FIG. 1.

R2 elements and the TPRT mechanism. (A) Diagram of the R2 TPRT mechanism. RNA corresponding to the 3′-UTR RNA of the R2 element and the R2 protein form a specific RNP complex. The RNP complex binds the target DNA and cleaves the first (bottom) strand. The 3′ DNA end generated by this nick is used to prime first-strand cDNA. Second (top)-strand cleavage occurs after reverse transcription. (B) R2 elements insert into a specific site within the 28S rRNA gene. R2 elements encode a 120-kDa protein containing an N-terminal domain with cysteine-histidine zinc fingers and c-Myb DNA binding motifs, a central reverse transcriptase domain (RT), and a C-terminal domain with a restriction-like-endonuclease domain and putative nucleic acid binding motif(s). Indicated below the R2 diagram is the 3′-UTR sequence used as R2 RNA in this study and the N-terminal peptide used for Fig. 3B and C.