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Mechanism of low CO2‐induced activation of the cmp bicarbonate transporter operon by a LysR family protein in the cyanobacterium Synechococcus elongatus strain PCC 7942

The cmp operon of the cyanobacterium Synechococcus elongatus strain PCC 7942, encoding the subunits of the ABC‐type bicarbonate transporter, is activated under CO2‐limited growth conditions in a manner dependent on CmpR, a LysR family transcription factor of CbbR subfamily. The 0.7 kb long regulatory region of the operon carried a single promoter, which responded to CO2 limitation. Using the luxAB reporter system, three cis‐acting elements involved in the low‐CO2 activation of transcription, each consisting of a pair of LysR recognition signatures overlapping at their ends, were identified in the regulatory region. CmpR was shown to bind to the regulatory region, yielding several DNA–protein complexes in gel shift assays. Addition of ribulose‐1,5‐bisphosphate (> 1 mM) or 2‐phosphoglycolate (> 10 μM) enhanced the binding of CmpR in a concentration‐dependent manner, promoting formation of large DNA–protein complexes. Given the involvement of O2 in adaptive responses of cyanobacteria to low‐CO2 conditions, our results suggest that 2‐phosphoglycolate, which is produced by oxygenation by ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) of ribulose‐1,5‐bisphosphate under CO2‐limited conditions, acts as the co‐inducer in the activation of the cmp operon by CmpR.