Final answer:
When the tryptophan level in the cell is low, the ribosome pauses in region 1 of the trp mRNA leader, allowing for transcription to continue due to an antiterminate loop formed between regions 2 and 3 as part of the trp operon's regulatory mechanism.
Step-by-step explanation:
When the tryptophan level in the cell is low, the ribosome pauses in region 1 of the trp mRNA leader, causing region 2 to bind to region 3 and transcription to continue. This regulatory mechanism is part of the trp operon system in bacteria, which controls the synthesis of the amino acid tryptophan. When tryptophan levels are depleted, the synthesis of the short leader peptide stalls and an antiterminator loop is formed between regions 2 and 3, allowing RNA polymerase to transcribe the structural genes of the tryptophan operon. Conversely, when tryptophan levels are high, the tryptophan-repressor complex binds to the operator site, which prevents RNA polymerase from transcribing the operon, thereby inhibiting the synthesis of tryptophan.