Chapter 6, Problem 19P

An Hfr strain with the genotype cys⁺leu⁺met⁺str^S is mated with an F^- strain carrying the genotype cys^- leu^- met^- str^R. In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every $\text{3}$ minutes for $\text{30}$ minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows:

Medium $\text{1}$: Minimal medium plus leucine, methionine, and streptomycin

Medium $\text{2}$: Minimal medium plus cysteine, methionine, and streptomycin

Medium $\text{3}$: Minimal medium plus cysteine, leucine, and streptomycin

a. What donor gene is the selected marker in each medium?

b. List all possible bacterial genotypes growing on each medium.

c. What is the purpose of adding streptomycin to each selection medium? The table on next page shows the number of colonies growing on each selection medium. The sampling time indicates how many minutes have passed since conjugation began

d. Determine the order of donor genes cys, leu, and met from the interrupted mating data.

e. Suppose a fourth selection medium containing leucine and streptomycin is prepared. At what sampling time do you expect the first-growing colonies to appear? Explain your reasoning.