Chapter 15, Problem 17P

Six strains of E.coli (mutants 1–6) that had one of the following mutations (i–vi) affecting the lac operon were isolated.

i. deletion of lacY

ii. oc mutation

iii. missense mutation in lacZ

iv. inversion of the lac operon (but not an inversion of the lacI gene)

v. superrepressor mutation

vi. inversion of lacZ, Y, and A but not lacI, P, o

a. Which of these mutations would prevent the strain from utilizing lactose?

b. The entire lac operon (including the lacI gene and its promoter) from each of the six E. coli strains was cloned into a plasmid vector containing an ampicillin resistance gene. Each recombinant plasmid was transformed into each of the six strains to create partial diploids. In analysis of these strains, mutant 1 was found to carry a deletion of lacY, so this strain corresponds to mutation i in the list above. Which of the other types of mutations would be expected to complement mutant 1 in these partial diploids so as to allow lactose utilization?

c. In part (b), each strain was plated on ampicillin media in which lactose was the only carbon source. (Ampicillin was included to ensure maintenance of the plasmid.) Growth of the transformants is scored below (a + sign indicates growth, a − sign means no growth). Synthesis of β-galactosidase and permease are both required for growth on this medium. Results of this merodiploid analysis are shown here. Which mutant bacterial strain (1–6) contained each of the alterations (i–vi) listed previously?