From: Science 284, 598-599 (1999). During transcription of DNA (black) by RNA polymerase (RNAP; green), a complex is formed between RNAP, DNA, and the elongating RNA chain (uracil-rich sequence, yellow; potential hairpin, red). When RNAP encounters a pause, arrest, or termination signal in the hesitant state, an unstable RNA:DNA hybrid pauses the enzyme long enough to allow rearrangement of the transcription complex either by formation of a hairpin in the mRNA or by backtracking of the RNAP. Whether the initial pause is caused by backtracking of RNAP by one or more nucleotides or by displacement of the mRNA's 3' end, it dissociates the transcription complex through either the collapse or pull-out pathways. However, when switched into transcription overdrive by binding of an antiterminator (AT, purple), RNAP ignores pause, arrest, or termination signals. AT may prevent hairpin formation or stablilize the RNA:DNA hybrid (pink areas within AT) either directly or by inducing a conformational change in RNAP. NTP, nucleoside triphosphate.