Practice question

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 trpEDCBA structural genes, producing full polycistronic messenger providing additional enzymes to synthesize amino acid during starvation and restoring pools.