Group II intron

Group II intron is a class of intron found in rRNA, tRNA, mRNA of organelles in fungi, plants, protists, and mRNA in bacteria. Self-splicing occurs in vitro (for a few of the introns studied to date), but protein machinery is probably required in vivo. In contrast to group I introns, intron excision occurs in the absence of GTP and involves the formation of a lariat, with a branchpoint strongly resembling that found in lariats formed during splicing of nuclear pre-mRNA. Although the resemblance to pre-mRNA splicing is strong, group II introns are not true catalysts as the entire intron is degraded once self-splicing has occurred. It is thought that pre-mRNA splicing may have evolved from group II introns.

tructure and catalytic site

The secondary structure of group II introns is characterized by six typical stem-loop structures, also called domains I to VI or D1 to D6. The domains radiate from a central core that brings the 5' and 3' splice junctions into close proximity. The proximal helix structures of the six domains are connected by a few nucleotides in the central region (linker or joiner sequences). Due to its enormous size, the domain 1 was divided further into subdomains a, b, c, and d. Sequence differences of group II introns were identified which led to a further division into subgroups IIA and IIB.

Group II introns possess only a very few conserved nucleotides, and the nucleotides important for the catalytic function are spread over the complete intron structure. The few strictly conserved primary sequences are the consensus at the 5' and 3' splicing site (...↓GUGYG&... and ...AY↓...), some of the nucleotides of the central core (joiner sequences), a relatively high number of nucleotides of D5 and some short sequence stretches of D1. The unpaired adenosine in D6 marked by an asterisk (7 or 8 nt away from the 3' splicing site, respectively) is also conserved and plays a central role in the splicing process.

In 2005, AD. Lencastre et al. found that during splicing of Group II introns, all reactants are preorganized before the initiation of splicing. The branch site, both exons, the catalytically essential regions of D5 and J2/3, and epsilon−epsilon' are in close proximity before the first step of splicing occurs. In addition to the bulge and AGC triad regions of D5, the J2/3 linker region, the epsilon−epsilon' nucleotides and the coordination loop in D1 are crucial for the architecture and function of the active-site.

Group II catalytic intron

Group II catalytic introns are found in rRNA, tRNA and mRNA of organelles in fungi, plants and protists, and also in mRNA in bacteria. They are large self-splicing ribozymes and have 6 structural domains (usually designated dI to dVI). This model and alignment represents only domains V and VI. A subset of group II introns also encode essential splicing proteins in intronic ORFs. The length of these introns can therefore be up to 3kb. Splicing occurs in almost identical fashion to nuclear pre-mRNA splicing with two transesterification steps. The 2' hydroxyl of a bulged adenosine in domain VI attacks the 5' splice site, followed by nucleophilic attack on the 3' splice site by the 3' OH of the upstream exon. Protein machinery is required for splicing "in vivo", and long range intron-intron and intron-exon interactions are important for splice site positioning. Group II introns are further sub-classified into groups IIA and IIB which differ in splice site consensus, distance of bulged

ee also

*Intron
*Splice site
*Nuclear introns
*Group I intron
*Group III intron
*Twintron
*"LtrA"
*"LtrB"

References

*AD. Lencastre et al., "Nature Structural & Molecular Biology" 12, 626 - 627 (2005)
*Karola Lehmann and Udo Schmidt, "Critical Reviews in Biochemistry and Molecular Biology", Vol 38, Issue 3, 249-303 (2003)
*cite journal | last = Bonen | first = L | coauthors = Vogel J | year = 2001 | title = The ins and outs of group II introns | journal = Trends Genet | volume = 17 | pages = 322–331 | pmid = 11377794 | doi = 10.1016/S0168-9525(01)02324-1
*cite journal | last = Chu | first = VT | coauthors = Adamidi C, Liu Q, Perlman PS, Pyle AM | year = 2001 | title = Control of branch-site choice by a group II intron | journal = EMBO J | volume = 20 | pages = 6866–6876 | pmid = 11726522 | doi = 10.1093/emboj/20.23.6866
*cite journal | last = Lehmann | first = K | coauthors = Schmidt U | year = 2003 | title = Group II introns: structure and catalytic versatility of large natural ribozymes | journal = Crit Rev Biochem Mol Biol | volume = 38 | pages = 249–303 | pmid = 12870716 | doi = 10.1080/713609236

External links

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