ABSTRACT
In addition to the ATP-binding cassette (ABC)-type nitrate/nitrite-bispecific transporter, which has a high affinity for both substrates (Km
, ∼1 μM), Synechococcus elongatus has an active nitrite transport system with an apparent Km
(NO2
−) value of 20 μM. 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 Ki
value of 0.025 μM. The transporter was induced under conditions of nitrogen deficiency, and the induced cells showed a V
max value of 11 to 13 μmol/mg of chlorophyll per h for cyanate or nitrite, which could supply ∼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.
Role of KaiC phosphorylation in the circadian clock system of Synechococcus elongatus PCC 7942
