Chloroplast sensor kinase

Chloroplast Sensor Kinase (CSK) is a protein in chloroplasts and cyanobacteria, bacteria from which chloroplasts evolved by endosymbiosis. It is part of a two-component system.^[1] In the plant Arabidopsis thaliana (thale cress) CSK is the product of the gene At1g67840. CSK is known in cyanobacteria as the histidine kinase 2 (Hik2; P73276).

Chloroplast sensor kinase
Identifiers
Organism	Arabidopsis thaliana
Symbol	CSK
UniProt	F4HVG8
Search for Structures Swiss-model Domains InterPro

CSK is an iron-sulfur protein with 3 iron and 4 sulphur atoms in its redox-active site.^[2] It has a midpoint redox potential of −15 mV at pH 8, which is consistent with its autophosphorylation communicating the redox state of the plastoquinone pool to regulation of chloroplast or cyanobacterial DNA transcription^[3] – specifically of genes for proteins at the photochemical reaction center of photosystem I.

In cyanobacteria and non-green algae, it is a histidine kinase 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, where CSK might work as a serine/threonine kinase passing the group to sigma factor 1 (SIG1) instead.^[2]

CSK is a prediction of the CoRR Hypothesis for genes in organelles.^[4][5][6] CSK is intrinsic to chloroplasts, targeted to chloroplast genes, and may have been required for the retention, in evolution, of chloroplast DNA.