Group I catalytic introns
are large self-splicing ribozymes
. They catalyze their own excision from mRNA
precursors in a wide range of organisms. The core secondary structure
consists of nine paired regions (P1-P9). These fold to essentially two domains
- the P4-P6 domain (formed from the stacking of P5, P4, P6 and P6a helices) and the P3-P9 domain (formed from the P8, P3, P7 and P9 helices). The secondary structure mark-up for this family represents only this conserved core. Group I catalytic introns
often have long open reading frames
inserted in loop regions
of group I introns
is processed by two sequential ester-transfer
reactions. The exogenous guanosine
or guanosine nucleotide
) first docks onto the active G-binding site located in P7, and its 3'-OH is aligned to attack the phosphorester bond
at the 5' splice site located in P1, resulting in a free 3'-OH group at the upstream exon
and the exoG being attached to the 5' end of the intron. Then the terminal G (omega G) of the intron swaps the exoG and occupies the G-binding site to organize the second ester-transfer reaction, the 3'-OH group of the upstream exon in P1 is aligned to attacks the 3' splice site in P10, leading to the ligation
of the adjacent upstream and downstream exons and free of the catalytic intron.
Two-metal-ion mechanism seen in protein polymerases and phosphatases was proposed to be used by group I and group II introns to process the phosphoryl transfer reactions, which was unambiguously proven by a recently resolved high-resolution structure of the Azoarcus group I intron.
Since early 1990s, scientists started to study how the group I intron achieves its native structure in vitro, and some mechanisms of RNA folding
have been appreciated thus far. It is agreed that the tertiary structure is folded after the formation of the secondary structure. During folding, RNA molecules are rapidly populated into different folding intermediates, the intermediates containing native interactions are further folded into the native structure through a fast folding pathway, while those containing non-native interactions are trapped in metastable or stable non-native conformations, and the process of conversion to the native structure occurs very slowly. It is evident that group I introns differing in the set of peripheral elements display different potentials in entering the fast folding pathway. Meanwhile, cooperative assembly of the tertiary structure is important for the folding of the native structure. Nevertheless, folding of group I introns in vitro encounters both thermodynamic and kinetic challenges. A few RNA binding proteins and chaperones have been shown to promote the folding of group I introns in vitro and in bacteria by stabilizing the native intermediates, and by destabilizing the non-native structures, respectively.
Distribution, Phylogeny and Mobility
Group I introns
are distributed in bacteria
, lower eukaryotes
. However, their occurrence in bacteria
seems to be more sporadic than in lower
, and they have become prevalent in higher plants
. The genes that group I
introns interrupt differ significantly: They interrupt rRNA
genes in bacterial
genomes, as well as in mitochondrial
genomes of lower eukaryotes
, but only invade rRNA
genes in the nuclear genome of
. In higher plants
, these introns seem to be restricted to a few
genes of the chloroplasts
. Both intron-early
and intron-late theories have found evidences in explaining the origin of group I introns.
Some group I introns encode homing
(HEG), which catalyzes intron mobility. It is proposed that HEGs move the
intron from one location to another, from one organism to another and thus account for the
wide spreading of the selfish group I introns. No biological role has been
identified for group I introns thus far except for splicing of themselves from the precursor
to prevent the death of the host that they live by. A small number of group I introns are
also found to encode a class of proteins called maturases that facilitate the intron
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