Low tryptophan level favors pairing between
Low intracellular tryptophan causes shortage of charged Trp-tRNA, ribosome encountering tandem UGG codons within region one of leader transcript stalls due to lack of cognate aminoacyl tRNA. Stalled ribosome physically masks region one, preventing formation of one-two hairpin structure, freeing region two unpaired to base pair with region three forming two-three antiterminator hairpin. Antiterminator structure sequesters nucleotides that would otherwise participate in three-four terminator hairpin, so terminator cannot fold. Consequently RNA polymerase reads through attenuator into downstream
Ref: CCBC Biology Lecture – low tryptophan ribosome stalls at Trp codons allowing 2-3 antiterminator formation and readthrough.