pob5e_solutions_ch28 - 2608T_ch28sm_S310-S318 02/26/2008...

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Regulation of Gene Expression S-310 1. Effect of mRNA and Protein Stability on Regulation E. coli cells are growing in a medium with glucose as the sole carbon source. Tryptophan is suddenly added. The cells continue to grow, and divide every 30 min. Describe (qualitatively) how the amount of tryptophan synthase activity in the cells changes under the following conditions: (a) The trp mRNA is stable (degraded slowly over many hours). (b) The trp mRNA is degraded rapidly, but tryptophan synthase is stable. (c) The trp mRNA and tryptophan synthase are both degraded rapidly. Answer The mRNA from the trpEDCBA operon encodes several enzymes for tryptophan biosynthesis; the trpB and trpA genes encode tryptophan synthase. The complete trp mRNA is synthesized only when the concentration of tryptophan (actually, that of charged Trp-tRNA) is low. This is the result of repression and attenuation (see Problem 6). There is no strong regulation at translation, so when trp mRNA is present, tryptophan synthase is produced. Although the regulation of tryptophan biosynthesis, like most biosynthetic pathways, is fine-tuned by feedback control, feedback inhibition by tryptophan is exerted at the branch point anthranilate synthase (the product of the trpE and trpD genes; see Fig. 28–19), so the activity of tryptophan synthase is not strongly affected by [tryptophan]. (a) If the trp mRNA is stable relative to cell generation time, it persists in the population of bacteria even after tryptophan has been added, and tryptophan synthase continues to be synthesized and active. In a simple model, we would expect to see the enzyme activity per cell roughly halved for each generation (30 min); that is, the activity would be slowly diluted out by the increasing numbers of cells. (b) Again, if the enzyme is stable relative to the generation time, it persists in the population, even after the addition of tryptophan, and remains active. (c) If the mRNA and enzyme are unstable (degraded rapidly relative to cell generation time), attenuation of transcription of the trp operon caused by addition of tryptophan leads to an abrupt decrease in levels of trp mRNA and tryptophan synthase. 2. Negative Regulation Describe the probable effects on gene expression in the lac operon of a mutation in (a) the lac operator that deletes most of O 1 ; (b) the lacI gene that inactivates the repressor; and (c) the promoter that alters the region around position ± 10. Answer The lac operon is negatively regulated by a repressor, the product of the lacI gene. The Lac repressor binds to specific DNA sequences called the operators (O 1 and pseudo- operators O 2 and O 3 ). Binding of the repressor prevents efficient initiation of transcription by RNA polymerase from the promoter. An inducer (allolactose or an analog) binds to the repres- sor and prevents its binding to the operator, thereby relieving the repression and allowing transcription of the lac operon.
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This note was uploaded on 09/24/2011 for the course CHEM 369 taught by Professor Wang during the Spring '11 term at University of Houston.

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pob5e_solutions_ch28 - 2608T_ch28sm_S310-S318 02/26/2008...

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