Mutations at three different nuclear loci of Chlamydomonas suppress a defect in chloroplast psbD mRNA accumulation

In the photosynthetic chloroplast mutant PRB2A of Chlamydomonas reinhardtii the psbD mRNA is unstable. Three strains were isolated, in which the under lying site-directed mutation within the psbD 5' UTR (untranslated region) i s suppressed. In all three suppressors, psbD RNA levels and RNA 5' maturati on are restored to a varying extent, suggesting a tight coupling of RNA sta bilization and 5' processing. Expression of the psbA gene is not compromise d in these strains. Genetic crosses revealed that the suppressor mutations affect three unlinked nuclear loci, which may encode new factors involved i n psbD gene expression.