An Hfr strain with the genotype cys⁺ lue⁺ met⁺ strˢ is mated with an  strain carrying the genotype cys⁻ lue⁻ met⁻ strᴿ. In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows:

Medium 1: Minimal medium plus leucine, methionine, and streptomycin
Medium 2: Minimal medium plus cysteine, methionine, and streptomycin
Medium 3: Minimal medium plus cysteine, leucine, and streptomycin

The following table shows the number of colonies growing on each selection medium. The sampling time indicates how many minutes have passed since conjugation began.

Sampling Time (minutes)              Number of Colonies
                                            Plate 1         Plate 2         Plate 3
           3                                   0                  0                   0
           6                                   0                  0                   0
           9                                   0                 62                  0
         12                                   0                 87                  0
         15                                  51               124                 0
         18                                  79               210                 62
         21                                 109               250                85
         24                                 144               250               111
         27                                 152               250               122
         30                                 152               250               122

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.