Practice question

Formation of terminator hairpin three-four results in transcription attenuation mediated by intrinsic termination mechanism. Stem region contains approximately eight consecutive GC base pairs providing strong thermodynamic stability plus tetraloop capping, while downstream element includes stretch of six to eight uridine residues forming weak riboU-deoxyA hybrid inside transcription bubble facilitating polymerase pausing and release factor independent dissociation. When three-four structure forms under high tryptophan conditions, polymerase terminates elongation producing short 140-nucleotide leader RNA without coding sequences for biosynthetic enzymes. Alternative antiterminator hairpin two-three forming under low tryptophan sequesters nucleotides preventing three-four formation, allowing continued readthrough. Hence three-four dictates aborted transcription outcome.