​The modular photoactive Orange Carotenoid Protein (OCP), which has a crucial role in cyanobacterial photoprotection, has gained a great interest in the recent years due to its ability to act as a molecular photoswitch and to its 12 Å translocation of the carotenoid upon photoactivation. Upon blue light absorption, the inactive orange form (OCP^O) converts to the active red state (OCP^R) which is able to interact with the phycobilisome, the cyanobacterial antenna, and to decrease the energy arriving at the photochemical centers. Due to its role in the thermal dissipation of excess energy, the expression of OCP must be tightly controlled to avoid a loss of energy under non-stressing conditions and to increase this dissipation under stressing ones.

In the present study, researchers discovered a molecular factor involved in the regulation of OCP expression. They show for the first time that a small RNA appended to a long operon mRNA functions in the regulatory network of cyanobacteria. This factor is the first example in cyanobacteria of a sRNA derived from a polycistronic mRNA regulating at least one other mRNA in trans. The researchers demonstrated that ApcZ, a sRNA originating from the 3'end of the apcABC operon encoding the core phycobilisome proteins, is responsible for the repression of ocp translation under non-stress conditions. The transcription of the apcABC operon decreases under most stress conditions and as a consequence ApcZ (free and as part of the entire operon transcript) concentration decreases leading to a de-repression of ocp mRNA translation. Thus, light harvesting and photoprotection are connected directly and in an inverse fashion by a single regulatory sRNA. If, under stress conditions, less energy must arrive at the photochemical centers, the transcription of phycobilisome genes decreases and synthesis of OCP increases. Hence, the OCP concentration is controlled in a simple and elegant way.

Adapted from the June 3, 2021 I2BC news (B3S department)