1. (a) Cells will express β-galactosidase, lac permease, and thiogalactoside transacetylase even in the absence of lactose. (b) Cells will express β-galactosidase, lac permease, and thiogalactoside transacetylase even in the absence of lactose. (c) The levels of catabolic enzymes such as β-galactosidase and arabinose isomerase will remain low even at low levels of glucose.

2. The concentration is 1/(6 × 10²³) moles per 10⁻¹⁵ liter = 1.7 = 10⁻⁹ M. Because K_d = 10⁻¹³ M, the single molecule should be bound to its specific binding site.

3. The number of possible 8-bp sites is 4⁸ = 65,536. In a genome of 4.6 = 10⁶ base pairs, the average site should appear 4.6 × 10⁶/65,536 = 70 times. Each 10-bp site should appear 4 times. Each 12-bp site should appear 0.27 times (many 12-bp sites will not appear at all).

4. The lac repressor does not bind DNA when the repressor is bound to a small molecule (the inducer), whereas the pur repressor binds DNA only when the repressor is bound to a small molecule (the corepressor). The E. coli genome contains only a single lac repressor-binding region, whereas it has many sites for the pur repressor.

A45

5. Anti-inducers bind to the conformation of repressors, such as the lac repressor, that are capable of binding DNA. They occupy a site that overlaps that for the inducer and, therefore, compete for binding to the repressor.

6. The inverted repeat may be a binding site for a dimeric DNA-binding protein or it may correspond to a stem-loop structure in the encoded RNA.

7. Bacteriophage λ would be more likely to enter the lytic phase because the cooperative binding of the λ repressor to O_R2 and O_R1, which supports the lysogenic pathway, would be disrupted.

8.

There are four differences in the −10 region and three differences in the −35 region.

9. Increased Cro concentration reduces the expression of the λ repressor gene. Increased λ repressor concentration reduces the expression of the Cro gene. At low λ repressor concentrations, increased λ repressor concentration increases the expression of the λ repressor gene. At higher λ repressor concentrations, increased λ repressor concentration decreases the expression of the λ repressor gene.

10. Normally, bacterial mRNAs have a leader sequence in which a Shine–Dalgarno sequence precedes the AUG start codon. The absence of a leader would be expected to lead to inefficient translation.

11. Add each compound to a culture of V. fischeri at low density and look for the development of luminescence.

12. ACC, 7; ACA, 1; ACU, 0; ACG, 0.

13. Retention. The reaction likely proceeds in two steps: Attack of Glu 537 on the carbon of the galactose moiety (with inversion) followed by attack of water on this carbon (with a second inversion) to release the galactose from the enzyme.

14. The footprint appears to have approximately 50% of its intensity near 3.7 nM so that the dissociation constant is approximately 3.7 nM, corresponding to a standard free energy of binding of −48 kJ/mol (−11 kcal/mol) at T = 298K.