Plant Biology Lighting Up Gene Expression in Chloroplasts

Chloroplasts are the energy-producing, photosynthesizing organelles of plants. These organelles have their own genome, which is regulated by light and which is not part of the nuclear genome of the plant. Baena-González et al. show that, under physiological concentrations of reduced glutathione (GSH) in the presence of low concentrations of adenosine triphosphate (ATP), the activity of plastid transcription kinase (PTK) is inhibited in vitro. PTK phosphorylates the chloroplast RNA polymerase PEP-A, inhibiting its activity and suppressing chloroplast gene expression. In an invitro transcription assay, preincubation of PEP-A with PTK inhibits expression of both photosynthetic and nonphotosynthetic chloroplast genes. Exposure of mustard plants to high-intensity light (HL) led to a higher ration of GSH to oxidized glutathione in chloroplasts and enhanced transcription rates for all chloroplast genes tested compared with those in plants grown under light conditions optimal for growth (GL). Biochemical analysis of a partially purified RNA polymerase complex from plants exposed to GL or GL showed that the intrinsic kinase activity was lost in the HL samples. The authors suggest that under high-intensity light conditions, photosynthesis is stimulated, producing high concentrations of reducing equivalents and thus increasing the formation of GSH. GSH, in turn, may reduce PTK, inhibiting the enzyme activity and allowing the chloroplast RNA polymerase activity to increase, leading to increased chloroplast gene expression.

E. Baena-González, S. Baginsky, P. Mulo, H. Summer, E.-M. Aro, G. Link, Chloroplast transcription at different light intensities. Glutathione-mediated phosphorylation of the major RNA polymerase involved in redox-regulated organellar gene expression. Plant Physiol. 127, 1044-1052 (2001). [Abstract] [Full Text]