Figure 7.

Model showing the effect of DNA damage on transcription elongation. (A) A stable triplex structure is difficult to melt during elongation by RNA pol II and destabilizes the elongation complex. This leads to termination 4–8 nucleotides before the elongation complex encounters triplex. (B) Psoralen monoadduct covalently staples the third strand to the double helix without significantly destabilizing the triplex structure. Elongating RNA pol II cannot unwind the triplex, therefore, it leaves the template 4–8 nucleotides before reaching the damage site. (C) An interstrand psoralen crosslink contracts the major groove and disrupts the triplex structure. RNA pol II elongation complex can unwind this DNA template and transcribes RNA until it reaches psoralen crosslink site, at that point RNA pol II gets arrested and waits for DNA damage repair.