Serine/threonine protein kinase (At5g01920), known as STN8 in Arabidopsis, and its Stl1 homologue in Chlamydomonas, are STN7/Stt7-like proteins that are not required for state transitions. The Arabidopsis mutants deficient in stn8 gene demonstrated highly reduced levels of the phosphorylated proteins of the photosystem II (Bonardi et al., 2005, Vaionen et al., 2005).