Info

Position of Ribosome

Secondary

Termination of

Level of

Ribosome Stalls When Region 3

Structure

Transcription

Tryptophan

at Trp Codons

Is Transcribed

of 5' UTR

of trp Operon

High

No

Covers region 2

3 + 4 hairpin

Yes

Low

Yes

Covers region 1

2 + 3 hairpin

No

their availability is what determines whether the ribosome stalls at the tryptophan codons. A second point concerns the synchronization of transcription and translation, which is critical to attenuation. Synchronization is achieved through a pause site located in region 1 of the 5' UTR. After initiating transcription, RNA polymerase stops temporarily at this site, which allows time for a ribosome to bind to the 5' end of the mRNA so that translation can closely follow transcription. A third point is that ribosomes do not traverse the convoluted hairpins of the 5' UTR to translate the structural genes. Ribosomes that attach to the ribosome-binding site at the 5' end of the mRNA encounter a stop codon at the end of region 1. Ribosomes translating the structural genes attach to a different ribosome-binding site located near the beginning of the trpE gene.

Why does attenuation occur? Why do bacteria need attenuation in the trp operon? Shouldn't repression at the operator site prevent transcription from taking place when tryptophan levels in the cell are high? Why does the cell have two types of control? Part of the answer is that repression is never complete; some transcription is initiated even when the trp repressor is active; repression reduces transcription only as much as 70-fold. Attenuation can further reduce transcription another 8- to 10-fold; so together the two processes are capable of reducing transcription of the trp operon more than 600-fold. Both mechanisms provide E. coli with a much finer degree of control over tryptophan synthesis than either could achieve alone.

Another reason for the dual control is that attenuation and repression respond to different signals: repression responds to the cellular levels of tryptophan, whereas attenuation responds to the number of tRNAs charged with trypto-phan. There may be times when it is advantageous for the cell to be able to respond to these different signals. Finally, the trp repressor affects several operons other than the trp operon. It's possible that at an earlier stage in the evolution of E. coli, the trp operon was controlled only by attenuation. The trp repressor may have evolved primarily to control the other operons and only incidentally affects the trp operon.

Attenuation is a complex process to grasp because you must simultaneously visualize how two dynamic processes— transcription and translation—interact, and it's easy to get the two processes confused. Remember that attenuation entails the early termination of transcription, not translation (although events in translation bring about the termination of transcription). Attenuation often causes confusion because we know that transcription must precede translation. We're comfortable with the idea that transcription might affect translation, but it's harder to imagine that the effects of translation could influence transcription, as it does in attenuation. The reality is that transcription and translation are closely coupled in prokaryotic cells, and events in one process can easily affect the other.

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