Chapter 16

In this chapter, we focused on the regulation of gene expression in prokaryotes. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How do we know that bacteria regulate the expression of certain genes in response to the environment? (b) What evidence established that lactose serves as the inducer of a gene whose product is related to lactose metabolism? (c) What led researchers to conclude that a repressor molecule regulates the lac operon? (d) How do we know that the lac repressor is a protein? (e) How do we know that the trp operon is a repressible con- trol system, in contrast to the lac operon, which is an inducible control system?

Contrast the role of the repressor in an inducible system and in a repressible system.

The locations of numerous $\mathrm{lac}I$ and $\mathrm{lac}{I}^{\mathrm{S}}$ mutations have been determined within the DNA sequence of the lacI gene. Among these, $lacI$ mutations were found to occur in the $5^{\prime }$ -upstream region of the gene, while $\mathrm{lac}{I}^{\mathrm{S}}$ mutations were found to occur farther downstream in the gene. Are the locations of the two types of mutations within the gene consistent with what is known about the function of the repressor that is the product of the lacI gene?

Describe the experimental rationale that allowed the lac repressor to be isolated.

What properties demonstrate the lac repressor to be a protein? Describe the evidence that it indeed serves as a repressor within the operon system.

Predict the effect on the inducibility of the $\mathrm{lac}$ operon of a mutation that disrupts the function of (a) the crp gene, which encodes the CAP protein, and (b) the CAP-binding site within the promoter.

Erythritol, a natural sugar abundant in fruits and fermenting foods, is about 65 percent as sweet as table sugar and has about 95 percent fewer calories. It is "tooth friendly" and generally devoid of negative side effects as a human consumable product. Pathogenic Brucella strains that catabolize erythritol contain four closely spaced genes, all involved in erythritol metabolism. One of the four genes $(eryD)$ encodes a product that represses the expression of the other three genes. Erythritol catabolism is stimulated by erythritol. Present a simple regulatory model to account for the regulation of erythritol catabolism in Brucella. Does this system appear to be under inducible or repressible control?

Describe the role of attenuation in the regulation of tryptophan biosynthesis.

Neelaredoxin is a 15 -kDa protein that is a gene product common in anaerobic prokaryotes. It has superoxide-scavenging activity, and it is constitutively expressed. In addition, its expression is not further induced during its exposure to ${\mathrm{O}}_2$ or ${\mathrm{H}}_2{\mathrm{O}}_2$ (Silva, G., et al. $2001.$ J. Bacteriol. $183:4413-4420$ ). What do the terms constitutively expressed and induced mean in terms of neelaredoxin synthesis?

Assume that the structural genes of the lac operon have been fused, through recombinant DNA techniques, to the regulatory apparatus of the ara operon. If arabinose is provided in a minimal medium to $E$. coli carrying this gene fusion, would you expect $\beta$ -galactosidase to be produced at induced levels? Explain.