Engineering of a green-light inducible gene expression system in Synechocystis sp. PCC6803

In order to construct a green-light-regulated gene expression system for cyanobacteria, we characterized a green-light sensing system derived from Synechocystis sp. PCC6803, consisting of the green-light sensing histidine kinase CcaS, the cognate response regulator CcaR, and the promoter of cpcG2 (P(cpcG)(2)). CcaS and CcaR act as a genetic controller and activate gene expression from P(cpcG)(2) with green-light illumination. The green-light induction level of the native P(cpcG)(2) was investigated using GFPuv as a reporter gene inserted in a broad-host-range vector. A clear induction of protein expression from native P(cpcG)(2) under green-light illumination was observed; however, the expression level was very low compared with P(trc), which was reported to act as a constitutive promoter in cyanobacteria. Therefore, a Shine-Dalgarno-like sequence derived from the cpcB gene was inserted in the 5′ untranslated region of the cpcG2 gene, and the expression level of CcaR was increased. Thus, constructed engineered green-light sensing system resulted in about 40-fold higher protein expression than with the wild-type promoter with a high ON/OFF ratio under green-light illumination. The engineered green-light gene expression system would be a useful genetic tool for controlling gene expression in the emergent cyanobacterial bioprocesses.