The base pairing of complementary regions of RNA leader sequence determines if the transcription will be attenuated at the end of leader sequence or will continue for structural genes.
Explanation:
The RNA leader sequence of trp operon is the site where the transcription is initiated.
When tryptophan levels are high, translation of sequence 1 of the leader sequence is followed by base pairing between the complementary 3 and 4 regions of the leader sequence. The base pairing forms a terminator like attenuator and stops the expression of structural genes.
When tryptophan is not present, ribosome pauses at tryptophan codon of region 1 of the leader sequence. Hence, the regions 2 and 3 of the leader sequence base pair and prevents the formation of attenuator by not allowing the region 3 to pair with region 4.
