title: Integration of light and circadian signals that regulate chloroplast transcription by a nuclear-encoded sigma factor.
abstract: We investigated the signalling pathways that regulate chloroplast transcription in response to environmental signals. One mechanism controlling plastid transcription involves nuclear-encoded sigma subunits of plastid-encoded plastid RNA polymerase. Transcripts encoding the sigma factor SIG5 are regulated by light and the circadian clock. However, the extent to which a chloroplast target of SIG5 is regulated by light-induced changes in SIG5 expression is unknown. Moreover, the photoreceptor signalling pathways underlying the circadian regulation of chloroplast transcription by SIG5 are unidentified. We monitored the regulation of chloroplast transcription in photoreceptor and sigma factor mutants under controlled light regimes in Arabidopsis thaliana. We established that a chloroplast transcriptional response to light intensity was mediated by SIG5; a chloroplast transcriptional response to the relative proportions of red and far red light was regulated by SIG5 through phytochrome and photosynthetic signals; and the circadian regulation of chloroplast transcription by SIG5 was predominantly dependent on blue light and cryptochrome. Our experiments reveal the extensive integration of signals concerning the light environment by a single sigma factor to regulate chloroplast transcription. This may originate from an evolutionarily ancient mechanism that protects photosynthetic bacteria from high light stress, which subsequently became integrated with higher plant phototransduction networks.
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol Life Sciences Building, 24 Tyndall Avenue, Bristol, BS8 1TQ, UK.
authors.forename: Fiona E
authors.initials: FE
authors.lastname: Belbin
authors.affiliation: Department of Botany and Plant Biology, University of Geneva, Geneva, CH-1211, Switzerland.
authors.forename: Zeenat B
authors.initials: ZB
authors.lastname: Noordally
authors.affiliation: Department of Biology, University of York, York, YO10 5DD, UK.
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Wetherill
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol Life Sciences Building, 24 Tyndall Avenue, Bristol, BS8 1TQ, UK.
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Atkins
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol Life Sciences Building, 24 Tyndall Avenue, Bristol, BS8 1TQ, UK.
authors.forename: Keara A
authors.initials: KA
authors.lastname: Franklin
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol Life Sciences Building, 24 Tyndall Avenue, Bristol, BS8 1TQ, UK.
authors.forename: Antony N
authors.initials: AN
authors.lastname: Dodd
mesh_terms: D017360:Arabidopsis / Q000235:genetics* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism* / Q000528:radiation effects; D002736:Chloroplasts / Q000235:genetics* / Q000378:metabolism / Q000528:radiation effects; D002940:Circadian Rhythm / Q000528:radiation effects*; D056931:Cryptochromes / Q000378:metabolism; D054627:Genome, Plastid; D008027:Light; D008156:Luciferases / Q000378:metabolism; D055519:Photoreceptors, Plant / Q000378:metabolism; D010788:Photosynthesis / Q000528:radiation effects; D010834:Phytochrome / Q000378:metabolism; D012808:Sigma Factor / Q000378:metabolism*; D015398:Signal Transduction / Q000528:radiation effects*; D014158:Transcription, Genetic
chemical_list: D029681:Arabidopsis Proteins; D056931:Cryptochromes; D055519:Photoreceptors, Plant; C486979:Sig5 protein, Arabidopsis; D012808:Sigma Factor; D010834:Phytochrome; D008156:Luciferases
keywords: 
  Arabidopsis thaliana
  ; chloroplasts; circadian rhythms; photobiology; signal transduction