title: Evolutionary rewiring: a modified prokaryotic gene-regulatory pathway in chloroplasts.
abstract: Photosynthetic electron transport regulates chloroplast gene transcription through the action of a bacterial-type sensor kinase known as chloroplast sensor kinase (CSK). CSK represses photosystem I (PS I) gene transcription in PS I light and thus initiates photosystem stoichiometry adjustment. In cyanobacteria and in non-green algae, CSK homologues co-exist with their response regulator partners in canonical bacterial two-component systems. In green algae and plants, however, no response regulator partner of CSK is found. Yeast two-hybrid analysis has revealed interaction of CSK with sigma factor 1 (SIG1) of chloroplast RNA polymerase. Here we present further evidence for the interaction between CSK and SIG1. We also show that CSK interacts with quinone. Arabidopsis SIG1 becomes phosphorylated in PS I light, which then specifically represses transcription of PS I genes. In view of the identical signalling properties of CSK and SIG1 and of their interactions, we suggest that CSK is a SIG1 kinase. We propose that the selective repression of PS I genes arises from the operation of a gene-regulatory phosphoswitch in SIG1. The CSK-SIG1 system represents a novel, rewired chloroplast-signalling pathway created by evolutionary tinkering. This regulatory system supports a proposal for the selection pressure behind the evolutionary stasis of chloroplast genes.
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, , Mile End Road, London E1 4NS, UK. s.puthiyaveetil@wsu.edu
authors.forename: Sujith
authors.initials: S
authors.lastname: Puthiyaveetil
authors.forename: Iskander M
authors.initials: IM
authors.lastname: Ibrahim
authors.forename: John F
authors.initials: JF
authors.lastname: Allen
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D005075:Biological Evolution; D002736:Chloroplasts / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant / Q000502:physiology*; D008969:Molecular Sequence Data; D045331:Photosystem I Protein Complex / Q000502:physiology; D011387:Prokaryotic Cells / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D045331:Photosystem I Protein Complex
keywords: chloroplast gene regulation; chloroplast sensor kinase; co-location for redox regulation; redox; sigma factor 1 region 1.1; two-component system