Genetics: Analysis and Principles 5th Edition by Robert Brooker

Exercise 11

The 3-4 stem-loop and U-rich attenuator found in the trp operon (see Figure 14.12) is an example of -independent termination. The function of -independent terminators is described in Chapter 12. Would you expect attenuation to occur if the tryptophan levels were high and mutations changed the attenuator sequence from UUUUUUUU to UGGUUGUC Explain why or why not.FIGURE 14.12 Sequence of the trpL mRNA produced during attenuation. A second method of regulation of the trp operon is attenuation, which occurs when tryptophan levels are sufficient for protein synthesis. A short mRNA is made that includes the trpL coding sequence. As shown here, this mRNA has several regions that are complementary to each other. The possible hydrogen bonding between regions 1 and 2, 2 and 3, and 3 and 4 is also shown, but each region can hydrogen bond to only one other region at a given time. For example, if region 2 is hydrogen bonded to region 1, it cannot simultanteously hydrogen bond with region 3. The last U in the purple attenuator sequence is the last nucleotide that would be transcribed during attenuation. At very low tryptophan concentrations, however, transcription occurs beyond the end of trpL and proceeds through the trpE gene and the rest of the trp operon.