Figure 1

Components of the ancient basal (cyanobacterial-like, grey) and modern extended (grey and black) gene expression system of chloroplasts. We propose that the evolution of RpoTs in plants, the expansion of the gene families coding for chloroplast sigma factors and PPR proteins help to neutralize mutational lesions in the chloroplast genomes (black triangles). RpoTs and additional sigma factors with lower or altered promoter specificity compensate for degenerated promoters. The highly specific members of the versatile PPR family promote or are required for proper translation, coding and splicing of chloroplast mRNAs.