A unique C-terminal domain contributes to the molecular function of Restorer-of-fertility proteins in plant mitochondria

center dot Restorer-of-fertility (Rf) genes encode pentatricopeptide repeat (PPR) proteins that are targeted to mitochondria where they specifically bind to transcripts that induce cytoplasmic male sterility and repress their expression. In searching for a molecular signature unique to this class of proteins, we found that a majority of known Rf proteins have a distinct domain, which we called RfCTD (Restorer-of-fertility C-terminal domain), and its presence correlates with the ability to induce cleavage of the mitochondrial RNA target. center dot A screen of 219 angiosperm genomes from 123 genera using a sequence profile that can quickly and accurately identify RfCTD sequences revealed considerable variation in RFL/ RfCTD gene numbers across flowering plants. We observed that plant genera with bisexual flowers have significantly higher numbers of RFL genes compared to those with unisexual flowers, consistent with a role of these proteins in restoration of male fertility. center dot We show that removing the RfCTD from the RFL protein RNA PROCESSING FACTOR 2-nad6 prevented cleavage of its RNA target, the nad6 transcript, in Arabidopsis thaliana mitochondria. center dot We provide a simple way of identifying putative Rf candidates in genome sequences, new insights into the molecular mode of action of Rf proteins and the evolution of fertility restoration in flowering plants.

Automated summary

Restorer-of-fertility (Rf) genes encode pentatricopeptide repeat (PPR) proteins that target mitochondria and bind to transcripts inducing cytoplasmic male sterility. A distinct domain called RfCTD is found in most Rf proteins and is associated with cleavage of mitochondrial RNA targets. Plant genera with bisexual flowers have higher numbers of RFL genes, suggesting their role in male fertility.