Role and regulation of plastid sigma factors and their functional interactors during chloroplast transcription - recent lessons from Arabidopsis thaliana.

Chloroplasts and other plastids within plant cells together are responsible for autotrophic growth and biosynthesis of metabolic key components. Genetically, the plastids are hybrid organelles composed of proteins that are either products of their own organellar genes or are nucleus-encoded and imported from the cytosol. This dual genetic principle is evident in the case of the multi-subunit RNA polymerase, i.e. a major enzyme of the plastid transcription apparatus, consisting of an organelle-encoded core surrounded by imported regulatory proteins. Representatives of the latter are the members of the plastid sigma factor family as well as a Ser/Thr-protein kinase (PTK/cpCK2) that functionally modifies these factors and controls transcription. The plant sigma factors contain regulatory phosphoacceptor sites within their unconserved (factor-specific) portion that precedes the conserved catalytic region. Phosphorylation state changes of these regulatory sites help establish the activity and promoter selectivity of individual members of this plant transcription factor family. The protein kinase itself responds to SH-group regulation by glutathione and transmits the redox signal via its phosphorylation activity to the plastid transcription apparatus. Other functional interactors include a set of sigma binding proteins that confer enhanced promoter binding in vitro and are thought to be involved in pathogenic stress responses of the chloroplast in vivo.