The protein disulfide isomerase-like RB60 is partitioned between stroma and thylakoids in Chlamydomonas reinhardtii chloroplasts

Translation of psbA mRNA in Chlamydomonas reinhardtii chloroplasts is regul ated by a redox signal(s), RB60 is a member of a protein complex that binds with high affinity to the 5'-untranslated region of psbA mRNA. RB60 has be en suggested to act as a redox-sensor subunit of the protein complex regula ting translation of chloroplast psbA mRNA, Surprisingly, cloning of RB60 id entified high homology to the endoplasmic reticulum-localized protein disul fide isomerase, including an endoplasmic reticulum-retention signal at its carboxyl terminus. Here we show, by in vitro import studies, that the recom binant RB60 is imported into isolated chloroplasts of C, reinhardtii and pe a in a transit peptide-dependent manner. Subfractionation of C, reinhardtii chloroplasts revealed that the native RB60 is partitioned between the stro ma and the thylakoids, The nature of association of native RB60, and import ed recombinant RB60, with thylakoids is similar and suggests that RB60 is t ightly bound to thylakoids, The targeting characteristics of RB60 and the p otential implications of the association of RB60 with thylakoids are discus sed.