Models of group I and group II introns and their 'homing' mechanisms. (a,b) Schematic representations of (a) group I and (b) group II intron secondary structures [13,37]. In both cases, secondary structures are represented by solid lines indicating conserved stem-loop structures, named P1 to P10 for group I introns, and DI to DVI for group II introns. The positions of ORFs and other insertions are depicted by solid red lines. The asterisk (*) next to domain II of group II introns indicates bioinformatic predictions of the ORF start sites, but these remain uncharacterized. Dashed gray lines indicate joining regions of unpaired nucelotides, with arrows indicating a 5'-3' orientation. The 5' and 3' exons are indicated by grey rectangles. (c) Homing of a group I intron. In this DNA-based mobility pathway, the intron donor (D) expresses the intron endonuclease (red enzyme symbol) (step 1). After cleavage of the allelic intron recipient sequence (R) at the homing site (step 2) the donor and recipient engage in double-strand break (DSB) repair to generate two intron-containing alleles. (d) Group II intron retrohoming by means of an RNA intermediate. The intron donor (D) in this case is the spliced intron lariat RNA (dashed red line), whereas the recipient (R) can be either double-stranded DNA (dsDNA) or single-stranded DNA (ssDNA), as at a replication fork. A ribonucleoprotein complex between the RNA and the IEP catalyzes a reverse splicing (step 1). In the dsDNA pathway the IEP then cleaves the second strand to generate the primer for cDNA synthesis by the IEP, whereas in the ssDNA pathway an Okazaki fragment at the replication fork (solid gray line) acts as a primer (step 2). Second-strand cDNA synthesis followed by repair completes the retromobility reactions (step 3).
Edgell et al. BMC Biology 2011 9:22 doi:10.1186/1741-7007-9-22