If tryptophan is scarce,the ribosome pauses when attempting to translate the two trp codons in the leader sequence,which leaves the leader sequence available to form the anti-terminator stem-loop.

• 色氨酸存在时，前导序列的翻译可以完成。此时核糖体的位置可以允许形成终止子茎环，而阻止形成抗终止子茎环。