The GIR1 branching ribozyme is a 179 nt ribozyme with a structural resemblance to a group I ribozyme. It is found within a complex type of group I introns also termed twin-ribozyme introns. Rather than splicing, it catalyses a branching reaction in which the 2'OH of an internal residue is involved in a nucleophilic attack at a nearby phosphodiester bond. As a result, the RNA is cleaved at an internal processing site (IPS), leaving a 3'OH and a downstream product with a tiny lariat at its 5' end. The lariat has the first and the third nucleotide joined by a 2',5' phosphodiester bond and is referred to as 'the lariat cap' because it caps an intron-encoded mRNA. The resulting lariat cap seems to contribute by increasing the half-life of the HE mRNA, thus conferring an evolutionary advantage to the HE.
Read more about GIR1 Branching Ribozyme: Biological Context, Structural Organization of The Twin-ribozyme Introns, GIR1 Catalyzes Three Different Reactions, Modelling Structure of GIR1
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