Structures of phage transcriptional antitermination complexes define novel motifs for recognition of RNA hairpins by arginine-rich peptides. A bent α-helix in each case follows the contour of an induced GNRA-like fold. A phage-specific pattern of base pairing, base stacking and base flipping underlies biological specificity and permits engagement with RNA polymerase. The structures suggest a mechanism of RNA-mediated signaling in transcriptional regulation.
