Homeostatic control of plastid transcription and consequences for development and stress responses
Zusammenfassung der Projektergebnisse
Chloroplasts and other plastids contain the enzymatic setup for photosynthesis and/or other anabolic key reactions of plant cells. The multi-faceted function of these organelles is reflected by the complex expression of both their own genes and of nuclear genes for plastid constituents. This dual genetic origin, which accounts for mixed ensembles of plastid-own vs. imported nuclear-coded proteins, is a characteristic feature of the plastid gene expression apparatus itself. For instance, transcription (RNA synthesis) involves two different types of DNA-dependent RNA polymerases known as NEP (nucleus-encoded single-catalytic-polypeptide “phagetype” enzyme) and PEP (plastid-encoded “bacterial-type” enzyme with multi-subunit catalytic core): The resemblance of the latter enzyme with bacterial RNA polymerase extends to the fact that in both cases the catalytic core acts in concert with multiple regulatory proteins named sigma factors. The plant versions of these factors are nucleus-encoded and – unlike their bacterial counterparts – they have acquired “eukaryotic” features, including e.g. phosphorylation control by specialized protein kinase(s). Previous work has provided details contributing to an emerging view of plastid sigma factors as regulators of organellar gene transcription by defining functional determinants of individual members of this protein family. We now have proceeded by addressing questions of how the entire sigma factor network is balanced during normal development, or is disrupted and re-shaped following genetic manipulation and in stress-related situations. Molecular genetic strategies resulting in (conditional or permanent) imbalance of the network have been applied to Arabidopsis thaliana as a model plant system. These include both singleand double-knockout mutants in combination with RNAi as well as dominant-negatively acting sigma gene variants introduced into the Arabidopsis wildtype (“Protein-i”). Subsequent phenotypic and molecular analyses with a focus on plastid target gene expression have allowed insights into redundant vs. specialized functions of sigma factors. For instance, pairs of functionally redundant sigma genes were identified, a lower limit of the number of intact members of the sigma gene family was assigned, and both similar and different features in sigma mutant vs. RNAi (or protein-i) lines were characterized. This knowledge can be considered as a prerequisite for success in continued studies on the role of homeostatic vs. transitory states in plastid gene expression. In addition to progress in our understanding of the basics of plant cell gene regulation, the current strategies and results may also be helpful within the context of applied plastid biotechnology.
Projektbezogene Publikationen (Auswahl)
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(2010) AtSIG6, a plastid sigma factor from arabidopsis, reveals functional impact of cpCK2 phosphorylation. Plant J. 62:192-202
Schweer,J., Türkeri,H., Link,B., and Link,G.
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(2010) Plant sigma factors come of age: flexible transcription factor network for regulated plastid gene expression. Endocytobiosis Cell Res. 20:1-20
Schweer,J.
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(2010) Role and regulation of plastid sigma factors and their functional interactors during chloroplast transcription - recent lessons from Arabidopsis thaliana. Eur.J.Cell Biol. 89:940- 946
Schweer,J., Türkeri,H., Kolpack,A., and Link,G.
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(2012) Phylogenetic and functional features of the plastid transcription kinase cpCK2 from Arabidopsis signify a role of cysteinyl SH-groups in regulatory phosphorylation of plastid sigma factors. FEBS J. 279:395-409
Türkeri,H., Schweer,J., and Link,G.
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(2014) Plastid gene transcription: promoters and RNA polymerases. In: Chloroplast Biotechnology – Methods and Protocols (P. Maliga, ed.). Methods Mol. Biol. 1132:47-72, Humana Press, Springer, New York
Ortelt,J. and Link,G.
