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.
Highlights
Plant cells are tripartite genetic systems consisting of three transcriptionally active compartments, i.e. the nucleus, the mitochrondria and the plastids
Biochem. 269, 2002, 3329; Planta, 219, 2004, 298). 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 requiresphosphorylation at the n + 3 serine residue Ser177, pointing to ‘pathfinder’ kinase activity capable of generating a functional cpCK2 substrate site
Summary
Plant cells are tripartite genetic systems consisting of three transcriptionally active compartments, i.e. the nucleus, the mitochrondria and the plastids The latter contain two principally different forms of RNA polymerases for transcription of a full complement of organellar genes in normal (wildtype) plants. Plastid-encoded polymerase (PEP) is a multi-subunit bacterial-type enzyme with a-, b- and b¢-equivalent core subunits that are encoded by plastid genes (Maliga, 1998; Hess and Borner, 1999). It has become clear, that the core polypeptides are embedded into a much larger functional complex, made up of nucleusencoded polypeptides, most of which seem to represent. The role of UCR has long remained enigmatic, recent work suggests that it is critically involved in specifying the visual and molecular phenotype (Kubota et al, 2007; Schweer et al, 2009)
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