OPR-RAP proteins and nucleotidyl transferases : role in organellar gene expression in Chlamydomonas reinhardtii

Overview & Synopsis

In the green alga Chlamydomonas reinhardtii, a family of RNA-binding proteins called OctotricoPeptide Repeat (OPR) proteins plays a role in controlling gene expression within organelles. These proteins, which are encoded in the nucleus, are characterized by a repeated sequence of 38 amino acids that fold into a structure resembling a spiral. OPR proteins are involved in many processes, such as stabilizing and maturing mRNA, translating it into proteins, and even splicing. Two separate dominant mutations in the nuclear genes ncc1 and ncc2 have been discovered. These mutations enable the proteins to recognize and destabilize new RNA targets, specifically two chloroplast mRNAs: atpA and petA, which encode essential components of the cell's photosynthetic chains. Interestingly, the NCC1 and NCC2 proteins have a RAP domain at their C-terminus, a feature also found in human mitochondrial proteins, which suggests they might act as endonucleases—enzymes that can cut RNA.NCC1 and NCC2 are part of a closely related group of genes unique to C. reinhardtii, known as the NCL genes. These genes, located on chromosome 15, are highly similar in sequence and appear to be evolving rapidly, particularly in a key part of the OPR repeats that determine RNA target specificity. We propose that these NCL genes may be adapted to avoid cutting their own chloroplast RNA but could potentially do so when crossed with a distantly related strain. This function might be crucial in ensuring that the chloroplast DNA is inherited from the correct parent during reproduction. In my thesis, I explored the role of the RAP domain in these proteins, aiming to demonstrate that they function as endonucleases by creating several variants of the RAP domains, that shows that the protein's ability to cut totally dependes on the presence of a functional RP domain.