Book of Abstracts: Albany 2005
Crystal Structure of an Active Group I Ribozyme-Product Complex
Group I introns are catalytic RNAs capable of orchestrating two sequential phosphotransesterification reactions and several structural rearrangements that result in self-splicing. Group I introns are metalloenzymes and a cluster of three catalytic metals at the active site has been described (1). To understand how the group I intron active site facilitates catalysis, we have solved the structure of an active ribozyme derived from the orf142-I2 intron from Staphyloccocus aureus phage Twort at 3.6 Å resolution bound to a four nucleotide product RNA (2).
Group I introns fold into three domain-like structures, each consisting of collection of co-axially stacked helices. In addition to the three conserved domains characteristic of all group I introns, the Twort ribozyme possesses peripheral insertions characteristic of phage T4 introns. These elements form a ring that completely envelops the active site. Although these insertions are not part of the catalytic core of the ribozyme, many introns have evolved similar strategies to aid in the folding process or to stabilize the folded structure of the RNA.
The base of the guanosine nucleotide substrate (ωG) docks in the major groove of the active site, and lies in the center of a series of stacked base triples, a recurring theme in RNA-base recognition (3-6). The ribose of the guanosine substrate is recognized by functional groups in the backbone of the RNA. These contacts include a ribose zipper interaction and coordination to at least one of the catalytic metals. These interactions likely mediate interplay between the ribozyme active site, the guanosine substrate and the catalytic metals and, thus, are critical for catalysis.
References and Footnotes
Barbara L. Golden
Department of Biochemistry