Optimization of expression of orange carotenoid protein in Escherichia coli

Naturally-occurring orange carotenoid protein (OCP) is synthesized in cyanobacteria and red algae for photoprotection. Holo-OCP can be produced with three plasmids in E. coli, which needs two inducers (arabinose and isopropyl β-D-thiogalactoside) to initiate two processes: one for generation of carotenoid and the other for generation of apo-OCP, so takes about two days. Afterwards, a two-plasmid method using two plasmids in E. coli is established, in which E. coli cells are induced only by isopropyl β-D-thiogalactoside, so can yield different holo-OCPs from several cyanobacteria within three days. In this work, we optimized the two-plasmid method as follows: (1) re-organization of the two plasmids, letting carotenoid-generating gene, crtW, be arranged together with apo-OCP-generating gene, ocp, in a single plasmid, which causes that both carotenoid and apo-protein were properly produced, (2) modification of several amino acids at the N-terminus of apo-OCP, in this way increasing the yield and purity of holo-OCP. After these optimizations, we can generate much more amount of holo-OCP within shorter time of only 16 h, and pure holo-OCP be conveniently prepared after routine purification. Comparing with the reported data, the general yield of holo-OCP is increased by ∼10-fold under similar conditions. The high quality of the prepared holo-OCPs is verified by fluorescence quenching of the phycobilisomes.