Chloroplasts have their own genetic system with both pro‐ and eukaryotic features. This is reflected by the organellar transcription apparatus itself, which has turned out to be a complex protein network that has not yet completely been resolved. Plastid transcription involves at least two DNA‐dependent RNA polymerases with different promoter‐, development‐ and environment‐ selective roles: NEP, the nucleus‐encoded phage‐type enzyme, and PEP, the plastid‐encoded multi‐subunit bacterial‐type enzyme. Like in bacteria, PEP is initiation‐competent only when associated with a sigma factor. Typically for higher plants, Arabidopsis thaliana has a set of six ‐ nucleus‐encoded ‐ sigma factors (SIG1‐6). The regulatory repertoire of plastid transcription is further increased by other PEP accessory proteins that can modify, and affect the activity level of, these transcription factors. The C-terminal part of the sigma proteins is very conserved and harbors the basic functions including promoter (‐10/‐35 element) and polymerase binding. In contrast, the N‐terminal half is highly variable in sequence and length and contains critical factor‐specific determinants (Schweer et al., 2009, 2010). Recent data will be presented that underpin the regulatory function which these factor‐specific sites and motifs can have on chloroplast transcription. For instance, a PEP‐associated plastid transcription kinase phosphorylates sigma factors and, this way, modulates transcription activity and specificity. Putative and experimentally confirmed phosphorylation sites on individual members of the Arabidopsissigma factor family will be discussed (Schweer, 2010).




7th Tri-national Arabidopsis Meeting, 15-18 September 2010, Salzburg, Austria

Effects of light and salt stress on the expression of photosynthesis genes in A. thaliana sigma mutants

Thomas Pieta and Gerhard Link

Ruhr‐University Bochum, Laboratory of Plant Cell Physiology & Molecular Biology
(Email: thomas.pieta at rub.de)

Chloroplasts and other plastids are genetically semiautonomous organelles, i.e. they have their own DNA and an active transcription apparatus with a complex and dynamic architecture. The latter is built up from both chloroplast‐own and nucleus‐encoded proteins, reflecting the stringent interaction network between different cell compartments. The basic catalytic process of RNA synthesis by plastid RNA polymerase(s) is part of a much larger molecular machinery that integrates various regulatory interactions. The impact of individual transcription regulatory proteins to a large part depends on developmental stage as well as on environmental cues. Like in bacteria, a small family of (nucleus‐encoded) proteins, so called sigma‐factors, seems to play a major role in 'landscaping' plastid transcription. To analyse the consequences resulting from defects in plastid transcription, sigma factor mutant lines of A. thaliana were tested for changes in plastid gene expression under different biotic and abiotic stress conditions, with emphasis on photosystem I and II components. Environmentally‐related stress conditions included redox‐active variation of light quality and intensity, salt, and biotic stress mediated by the plant pathovar Pseudomonas syringae DC3000.

Chloroplasts are the essential plant cell organelles that carry out photosynthesis and contain a complete genetic system composed of plastid- and nucleus-encoded proteins. Two different types of organellar RNA polymerases participate in transcription, i.e the phage T7-type nucleus-encoded NEP and the bacterial-type PEP with a plastid-encoded core. The latter is embedded, however, into a complex with more than 50 accessory - mostly regulatory - proteins that are imported from the cytosol. The nucleus-encoded regulatory PEP partners include a set of sigma factors, which can confer the ability for promoter binding and transcription initiation to the core enzyme. Much current interest is centered on the members of the plant sigma family, their function and regulatory importance.
   To define the role of individual members of this transcription factor family in Arabidopsis, sigma knockout plants as well as double mutants and chimeric constructs were analysed. In the work described here, we have studied the impact on plastid expression by chimeric (hybrid) factors that contain or lack critical motifs. We observe a dramatic decrease or even complete loss of certain chloroplast transcripts and, conversely, the appearance of new RNAs not seen in wildtype, which seem to be initiated from previously unrecognized promoter(s) far-upstream of known coding regions. These distal promoter regions reveal conserved sequence elements used by the nucleus-encoded NEP polymerase (RpoTp and/or RpoTmp). Molecular adaptation to NEP promoter clusters can be viewed as a way to ensure RNA synthesis in situations where the regulatory fine-tuning of PEP-sigma complexes is unbalanced, thus resulting in sufficient transcript level for rescue and maintenance of fundamental processes (SOS response). Hence, these data provide information on plastid sigma factors themselves as well as on the NEP/PEP interplay, which
seems to be more complex than might be expected from the relatively small size of the chloroplast genome.




ISE (International Society of Endocytobiology)
XIth International Colloquium on Endocytobiology and Symbiosis
Tromsø, Norwegen 29.08.- 03.09.2010

Balance and Fine-Tuning within the Sigma Factor Family in Arabidopsis thaliana

Sylvia Bock, Jennifer Schweer, Thomas Pieta, Brigitte Link and Gerhard Link
Ruhr University Bochum, Bochum

In terms of origin and function, plastids are genetically semiautonomous cell organelles with both prokaryotic and eukaryotic features in their gene expression maschinery. This is evident at the level of transcription, which is shared by two types of RNA polymerases: the nucleus-encoded phage-type polymerase (NEP) and the „plastid-encoded“ multisubunit bacterial-type enzyme (PEP). Its catalytic core is embedded in a complex with up to 50 accessory proteins, most of which are nucleus-encoded. The perhaps best-known representatives of the latter are the sigma factors, i.e. prokaryotic-type transcription regulatory proteins necessary for transcription initiation. The plastids of higher plants contain multiple sigma factors (e.g. six in Arabidopsis) and recent efforts have centered on their role and regulation during plant development and in response to variable environmental conditions. In contrast to the situation in bacteria, plant sigma factors cannot easily be grouped into either primary (essential) or alternative factors; instead, both functional redundancy and specialization is noticeable. To reduce the complexity of the system, we have addressed the question of function by using single and multiple sigma knockout lines. Recent results suggest a flexible network of interactors and regulators that can affect the efficiency of individual sigma factors and the entire family. Using multiple knockout mutants, progress has been made towards a minimal system with regard to the plastid sigma factor(s). This coarse-control strategy, along with fine-tuning by RNAi, can be expected to further help answer the question of sigma specialization versus functional redundancy.