Group I introns are large self-splicing
ribozymes. They
catalyze their own excision from
mRNA,
tRNA and
rRNA 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
introns often have long
open reading frames inserted in
loop regions.