The Nac2 gene of Chlamydomonas encodes a chloroplast TPR-like protein involved in psbD mRNA stability

The psbD mRNA, which encodes the D2 reaction center polypeptide of photosys tem II, is one of the most abundant chloroplast mRNAs, We have used genomic complementation to isolate the nuclear Nac2 gene, which is required for th e stable accumulation of the psbD mRNA in Chlamaydomonas reinhardtii. Nac2 encodes a hydrophilic polypeptide of 1385 amino acids with nine tetratricop eptide-like repeats (TPRs) in its C-terminal half. Cell fractionation studi es indicate that the Nac2 protein is localized in the stromal compartment o f the chloroplast, It is part of a high molecular weight complex that is as sociated with nonpolysomal RNA. Change of a conserved alanine residue of th e fourth TPR motif by site-directed mutagenesis leads to aggregation of Nac 2 protein and completely abrogates its function, indicating that this TPR i s important for proper folding of the protein and for psbD mRNA stability, processing and/or translation.