(Solved): To probe the detailed features of the active site of RNA polymerase, an analog of U carrying a rea ...

To probe the detailed features of the active site of RNA polymerase, an analog of U carrying a reactive chemical group (see the figure Part A) was incorporated at one of two positions $$(+20$$ or $$+41$$ ) in a radiolabeled RNA and then moved to specific locations relative to the 3' end of the RNA chain at the RNA polymerase active site (indicated by the numbers in Part B). When the analog was then activated, it reacted either with the adenine it was paired with in the DNA template strand (as shown by the arrow in Part A) or with the RNA polymerase (Part C). The pattern of reactivity with DNA and protein varied depending on the position of the reactive analog relative to the 3' end of the RNA. Why do you suppose that the activated U reacts so strongly with DNA up to a point and then abruptly begins to react more strongly with protein? (Part C from E. Nudler et al., Cell 89:33-41, 1997. With permission from Elsevier.) Choose one: A. The $$U$$ analog loses its reactivity shortly after it becomes incorporated into the RNA chain. B. The $$U$$ analog loses its reactivity when it is located too close to the polymerase active site. C. The RNA transcript pairs with the DNA only after it emerges from the RNA polymerase. D. The RNA transcript pairs with 10 template nucleotides and is single-stranded afterward.