INTRON-ENCODED ENDONUCLEASE I-TEVII BINDS ACROSS THE MINOR-GROOVE ANDINDUCES 2 DISTINCT CONFORMATIONAL-CHANGES IN ITS DNA SUBSTRATE

I-TevII is the homing endonuclease encoded by the sun Y intron of bact eriophage T4. The enzyme cleaves an intronless sun Y gene near the exo n I-exon II junction, thereby initiating intron homing into its cognat e intronless allele. Specifically, I-TevII cleaves its DNA target 13 t o 15 nucleotides (nt) downstream of the sun Y intron insertion site, g enerating 2-nt 3'-OH extensions. Here, we present evidence that I-TeVI I makes predominantly minor groove contacts in two regions of its reco gnition sequence, as does I-TeVI, the other homing endonuclease encode d by phage T4. Following cleavage, I-TeVII was shown to remain bound t o one of its DNA products, suggesting possible additional roles for th e endonuclease in the mobility process. Interestingly, two distinct co nformational changes were detected by gel analysis in the DNA substrat e following binding by I-TeVII, one occurring in the absence of Mg2+, the second being dependent on the presence of Mg2+. The Mg2+-induced d istortion accompanies a nick in one strand, and may serve to bring the cleavage site on the ether strand into proximity with the catalytic d omain of the protein. (C) 1996 Academic Press Limited