Discussion

A bacterial strain that is lys⁺ his⁺ val⁺ is used as a donor, and another strain that is lys^- his^- val^- as the recipient for setting up an experiment on transformation. Initial transformants were isolated on a minimal medium with supplemented with lysine and valine. Which of the following maximum genotypes will grow on this medium ?

A.\(\text{lys}^{-}\text{his}^{+}\text{val}^{+}\); \(\text{lys}^{+}\text{his}^{+}\text{val}^{-}\); \(\text{lys}^{+}\text{his}^{+}\text{val}^{+}\); \(\text{lys}^{-}\text{his}^{+}\text{val}^{-}\)
B.\(\text{lys}^{+}\text{his}^{+}\text{val}^{+}\); \(\text{lys}^{-}\text{his}^{-}\text{val}^{+}\)
C.\(\text{lys}^{-}\text{his}^{-}\text{val}^{-}\) only
D.\(\text{lys}^{+}\text{his}^{-}\text{val}^{+}\) only

Answer: A

Every transformant that has become his⁺ will grow, in any combination with lys and val, which is the first option.

The medium is minimal medium supplemented with lysine and valine. Lysine and valine are supplied, so the lys and val alleles do not matter; histidine is not supplied, so a cell can grow only if it has become his⁺. The selection is for his⁺ alone.

Transformation can bring in his⁺ alone or together with linked donor genes, so all four his⁺ classes can appear: lys^- his⁺ val⁺, lys⁺ his⁺ val^-, lys⁺ his⁺ val⁺ and lys^- his⁺ val^-. The other options include his^- genotypes, which cannot grow without histidine, or list too few classes.

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