A nuclear gene of Chlamydomonas reinhardtii, Tba1, encodes a putative oxidoreductase required for translation of the chloroplast psbA mRNA.

Biosynthesis of chloroplast proteins is to a large extent regulated post-transcriptionally, and a number of nuclear-encoded genes have been identified that are required for translation or stability of specific chloroplast mRNAs. A nuclear mutant of Chlamydomonas reinhardtii, hf261, deficient in the translation of the psbA mRNA, has reduced association of the psbA mRNA with ribosomes and is deficient in binding of the chloroplast localized poly (A) binding protein (cPAB1) to the psbA mRNA. Cloning of the hf261 locus and complementation of hf261 using a wt genomic clone has identified a novel gene, Tba1, for translational affector of psbA. Strains complemented with the wt Tba1 gene restore the ability of the psbA mRNA to associate with ribosomes, and restores RNA binding activity of cPAB1 for the psbA mRNA. Analysis of the Tba1 gene identified a protein with significant homology to oxidoreductases. The effect of Tba1 expression on the RNA binding activity of cPAB1, and on the association of psbA mRNA with ribosomes, implies that Tba1 functions as a redox regulator of cPAB1 RNA binding activity to indirectly promote psbA mRNA translation initiation. A model of chloroplast translation incorporating Tba1 and other members of the psbA mRNA binding complex is presented.