| Chloroplast sensor kinase | |
| Symbol: | CSK |
| Uniprot: | F4HVG8 |
| Organism: | Arabidopsis thaliana |
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;).
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.