Book Viewer

Short Contents | Full Contents | Other books @ NCBI

Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription
Problems

1. | Explain why I mutations in the lac system are normally recessive to I+ mutations and why I+ mutations are recessive to Is mutations.

2. | What do we mean when we say that Oc mutations in the lac system are cis-acting? See answer

3.

The genes shown in the table below are from the lac operon system of E. coli. The symbols a, b, and c represent the repressor (I) gene, the operator (O) region, and the structural gene (Z) for b -galactosidase, although not necessarily in that order. Furthermore, the order in which the symbols are written in the genotypes is not necessarily the actual sequence in the lac operon.

a. State which symbol (a, b, or c) represents each of the lac genes I, O, and Z.
b. In the table, a superscript minus sign on a gene symbol merely indicates a mutant, but you know that some mutant behaviors in this system are given special mutant designations. Use the conventional gene symbols for the lac operon to designate each genotype in the table.

(Problem 3 is from J. Kuspira and G. W. Walker, Genetics: Questions and Problems. Copyright © 1973 by McGraw-Hill.)

4.

The map of the lac operon is

The promoter (P) region is the start site of transcription through the binding of the RNA polymerase molecule before actual mRNA production. Mutationally altered promoters (P) apparently cannot bind the RNA polymerase molecule. Certain predictions can be made about the effect of P mutations. Use your predictions and your knowledge of the lactose system to complete the table on the following page. Insert a "+" where an enzyme is produced and a "" where no enzyme is produced.

See answer

5. | In Neurospora, all mutants affecting the enzymes carbamyl phosphate synthetase and aspartate transcarbamylase map at the pyr-3 locus. If you induce pyr-3 mutations by ICR-170 (a chemical mutagen), you find that either both enzyme functions are lacking or only the transcarbamylase function is lacking; in no case is the synthetase activity lacking when the transcarbamylase activity is present. (ICR-170 is assumed to induce frameshifts.) Interpret these results in regard to a possible operon.

6. | Certain lacI mutations eliminate operator binding by the Lac repressor but do not affect the aggregation of subunits to make a tetramer, the active form of the repressor. These mutations are partially dominant to wild type. Can you explain the partially dominant I phenotype of the I / I+ heterodiploids?

7. | Explain the fundamental differences between negative control and positive control. See answer

8. | Mutants that are lacY retain the capacity to synthesize b -galactosidase. However, even though the lacI gene is still intact, b -galactosidase can no longer be induced by adding lactose to the medium. How can you explain this?

9. | What analogies can you draw between transcriptional trans-acting factors that activate gene expression in eukaryotes and prokaryotes? Give an example.

10. | Compare the arrangement of cis-acting sites in the control regions of eukaryotes and prokaryotes. See answer

11. | Explain how models for bacterial operons such as ara relate to eukaryotic trans-acting proteins and their mechanism of action.

12.

One interesting mutation in lacI results in repressors with 100-fold increased binding to both operator and nonoperator DNA. These repressors display a "reverse" induction curve, allowing b -galactosidase synthesis in the absence of an inducer (IPTG) but partly repressing b -galactosidase expression in the presence of IPTG. How can you explain this?

(Note that, when IPTG binds a repressor, it does not completely destroy operator affinity, but rather it reduces affinity 1000-fold. Additionally, as cells divide and new operators are generated by the synthesis of daughter strands, the repressor must find the new operators by searching along the DNA, rapidly binding to and dissociating from nonoperator sequences.)

13. | You are studying a mouse gene that is expressed in the kidneys of male mice. You have already cloned this gene. Now you wish to identify the segments of DNA that control the tissue-specific and sex-specific expression of this gene. Describe an experimental approach that would allow you to do so. See answer

© 1999 by W. H. Freeman and Company. All rights reserved.