Group II introns are large catalytic RNAs that are ancestrally related to nuclear spliceosomal introns. Sequences corresponding to group II RNAs are found in many prokaryotes and are particularly prevalent within plants organellar genomes. Proteins encoded within the introns themselves (maturases) facilitate the splicing of their own host pre-RNAs. Mitochondrial introns in plants have diverged considerably in sequence and have lost their maturases. In angiosperms, only a single maturase has been retained in the mitochondrial DNA: the matR gene found within NADH dehydrogenase 1 (nad1) intron 4. Its conservation across land plants and RNA editing events, which restore conserved amino acids, indicates that matR encodes a functional protein. However, the biological role of MatR remains unclear. Here, we performed an in vivo investigation of the roles of MatR in Brassicaceae. Directed knockdown of matR expression via synthetically designed ribozymes altered the processing of various introns, including nad1 i4. Pull-down experiments further indicated that MatR is associated with nad1 i4 and several other intron-containing pre-mRNAs. MatR may thus represent an intermediate link in the gradual evolutionary transition from the intron-specific maturases in bacteria into their versatile spliceosomal descendants in the nucleus. The similarity between maturases and the core spliceosomal Prp8 protein further supports this intriguing theory.
Bibliographical noteFunding Information:
We thank Christian Schmitz-Linneweber (Humboldt University, Berlin) for help with mitochondriaRNA-seq data and dot-blot analysis ofRNAco-IPs, Ian Small (University of Western Australia) for the assistancewith RT-qPCR analyses, Yoram Eyal (The Volcani Research Instute) for providing the RACE protocols, Amalia Biran-Ostersetzer for assistance with mitochondria RNA-seq analysis, Maciej Szymanski (A. Mickiewicz University, Poznan) for help in ribozyme design, Marta Gabryelska (PAN-IBCH, Poznan) for help in ribozyme testing, Anne Cosset for help in transformant analysis, and Sam Aldrin for his careful reading of the manuscript and points raised. This work was supported by grants to O.O.-B. from the Israeli Science Foundation (No. 741/15) and in part from the German-Israeli Foundation (GIF 1213/2012) to A.D., by the French State program "Investments for the future" (LABEX ANR-11-LABX-0057_MITOCROSS), and by the French National Research Agency (ANR-06-MRAR-037-02 and ANR- 09-BLAN-0240-01). This work was also supported by a grant from the Polish National Science Centre (UMO-2013/09/B/NZ1/03359) to J.B. and by grants from the U.S. National Science Foundation (IOS-1027529 and MCB-1125386) to J.P.M. Regular funding to A.D. from the French National Center for Scientific Research (CNRS-UPR2357) and the University of Strasbourg is acknowledged.
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