An mRNA is capped by a 2′,5′ lariat catalyzed by a group I-like ribozyme

Twin-ribozyme introns are formed by two ribozymes belonging to the group I family and occur in some ribosomal RNA transcripts. The group l-like ribozyme, GIR1, liberates the S' end of a homing endonuclease messenger RNA in the slime mold Didymium iridis. We demonstrate that this cleavage occurs by a transesterification reaction with the joining of the first and the third nucleotide of the messenger by a 2',5'-phosphodiester linkage, Thus, a group l-like ribozyme catalyzes an RNA branching reaction similar to the first step of splicing in group II introns and spliceosomal introns. The resulting short lariat, by forming a protective 5' cap, might have been useful in a primitive RNA world.