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.