Tetrahymena thermophila introns

Group I intron phosphotransesterification reactions are carried out by a conserved active site that contains a set of imperfect double helices named PI through P9. (See Figure 6.4.) P1-P9 helices are organized into three domains P1-P2, P4-P6, and P3-P9. Specifically, the Tetrahymena thermophila intron contains two sets of coaxially stacked helices that overlap to create the active site. These helices reside in two domains of approximately equal size P4-P6 and P3-P9. P domains are defined as base-paired regions, whereas J domains... 

The Cech group described an X-ray crystallographic structure of the group I intron from Tetrahymem thermophila in a Science magazine research article published in 1998 (PDB IGRZ). The 5.0-A resolution crystal structure included 247 nucleotides comprising most of the Tetrahymena thermophila intron. At this resolution, clear density for the ribozyme backbone was seen, and stacked bases were visualized as continuous tubes of electron density. 

Table 2.1. Sequence context of mutation types and their frequencies observed after application of error-prone PCR as described herein and sequencing of cloned genes. Targets T7 RNAP, coding sequence of T7 RNA polymerase, Poll, coding sequence of E. coli DNA polymerase I, Inti on, cDNA of Tetrahymena thermophila intron,...

All four scientists whose work led to modification of the dogmas received the Nobel Prize. Thomas Cech (1987) was the first to observe enzyme-like reactions taking place at the same RNA strand, in ribosomal RNA (rRNA) from the proto-zoon Tetrahymena thermophila. The RNA produced, which is completely viable, is formed in a process in which certain sections (introns) of the primary copy (the transcription of DNA to mRNA) are cut out, the two remaining ends of the exon then being rejoined (spliced). 

Fig. 2. The P4-P6-domain of the group I intron of Tetrahymena thermophila. A Schematic representation of the secondary structure of the whole self-cleaving intron (modified after Cate et al. [34]). The labels for the paired regions P4 to P6 are indicated. The grey shaded region indicate the phylogenetically conserved catalytic core. The portion of the ribozyme that was crystallized is framed. B Three dimensional structure of the P4-P6 domain. Helices of the PSabc extension are packed against helices of the conserved core due to a bend of approximately 150° at one end of the molecule...

ID 1MME3. Ribozymes, or RNA enzymes, catalyze a variety of reactions, primarily in RNA metabolism and protein synthesis The complex three-dimensional structures of these RNAs reflect the complexity inherent in catalysis, as described for protein enzymes in Chapter 6. (c) A segment of mRNA known as an intron, from the ciliated protozoan Tetrahymena thermophila (derived from PDB ID 1GRZ). This intron (a ribozyme) catalyzes its own excision from between exons in an mRNA strand (discussed in Chapter 26). 

Lehnert, V., Jaeger, L., Michele, F., and Westhof, E. (1996). New loop-loop tertiary interactions in self-splicing introns of subgroup IC and ID A complete 3D model of the Tetrahymena thermophila ribozyme. Chem. Biol. 3, 993—1009. 

In the case of an rRNA from Tetrahymena thermophila, a single intron is autocatalytically sphced out in the presence of only GTP and certain cations in a process known as self-splicing. The RNA acts as an enzyme and is called a ribozyme (see Sec. 5.2). 

The sequence shown is for the Tetrahymena thermophila pre-tRNA. Exons and introns are shown in normal and bold letters respectively. The internal guide sequence (boxed) forms base pair with both exons to form a precise alignment structure for RNA splicing. Similar structures can be drawn for most of group I intervening sequences. 

Ribozymes Appeared. Tetrahymena thermophila revealed its group I intron ribozymes in Thomas Czeh s laboratory. Ribozymes catalyze their substrates ( like protein enzymes ) infra- and intermolecularly. The tetrahymena ribozyme s intramolecular catalysis consists of self-splicing (Golden BL et al Howard Hughes Medical Institute, Department of Chemistry and Biochemisby, University of Colorado, Boulder, CO. Science 1998 282 259-264). Alu ribonucleoproteins consists of polymerase Ill-transcribed Alu sequences and signal recognition proteins (SRP9/14) united. In the ribosome, these units inhibit IRES-mediated (internal ribosome enfry site) translation initiation (Ivanova E et al Nucleic Acids Res 2015 43 2874-2887). 

Ban G, Song MS, Lee SW. Cancer cell targeting with mouse TERT-specific group I introns of Tetrahymena thermophila. J Microbiol Biotechnol. 2009 19 1070-6. 

The crystal structure of the 160 nucleotide domain of the P4-P6 intron of Tetrahymena thermophila includes several internal loop regions (5). Of the four internal loops (there is also a bulge), three are asymmetric in the number of nucleotides on the opposing strands. This provides a good comparison to the symmetric internal loops observed in oligomer structures. The four internal loops are shown in Figure 6. 

---