Termination of transcription. (a) In ρ-independent termination, an mRNA sequence forms a hairpin, which is followed by three U residues, stalling the polymerase and separating it from the mRNA. (b) RNAs that include a rut site (purple) recruit the ρ helicase, which migrates in the 5′→3′ direction along the mRNA and separates it from the polymerase.