Question

Explain how the translation of an open reading frame in TrpL mRNA in the presence or absence of tryptophan affects the formation of a terminator stem-loop.

Answer 1

The leader sequence encodes a short polypeptide and also contains an attenuator sequence. The attenuator does not encode a polypeptide, but when transcribed into mRNA, it has self-complementary sections and can form various hairpin structures.

The polypeptide encoded by the leader is short, just 14 amino acids long, and it includes two tryptophan (Trp) residues. The tryptophans are important because:

• If there is plenty of tryptophan, the ribosome won't have to wait long for a tryptophan-carrying tRNA, and will rapidly finish the leader polypeptide.

• If there is little tryptophan, the ribosome will stall at the Trp codons (waiting for a Trp-carrying tRNA) and will be slow to finish the translation of the leader.

• If the ribosome translates slowly, it will pause, and its pausing causes formation of the antiterminator (non-terminating hairpin). This hairpin prevents the formation of the terminator and allows transcription to continue.

• If the ribosome translates quickly, it will fall off the mRNA after translating the leader peptide. This allows the terminator hairpin and an associated hairpin to form, making RNA polymerase detach and ending transcription.