GROUP-II INTRON ENDONUCLEASES USE BOTH RNA AND PROTEIN SUBUNITS FOR RECOGNITION OF SPECIFIC SEQUENCES IN DOUBLE-STRANDED DNA

Group II introns use intron-encoded reverse transcriptase, maturase an d DNA endonuclease activities for site-specific insertion into DNA, Re markably, the endonucleases are ribonucleoprotein complexes in which t he excised intron RNA cleaves the sense strand of the recipient DNA by reverse splicing, while the intron-encoded protein cleaves the antise nse strand. Here, studies with the yeast group II intron aI2 indicate that both the RNA and protein components of the endonuclease contribut e to recognition of an similar to 30 bp DNA target site. Our results l ead to a model in which the protein component first recognizes specifi c nucleotides in the most distal 5' exon region of the DNA target site (E2-21 to -11). Binding of the protein then leads to DNA unwinding, e nabling the intron RNA to base pair to a 13 nucleotide DNA sequence (E 2-12 to E3+1) for reverse splicing, Antisense-strand cleavage requires additional interactions of the protein with the 3' exon DNA (E3+1 to +10). Our results show how enzymes can use RNA and protein subunits co operatively to recognize specific sequences in double-stranded DNA.