Phage twort group I ribozyme (self-splicing intron) complexed with it's product
Bacteriophage Twort (virus)
During protein synthesis in eukaryotes, the RNA transcript produced from the DNA template is subjected to some modifications before it is used to synthesize the proteins. One such modification of the RNA is called splicing. Splicing is the removal of non-coding RNA fragments (introns) from the RNA transcript and the ligation of the coding RNA (exons). The mRNA that is used for protein synthesis of translation in eukaryotes is a collection of ligated exons. The removal of introns from most pre-mRNA molecules is a complex and accurate process that requires 5 additional RNA molecules (catalytic RNA or ribozymes) and more than 50 proteins. However, in some pre-mRNA molecules, introns can be spliced out without the presence of these additional proteins and RNA. Such introns are called self-splicing introns. There are two types of self-splicing introns. Group I introns are those that begin the splicing reaction by binding a guanine (G) nucleotide to the intron sequence, activating it and resulting in the cleavage of a phosphodiester bond to splice the intron. Group II introns have an active adenine (A) nucleotide that causes the cleavage of the phosphodiester bond beginning the splicing process. Both Group I and Group II introns achieve their catalytic activity by folding into highly specific structures. Although intron splicing is usually found in eukaryotes, it is also observed in viruses, as shown below.
The structure shown here is that of an active ribozyme derived from the Group I intron of the phage Twort. The structure has 3 conserved domains that form a ring which envelopes the active site. The active site is a small pocket for the G to bind formed by a series of stacked base triples. The structure also shows a small 4 nucleotide product RNA bound to it.
Protein Data Bank (PDB)
Golden, B.L. Kim, H. Chase, E.; "Crystal structure of a phage Twort group I ribozyme-product complex"; Nat.Struct.Mol.Biol.; (2005) 12:82-89 PubMed:15580277.
author: Ashwini Patil