Fig. S1.

Origin of +12 paused and abortive RNAs. The +12 RNA species from λpR′ is known to be not only abortive (i.e., released), when it is made ab initio from the open complex, but also to be a stable TEC component when it is derived through Gre-mediated (or endogenous) cleavage from longer transcripts (23). The abortive precursor of released +12 RNA is scrunched from the promoter and presumably paused due to an EPS, with its 3′ end in the i + 1 site; the cleavage-derived +12 RNA originates from a σ⁷⁰-dependent paused complex in which the scrunch is relaxed, and its 3′ end would be in the i site because cleavage was catalyzed by the active site.