Chloroplast Sensor Kinase (CSK) is a protein in

chloroplasts A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...

and cyanobacteria, bacteria from which chloroplasts evolved by

endosymbiosis An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "withi ...

. It is part of a

two-component system In the field of molecular biology, a two-component regulatory system serves as a basic stimulus-response coupling mechanism to allow organisms to sense and respond to changes in many different environmental conditions. Two-component systems t ...

. In the plant ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter ...

'' (thale cress) CSK is the product of the gene ''At1g67840''. CSK is known in cyanobacteria as the histidine kinase 2 (''Hik2''; ). CSK is an

iron-sulfur protein Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur c ...

with 3 iron and 4 sulphur atoms in its redox-active site.Ibrahim IM et al. (2020) An evolutionarily conserved iron-sulfur cluster underlies redox sensory function of the Chloroplast Sensor Kinase. Communications Biology 3: 13 https://doi.org/10.1038/s42003-019-0728-4 It has a midpoint

redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

of −15 mV at pH 8, which is consistent with its

autophosphorylation Autophosphorylation is a type of post-translational modification of proteins. It is generally defined as the phosphorylation of the kinase by itself. In eukaryotes, this process occurs by the addition of a phosphate group to serine, threonine or ...

communicating the redox state of the

plastoquinone Plastoquinone (PQ) is an isoprenoid quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. The most common form of plastoquinone, known as PQ-A or PQ-9, is a 2,3-dimethyl-1,4- benzoquinone m ...

pool to regulation of chloroplast or cyanobacterial

DNA transcription Transcription is the process of copying a segment of DNA into RNA. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). Other segments of DNA are copied into RNA molecules calle ...

– specifically of genes for proteins at the photochemical reaction center of

photosystem I Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that us ...

. In cyanobacteria and non-green algae, it is a

histidine kinase Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically transmembrane, proteins of the transferase class of enzymes that play a role in signal transduction across the cellular membrane. The vast majority of HKs are homod ...

that work by autophosphorylation on a conserved histidine residue, then in turn passing the phosphoryl group to Rre1 and Rppa. These components are not found in

green plants Viridiplantae (literally "green plants") are a clade of eukaryotic organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems. They are made up of the green algae, which a ...

, where CSK might work as a

serine/threonine kinase A serine/threonine protein kinase () is a kinase enzyme, in particular a protein kinase, that phosphorylates the OH group of the amino-acid residues serine or threonine, which have similar side chains. At least 350 of the 500+ human protei ...

passing the group to

sigma factor A sigma factor (σ factor or specificity factor) is a protein needed for initiation of transcription in bacteria. It is a bacterial transcription initiation factor that enables specific binding of RNA polymerase (RNAP) to gene promoters. It is ho ...

1 (SIG1) instead. CSK is a prediction of the

CoRR Hypothesis The CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation ("redox") state of its gene products. CoRR is short for "co-location for redox regulation ...

for genes in

organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...

.Allen JF (2017) The CoRR hypothesis for genes in organelles. Journal of Theoretical Biology, doi:10.1016/j.jtbi.2017.04.008 CSK is intrinsic to chloroplasts, targeted to chloroplast genes, and may have been required for the retention, in evolution, of

chloroplast DNA Chloroplast DNA (cpDNA) is the DNA located in chloroplasts, which are photosynthetic organelles located within the cells of some eukaryotic organisms. Chloroplasts, like other types of plastid, contain a genome separate from that in the cell n ...

Notes

Plant proteins {{protein-stub