TCP34, a nuclear-encoded response regulator-like TPR protein of higher plant chloroplasts.

We describe the identification of a novel chloroplast protein, designated TCP34 (tetratricopeptide-containing chloroplast protein of 34 kDa) due to the presence of three tandemly arranged tetratricopeptide repeat (TPR) arrays. The presence of the genes encoding TCP34 only on the genomes of higher plants but not in the photosynthetic cyanobacterial prokaryotes suggests that TCP34 evolved with eukaryotism. The in vitro translated precursor could be imported into intact spinach chloroplasts and showed stable association with thylakoid membranes. Using a specific polyclonal antiserum raised against TCP34 three protein variants were detected. Two forms, T1 and T2, were associated with the thylakoid membranes and one, S1, could be released in the stroma. TCP34 protein was not present in etioplasts and appeared only in developing chloroplasts. The ratio of membrane-bound and soluble forms was maximum at the onset of photosynthesis. The high molecular weight thylakoid TCP34 variant was found in association with a transcriptionally active protein/DNA complex (TAC) from chloroplasts and recombinant TCP34 showed specific binding to Spinacia oleracea chloroplast DNA. Two TCP34 forms, T1 and S1, were found to be phosphorylated. An as yet unidentified phosphorelay signal may modulate its capability for ptDNA binding through the phosphorylation state of the putative response regulator-like domain. Based on the structural properties and biochemical analyses we discuss the putative regulatory function of TCP34 in a plastid gene expression.