Photosystem 2 protein PsbS is a component of the molecular mechanism of non-photochemical quenching in green algae

The energy transfer of chlorophyll excited state during photosynthesis and electron transfer along the photosynthetic electron transport chain are the central processes of light energy conversion into chemical energy while also the sources of potential danger to the organism. Non-photochemical quenching (NPQ) of chlorophyll excited states is one of the key mechanisms for protecting the photosynthetic apparatus from the dangerous effects of excessive light energy. The main molecular components of NPQ are oxygen-containing carotenoids, xanthophylls, as well as proteins: a photosystem 2 component PsbS and its algal analogue, the LhcSR/LHCX protein. Previously, PsbS was believed to be typical only for higher plants, while in algae it should not be expressed or at least play any significant role in NPQ. Later it became clear that its expression is stress-induced, but it took place in a too short timescale. This led to an assumption that its protective role was limited only by an auxiliary role in transients. However, we have recently found long-term stress-induced expression of its gene in one of the of green algal species. In this paper, we attempted to expand the range of analyzed species and show that long-term stress-induced expression of PsbS (which could be indicative of its protective function) can be widespread among green algae. The variability of PsbS primary structure in green algae was also studied, which, in turn, is considered to correlate with the importance of protein functions for an organism.