When referring to attenuation in regulation of the trp operon, it would be safe to say that when there are high levels of tryptophan available to the organism, ________.
A) translational termination is likely
B) tryptophan is inactivating the repressor protein
C) ribosomes are stalling during translation of the attenuator region
D) the trp operon is being transcribed at relatively high levels
E) transcriptional termination is likely

When tryptophan is present in the medium, the organism does not make the enzymes for tryptophan synthesis and the expression of tryptophan operon is blocked. The tryptophan operon attenuation includes four sequences of the leader region at the 5' end of the mRNA. The leader sequence precedes the initiation codon of the first gene. The sequences 3 and 4 of the leader sequence serve as the attenuator as they base-pair to form a stem-and-loop structure which is followed by a series of U residues (genetic code for tryptophan is "UUU").

When sequences 2 and 3 base-pair, the attenuator structure cannot form and transcription of the operon continues. When tryptophan levels are high, the ribosome quickly translates sequence 1 and thereby blocks sequence 2 before sequence 3 is transcribed. The continued transcription of sequences 3 and 4 followed by the terminator-like attenuator structure blocks the transcription of the operon.