Attenuation of trp operon

Bacteria such as E.coli need amino acid to survive because, like us, they need to build protein. One of the amino acid they need is tryptophan. If tryptophan is available in the environment, E.coli will take it up and use it to build protein. However, E.coli can also make their own tryptophan using enzymes that are encoded by five genes. These five genes are located next to each other in what is called the trp operon.
Like regulation by trp repressors, attenuation is a process for reducing the expression of trp operon when levels of tryptophan is high. However, rather than blocking initiation of transcription, attenuation prevents completion of transcription.
When levels of tryptophan are high, attenuation causes RNA Polymerase to stop prematurely when it is transcribing the trp operon. Only a short, stubby mRNA is made, one that does not encode any tryptophan biosynthesis enzymes. Attenuation work through a mechanism that depends on coupling.
The structure of trp operon consists of operator and first gene of the operon is called leader. The leader encodes a shorter polypeptide and also contain an attenuator sequence . The attenuator does not encode a polypeptide, but when transcribed into mRNA, it has self-complementary sequences that can form various hairpin structures.
Once RNA Pol has started transcribing the operon, a ribosome can attack to the still-forming transcript and begin translating the leader region. The polypeptide encoded by leader is short, just 14 amino acids long and it included two tryptophan residues.

• 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 to at trp codons (waiting for a tryptophan carrying tRNA) and will be slow to finish the translation of the leader.

As we see earlier, the leader is followed by an attenuator region, which (in its mRNA form) can stick to itself to form  different hairpin structures. One structure include a transcription terminal signal, while the other does not end termination.

• If the ribosome translates slowly, it will pause and its pausing causes formation of anti-terminator (non-terminating hairpin). This hairpin prevent formation of terminator and allow transcription to continue.
• If the ribosome translate quickly, it will fall off the mRNA after translating the leader peptide. This allow the terminator hairpin and associated hairpin to form, makin RNA Pol detached and ending transcription. 

This mechanism may be complex, but the result is pretty straightforward. When tryptophan is abundant, the ribosome moves quickly along the leader, the terminator hairpin form and transcription of trp operon end. When tryptophan is scare, the ribosome moves slowly along the leader, the non-terminator hairpin form and transcription of the trp operon continues.

In other words, the logic of attenuation is the same as that of regulation by the trp repressor. In both cases, high level of tryptophan in cell shut down the expression of operon. This makes sense, since high level of tryptophan means cell does not need to make more biosynthetic enzymes to produce additional tryptophan.