Attenuation The Premature Termination of Transcription

We've now seen how both positive and negative control regulate the initiation of transcription in an operon. Some operons have an additional level of control that affects the continuation of transcription rather than its initiation. In attenuation, transcription begins at the start site, but termination takes place prematurely, before the RNA poly-merase even reaches the structural genes. Attenuation occurs in a number of operons that code for enzymes participating in the biosynthesis of amino acids.

We can understand the process of attenuation most easily by looking at one of the best-studied examples, which is found in the trp operon of E. coli. Several observations by Charles Yanofsky and his colleagues in the early 1970s indicated that repression at the operator site is not the only method of regulation in the trp operon. They isolated a series of mutants that possessed deletions in the transcribed region of the operon. Some of these mutants exhibited increased levels of transcription, yet control at the operator site was unaffected. Furthermore, they observed that two mRNAs of different sizes were transcribed from the trp operon: a long mRNA containing sequences for the structural genes and a much shorter mRNA of only 140 nucleotides. These observations led Yanofsky to propose that another mechanism—one that caused premature termination of transcription—also regulates transcription in the trp operon.

Close examination of the trp operon reveals a region of 162 nucleotides that corresponds to the long 5' UTR of the mRNA (mentioned earlier) transcribed from the trp operon (iFigure 16.15a). The 5' UTR (also called a leader) contains four regions: region 1 is complementary to region 2, region 2 is complementary to region 3, and region 3 is complementary to region 4. These complementarities allow the 5' UTR to fold into two different secondary structures (< Figure 16.15b). Which secondary structure is assumed determines whether attenuation will occur.

(a) Trp operon

Ribosome binding site

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