Nitrite Transport Activity of the ABC-Type Cyanate Transporter of the Cyanobacterium Synechococcus elongatus

In addition to the ATP-binding cassette (ABC)-type nitrate/nitrite-bispecific transporter, which has a high affinity for both substrates (K-m, similar to 1 mu M), Synechococcus elongatus has an active nitrite transport system with an apparent K-m (NO2-) value of 20 mu M. We found that this activity depends on the cynABD genes, which encode a putative cyanate (NCO-) ABC-type transporter. Accordingly, nitrite transport by CynABD was competitively inhibited by NCO- with a K-i value of 0.025 mu M. The transporter was induced under conditions of nitrogen deficiency, and the induced cells showed a V-max value of 11 to 13 mu mol/mg of chlorophyll per h for cyanate or nitrite, which could supply similar to 30% of the amount of nitrogen required for optimum growth. Its relative specificity for the substrates and regulation at transcriptional and posttranslational levels suggested that the physiological role of the bispecific cyanate/nitrite transporter in S. elongatus is to allow nitrogen-deficient cells to assimilate low concentrations of cyanate in the medium. Its contribution to nitrite assimilation was significant in a mutant lacking the ABC-type nitrate/nitrite transporter, suggesting a possible role for CynABD in nitrite assimilation by cyanobacterial species that lack another high-affinity mechanism(s) for nitrite transport.