Characterization of Mbb1, a nucleus-encoded tetratricopeptide-like repeat protein required for expression of the chloroplast psbB/psbT/psbH gene cluster in Chlamydomonas reinhardtii

Genetic analysis has revealed that the accumulation of several chloroplast mRNAs of the green alga Chlamydomonas reinhardtii requires specific nucleus -encoded functions. To gain insight into this process, we have cloned the n uclear gene encoding the Mbb1 factor by genomic rescue of a mutant specific ally deficient in the accumulation of the mRNAs of the psbB/psbT/psbH chlor oplast transcription unit. Mbb1 is a soluble protein in the stromal phase o f the chloroplast. It consists of 662 amino acids with a putative chloropla st-transit peptide at its N-terminal end. A striking feature is the presenc e of 10 tandemly arranged tetratricopeptide-like repeats that account for h alf of the protein sequence and are thought to be involved in protein-prote in interactions. The Mbb1 protein seems to have a homologue in higher plant s and is part of a 300-kDa complex that is associated with RNA. This comple x is most likely involved in psbB mRNA processing, stability, and/or transl ation.