Here, Erickson et al. studied whether paused RNA polymerase II (Pol II) that piles up near most human promoters is a stable or dynamic target of mechanisms that control entry into productive elongation. They demonstrate that most 5′ paused Pol II throughout the genome is turned over within 2 min and propose that Pol II occupancy near 5′ ends is governed by a cycle of ongoing assembly of preinitiated complexes that transition to pause sites followed by eviction from the DNA template.