispecic ABC transporter, making the nitrite transport activity of NrtP questionable. In this study, we used a double mutant of S. elongates, lacking both the ABC nitrate/nitrite- and cyanate/nitrite transporters to investigate the function of NrtP. The NrtP-expressing cells showed low but detectable activity of nitrite uptake as well as high activity of nitrate uptake, confirming its ability to transport nitrite. A strep-tagged version of NpNrtP was found only in the plasma membrane of S. elongatus, indicating occurrence of a common mechanism for targeting of the transporter to the plasma membrane in cyanobacteria.
