Figure 3

Variations in group II intron forms. RNA domains are depicted as stem-loops in red, ORF domains in blue or tan, and exons in green. The right column indicates whether the variants are found in bacteria (B), mitochondria (M), or chloroplasts (C). (A) Full-length retroelement form with standard RNA and IEP domains. Example: the IIA intron Ll.LtrB of Lactococcus lactis. ORF, open reading frame; RT, reverse transcriptase. (B) Intron lacking the endonuclease domain (found in all introns of classes C, D, E, and F and some of class CL). Example: the IIC intron B.h.I1. (C) Intron in which the IEP has lost RT motifs while maintaining the domain X/thumb domain required for maturase function. Example: the chloroplast IIA intron trnKI1, which encodes the ORF MatK. IEP, intron-encoded protein. (D) Intron encoding a LAGLIDADG homing endonuclease. Example: Grifola frondosa SSUI1 rRNA intron (fungi). (E) ORF-less, self-splicing intron. Example: S. cerevisiae aI5g. (F) ORF-less intron with a degenerated RNA sequence. Example: tobacco petDI1. (G) Group III intron. Example: Euglena gracilis rps11 (H) Trans-splicing group II introns. Examples: tobacco nad1I1 (bipartite) and Chlamydomonas psaAI1 (tripartite). (I) Altered 5′ splice site. Example: Grifola frondosa SSUI1 rRNA intron. (J) Altered 3′ splice site. Example: Bacillus cereus B.c.I4. (K) Alternatively splicing group II intron. Example: Clostridium tetani C.te.I1. (L) Twintron. Example: Euglena gracilis rps3.