In attenuation occurring at the trp operon, the ribosome will pause in the leader sequence before the stop codon is reached when

a. tryptophan levels are low.
b. tryptophan levels are high.
c. charged transfer RNAs bound with tryptophan are plentiful
d. The ribosome never pauses in the leader sequence.

a. tryptophan levels are low.

Explanation:

Attenuation occurs when the tryptophan level in the cell is high so there is no need to synthesize tryptophan any more. In this situation, tryptophan mRNA synthesis comes to an end.

Attenuation is based on two following attributes:

(1) Translation dependent transcription

(2) Ability of RNA to assume alternative secondary structures.

Leader peptide is 16o nucleotide long region which contains 4 regions named as R1, R2, R3 and R4. In R1 region there are 2 tryptophan codons while after R4 region there is a polyU sequence. It it only after leader peptide, tryptophan biosynthesis genes are present in tryptophan operon.

When tryptophan levels in the cell are high, ribosome at R1 region immediately clears the region without any halt as a result of which R1 is able to pair up with R2 and R3 is able to pair up with R4 making a secondary structure i.e. hair pin structure. Soon after that there is a polyU sequence. This is a perfect condition for rho independent transcription termination.

This phenomenon helps in conserving the energy by inhibiting transcription in cell when tryptophan is not required because it is already present in abundance.

In contrast to this, when tryptophan levels are low, transcript is supposed to be transcribed so as to enhance the level of tryptophan in the cell. In such situation, the tRNA charged with tryptophan amino acid is in scarcity so the ribosome keeps waiting for this charged tRNA in R1. Since ribosome has occupied this region, R1 will be unable to pair up with R2. So R2 will pair up with R3. Since R3 has paired up with R2, it will not be able to pair up with R4. So a secondary hair pin structure immediately followed up by polyU sequence will not form and hence transcription by RNA polymerase will go on.

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