• Orange Carotenoid binding Protein
• blue-green light
• molecular switch
• β-galactosidase α-complementation

A specific plant photoreceptor for green light has not been described yet. Indeed, in the region between blue-green light plants lack an endogenous response, although such a response is present in other organisms. This work takes advantage of a photoreceptor from cyanobacteria, dedicated to photoprotection mechanisms. Cyanobacteria, such as Synechocystis, possess a carotenoid associated protein named OCP (Orange Carotenoid binding Protein) which, when activated by blue-green light, undergoes reversible conformational changes that enable photoprotection mechanisms at the level of the phycobilisome. The aim of the present study is to engineer a molecular switch that responds to blue-green light exploiting OCP structural properties. As a proof of concept, I planned to fuse OCP with variable fragments (α-peptide) of the β-galactosidase enzyme to establish a blue-green light dependent complementation assay in Escherichia coli. At the same time, I expressed OCP from Fischerella thermalis and stimulated the synthesis of its specific binding carotenoid, canthaxanthin, in Arabidopsis thaliana for the future engineering of a blue-green light sensor in higher plants. I realized several α-peptide - OCP fusion protein formulations to be expressed in a strain of E. coli which I previously engineered to produce canthaxanthin. Since the first combinatorial attempts did not produce the desired blue-green light dependent staining of the bacterial colonies, I analysed β-galactosidase activity through a chemiluminescent assay; It showed a low basal β-gal activity, with a significant increase in luminescence when the samples were exposed to high blue-green light. I was able to obtain Arabidopsis transgenic lines expressing the genes for the synthesis of OCP and the carotenoid canthaxanthin, which were subsequently crossed to obtain double transgenic lines. I quantified the carotenoid content within the transgenic lines to assess whether they truly accumulate canthaxanthin. In conclusion, this study represents a first attempt to exploit the OCP photoreceptor to engineer blue-green light responses in higher plants and bacteria.