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We identify and functionally characterize MRL1, a conserved nuclear-encoded regulator of the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. The nonphotosynthetic mrl1 mutant of Chlamydomonas reinhardtii lacks ribulose-1,5-bisphosphate carboxylase/oxygenase, and the resulting block in electron transfer is partially compensated by redirecting electrons toward molecular oxygen via the Mehler reaction. This allows continued electron flow and constitutive nonphotochemical quenching, enhancing cell survival during illumination in spite of photosystem II and photosystem I photoinhibition. The mrl1 mutant transcribes rbcL normally, but the mRNA is unstable. The molecular target of MRL1 is the 5 ' untranslated region of rbcL. MRL1 is located in the chloroplast stroma, in a high molecular mass complex. Treatment with RNase or deletion of the rbcL gene induces a shift of the complex toward lower molecular mass fractions. MRL1 is well conserved throughout the green lineage, much more so than the 10 other pentatricopeptide repeat proteins found in Chlamydomonas. Depending upon the organism, MRL1 contains 11 to 14 pentatricopeptide repeats followed by a novel MRL1-C domain. In Arabidopsis thaliana, MRL1 also acts on rbcL and is necessary for the production/stabilization of the processed transcript, presumably because it acts as a barrier to 5 ' &gt;3 ' degradation. The Arabidopsis mrl1 mutant retains normal levels of the primary transcript and full photosynthetic capacity.

MRL1, a Conserved Pentatricopeptide Repeat Protein, Is Required for Stabilization of rbcL mRNA in Chlamydomonas and Arabidopsis