Abstract
Synechococcus elongatus possesses a sulfate permease family transporter (LtnT) having latent nitrate transport activity and whose C-terminal domain inhibits the transport activity. The physiological substrate of this transporter, whose affinity for nitrate is low, remains to be elucidated. To characterize the amino acid residues involved in determination of the affinity for substrates, we aimed to obtain the LtnT transporters with improved affinity for nitrate. A truncated ltnT gene lacking the region encoding the C-terminal domain was amplified by error-prone PCR, the resulting ltnT genes were introduced into a nitrate transport-deficient Synechococcus strain, and mutants that grew on medium containing low concentrations of nitrate were isolated. Characterization of the ltnT genes of the mutants showed that five amino acid residues are involved in determination of the affinity for nitrate of the transporter. Combinations of these mutations synergistically improved the affinity for nitrate of the transporter.
