These changes in photosystem stoichiometry represent an adaptation, or acclimation, that is complementary to state transitions, achieving balanced operation of photosystem I and photosystem II. While state transitions are a relatively rapid, reversible, post-translational solution to changing spectral composition, photosystem stoichiometry adjustment is a more long-term acclimatory response, taking hours or days to complete, and involving control of gene expression at the level of transcription and/or translation. State transitions are superimposed on different photosystem stoichiometries and occur apparently independently of the ratio of photosystem I to II, although the variable chlorophyll fluorescence often used to monitor state transitions in vivo is influenced by both reaction centre stoichiometry and light-harvesting antenna size. A major factor affecting fluorescence yield is the antenna size of photosystem II, since this is the origin of the variable component of chlorophyll fluorescence at room temperature. Photosystem stoichiometry adjustment has been shown to be initiated, like state transitions, by changes in redox state of plastoquinone. Thus a prolonged light 2 alters gene expression and results in an increase in the stoichiometry of photosystem I to photosystem II. In plants, this change may be monitored easily as an increase in the ratio of chlorophyll a to chlorophyll b. The core apoproteins of the photosystem I and II reaction centres are the products of genes in chloroplast DNA. Studies of transcription in isolated chloroplasts demonstrated that photosystem I transcription is induced, while photosystem II transcription is repressed, upon reduction of plastoquinone. Conversely, photosystem I is repressed, and photosystem II induced, upon oxidation of plastoquinone. These experiments introduced the possibility of studying early events in control of photosystem stoichiometry in vitro. A conserved redox sensor kinase, Chloroplast Sensor Kinase, has been shown to be required for the plastoquinone redox-state dependent regulation of chloroplast reaction centre gene transcription. Arabidopsis knockout mutants of the CSK gene are unable to repress photosystem I genes in light absorbed predominantly by photosystem I, and therefore cannot regulate the stoichiometry of photosystem I relative to photosystem II. CSK is a bacterial-type sensor LY2109761 kinase that belongs to the family of two-component signalling proteins. CSK has homologues in all major lineages of photosynthetic eukaryotes. In the complete genome sequences of the chlorophycean alga Chlamydomonas reinhardii and the haptophyte Emiliania huxleyi, however, no CSK gene is identified by similarity searches. Nevertheless, the possibility exists that the histidine kinase-like chlamyopsin protein replaces CSK in Chlamydomonas and that the plastid-encoded histidine kinase ycf26 compensates for the lack of CSK in Emiliania. The functional partner of CSK in plants and green algae is not a response regulator as in canonical bacterial two-component systems, but a eukaryotic serine/threonine protein kinase known as Plastid Transcription Kinase and a chloroplast sigma factor, SIG1.