[1] In the plant Arabidopsis thaliana (thale cress) CSK is the product of the gene At1g67840.
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.