Séminaire départemental Dr. Maria COSTA
Lundi 25 septembre 2017 à 11h
Amphi FAURRE , Ecole polytechnique
Dr. Maria COSTA,
Institute for Integrative Biology of the Cell (I2BC), Gif-sur-Yvette
Titre : "Crystal structures of a group II intron lariat: implications for the spliceosome"
Group II introns are catalytic RNAs able to splice from precursor RNA molecules in the absence of proteins. These large ribozymes, abundant in bacteria and in the bacterial-derived organelles of some higher organisms, are believed to have played an essential role in eukaryotic evolution as the ancestors of nuclear premessenger introns and their splicing machinery (the spliceosome). A hallmark of group II introns, which they share with spliceosomal introns, is their excision as branched molecules (‘lariats’). This particular conformation results from a specific 2’-5’ phosphodiester bond that links a conserved intron adenosine to the first intron nucleotide, most often a guanosine. Most bacterial group II introns encode a reverse transcriptase (RT) enzyme and, in association with their RT, behave as mobile retroelements. Their mobility pathway, by retrotransposition, is initiated through ‘reverse splicing’ of the excised intron lariat into a DNA target site.
Our recent crystal structures of a group II intron lariat illuminate the raisons for the evolutionary success of the lariat conformation by revealing that the 2’-5’ branch is essential in organizing the intron active site for efficient and faithful ligation of the flanking exons during the last stage of splicing. Moreover, the structures show that the terminal 3’-hydroxyl group of the excised lariat is activated by a metal ion and poised for catalysis of the reverse-splicing reaction, the first step of intron retrotransposition. Somewhat unexpectedly, the crystal structures also bring to light, (1) the necessity to rearrange the structure of the branchpoint and surrounding nucleotides in between the two catalytic steps of splicing and (2), the existence of a ‘sensing’ mechanism that allows the catalytic center to recognize proper positioning of the 5’ exon. Finally, our work reinforces the evolutionary relationship between group II and eukaryotic pre-mRNA splicing by suggesting new homologies between nucleotides conserved in both systems.
Lieu : Ecole Polytechnique, Amphithéâtre FAURRE
Contact : Yves Mechulam