Аннотация:Non-photochemical quenching (NPQ) of excited chlorophyll states is essential for protecting the photosyntheticapparatus (PSA) from the excessive light-induced damage in all groups of oxygenic photosynthetic organisms. The key componentof the NPQ mechanism in green algae and some other groups of algae and mosses is the LhcSR protein of the lightharvesting complex (LHC) protein superfamily. In vascular plants, LhcSR is replaced by PsbS, another member of the LHCsuperfamily and a subunit of photosystem II (PSII). PsbS also performs the photoprotective function in mosses. For a longtime, PsbS had been believed to be nonfunctional in green algae, although the corresponding gene was discovered in thegenome of these organisms. The first evidence of the PsbS accumulation in the model green alga Chlamydomonas reinhardtiiin response to the increase in irradiance was obtained only six years ago. However, the observed increase in the PsbS contentwas short-termed (on an hour-timescale). Here, we report a significant (more than three orders of magnitude) andprolonged (four days) upregulation of PsbS expression in response to the chilling-induced high-light stress followed by aless significant (~ tenfold) increase in the PsbS expression for nine days. This is the first evidence for the long-term upregulationof the PsbS expression in green alga (Chlorophyta) in response to stress. Our data indicate that the role of PsbSin the PSA of Chlorophyta is not limited to the first-line defense against stress, as it was previously assumed, but includesfull-scale participation in the photoprotection of PSA from the environmental stress factors.