12. Gene Regulation in Prokaryotes

The transcription of -galactosidase and permease is inducible in lac⁺ bacteria with a wild-type lac operon. Explain the mechanism by which lactose gains access to the cell to induce transcription of the genes.

The locations of numerous lacI⁻ and lacIˢ mutations have been determined within the DNA sequence of the lacI gene. Among these, lacI⁻ mutations were found to occur in the 5′-upstream region of the gene, while lacIˢ 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 role does cAMP play in transcription of lac operon genes? What role does CAP play in transcription of lac operon genes?

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

How would a cap⁻ mutation that produces an inactive CAP protein affect transcriptional control of the lac operon?

Predict the effect on the inducibility of the lac operon of a mutation that disrupts the function of:

(a) The CRP gene, which encodes the CAP protein

(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?

Consider the transcription of genes of the lac operon under two conditions: (1) when both glucose and lactose are present and (2) when glucose is absent and lactose is present. Describe the comparative levels of transcription of lac operon genes under these conditions, and explain the molecular basis for the difference.

Neelaredoxin is a 15-kDa protein that is a gene product common in anaerobic bacteria. It has superoxide-scavenging activity, and it is constitutively expressed. In addition, its expression is not further induced during its exposure to O₂ or H₂O₂ [Silva, G. et al. (2001). J. Bacteriol. 183:4413 4420]. What do the terms constitutively expressed and induced mean in terms of neelaredoxin synthesis?

The creation of milk products such as cheeses and yogurts is dependent on the conversion by various anaerobic bacteria, including several Lactobacillus species, of lactose to glucose and galactose, ultimately producing lactic acid. These conversions are dependent on both permease and β-galactosidase as part of the lac operon. After selection for rapid fermentation for the production of yogurt, one Lactobacillus subspecies lost its ability to regulate lac operon expression [Lapierre, L., et al. (2002). J. Bacteriol. 184:928–935]. Would you consider it likely that in this subspecies the lac operon is on or off? What genetic events would likely contribute to the loss of regulation as described above?