title: AtSIG6, a plastid sigma factor from Arabidopsis, reveals functional impact of cpCK2 phosphorylation.
abstract: Plastids contain sigma factors, i.e. gene-regulatory proteins for promoter binding and transcription initiation. Despite the physical and functional similarity shared with their prokaryotic counterparts, the plant sigma factors have distinguishing features: most notably the existence of a variable extra sequence comprising their N-terminal portions. This distinct architecture is reflected by functional differences, including phosphorylation control by organellar protein kinase(s) closely related to nucleocytosolic, rather than bacterial-type, enzymes. In particular, cpCK2, a nuclear-coded plastid-targeted casein kinase 2, has been implicated as a key component in plant sigma factor phosphorylation and transcriptional regulation (Eur. J. Biochem. 269, 2002, 3329; Planta, 219, 2004, 298). Although this notion is based mainly on biochemical evidence and in vitro systems, the recent availability of Arabidopsis sigma knock-out lines for complementation by intact and mutant sigma cDNAs has opened up new strategies for the study of transcription regulatory mechanisms in vivo. Using Arabidopsis sigma factor 6 (AtSIG6) as a paradigm, we present data suggesting that: (i) this factor is a substrate for regulatory phosphorylation by cpCK2 both in vitro and in vivo; (ii) cpCK2 phosphorylation of SIG6 occurs at multiple sites, which can widely differ in their effect on the visual and/or molecular phenotype; (iii) in vivo usage of the perhaps most critical cpCK2 site defined by Ser174 requires (pre-)phosphorylation at the n + 3 serine residue Ser177, pointing to 'pathfinder' kinase activity capable of generating a functional cpCK2 substrate site.
authors.affiliation: Laboratory of Plant Cell Physiology and Molecular Biology, University of Bochum, Universitaetsstr. 150, 44780 Bochum, Germany.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Schweer
authors.forename: Hacer
authors.initials: H
authors.lastname: Türkeri
authors.forename: Brigitte
authors.initials: B
authors.lastname: Link
authors.forename: Gerhard
authors.initials: G
authors.lastname: Link
mesh_terms: D020816:Amino Acid Motifs; D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D047390:Casein Kinase II / Q000378:metabolism*; D002736:Chloroplasts / Q000201:enzymology*; D018506:Gene Expression Regulation, Plant; D055786:Gene Knockout Techniques; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D018749:RNA, Plant / Q000235:genetics; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D012808:Sigma Factor / Q000235:genetics / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D018749:RNA, Plant; D011994:Recombinant Proteins; C502611:SIG6 protein, Arabidopsis; D012808:Sigma Factor; D047390:Casein Kinase II