Chloroplasts are essential sites of biosynthesis and gene activity in plants. Transcription, the first step in plastid gene expression, has recently become more fully understood despite its inherent complexity: At least two different organellar RNA polymerases are involved, i.e. NEP, the nucleus-encoded single-subunit (T7-like) enzyme, and PEP, the multi-subunit bacterial-type enzyme initially thought to be entirely coded by chloroplast genes. It has become clear, however, that this is true only for the subunits of the catalytic core, i.e. the equivalents of bacterial alpha, beta, and beta-prime. In contrast, regulatory components of the plastid transcription apparatus including a set of sigma initiation factors are encoded by the nucleus, thus placing the chloroplast under nuclear control at this key level. Our research efforts are centered on the plastid sigma factors and their genes, which have partially overlapping and specialized functions during plant development and in response to environmental and stress conditions. Moreover, sigma activity is under phosphorylation control by a master regulator named plastid transcription kinase (PTK), which is a CK2 kinase associated with the plastid transcription machinery. Current work is based on mutational function analyses of both sigma factor and kinase genes in bacterial cells and transgenic Arabidopsis plants. Switching on or off these genes in a controlled manner is expected to provide new insights into their roles both in a developmental and environmental context.

Chloroplasts provide the basis for most life on earth by the process widely known as photosynthesis. It has become clear that these unique plant cell organelles have their own gene expression system in close physical proximity to the photosynthetic apparatus, which is an ideal requirement for mutual regulation of these two macromolecular systems. Photosynthesis proteins such as those of the reaction centers are subject to accelerated turnover and degradation in response to light intensity and other environmental conditions and hence there is a need for controlled replenishment by gene expression. A key player identified in our group is a redox- responsive Ser/Thr protein kinase of the CK2 type, which is a functional part of the chloroplast transcription apparatus. Following the cloning and characterization of this redox mediator our current interest centers on the interaction partners, both upstream and downstream along the redox signaling pathway, its target genes, and the molecuar phenotype of CK2 knockout vs. wildtype plants under normal and redox stress conditions. In combination with a broader analysis of redox-active components involved in chloroplast gene expression, this can be expected to provide a better picture of redox control in this important plant cell organelle.