FIGURE 8-17 Proposed mechanism of LINE reverse transcription and integration. Only ORF2 protein is represented here. Newly synthesized LINE DNA is shown in black. ORF1 and ORF2 proteins, produced by translation of LINE RNA in the cytoplasm, bind to LINE RNA and transport it into the nucleus. Step 1: In the nucleus, ORF2 makes staggered cuts in AT-rich target-site DNA, generating the DNA 3′-OH ends indicated by blue arrowheads. Step 2: The 3′ end of the T-rich DNA strand hybridizes to the poly(A) tail of the LINE RNA and primes DNA synthesis by ORF2. Step 3: ORF2 extends the DNA strand using the LINE RNA as a template. Steps 4 and 5: When synthesis of the LINE DNA bottom strand reaches the 5′ end of the LINE RNA template, ORF2 extends the newly synthesized LINE DNA using as a template the top-strand cellular DNA generated by the initial ORF2 staggered cleavage. Step 6: A cellular DNA polymerase extends the 3′ end of the top strand generated by the initial ORF2 staggered cut, using the newly synthesized bottom-strand LINE DNA as a template. The LINE RNA is digested as the DNA polymerase extends the upper-strand DNA, just as occurs during removal of lagging-strand primer RNA during cellular DNA synthesis (see Figure 5-29). The 3′ ends of the newly synthesized DNA strands are ligated to the 5′ ends of the cellular DNA strands as in lagging-strand cellular DNA synthesis. See D. D. Luan et al., 1993, Cell 72:595.