Chapter 4, Problem 4P

Fifteen bacterial colonies growing on a complete medium are transferred to a minimal medium. Twelve of the colonies grow on minimal medium. a. Using terminology from the chapter, characterize the 12 colonies that grow on minimal medium and the 3 colonies that do not.

b. The three colonies that do not grow on minimal medium are transferred to minimal medium supplemented with the amino acid serine (min + Ser), and all three colonies grow. Characterize these three colonies.

c. The serine biosynthetic pathway is a three-step path-way in which each step is catalyzed by the enzyme product of a different gene, identified as enzymes A, B, and C in the diagram below.

$$ $\text{3-Phosphoglycerate }$ $\stackrel{\text{Enzyme A}}{\to }$ $\text{3-Phospho-hydroxypyruvate }$ $\text{(3-PHP)}$ $\stackrel{\text{Enzyme B}}{\to }$ $\text{3-Phosphoserine}$ $\text{(3-PS)}$ $\stackrel{\text{Enzyme C}}{\to }$ $\text{Serine (Ser)}$

Mutant 1 grows only on min $\text{+ Ser}$. In addition to growth on min $\text{+ Ser}$, mutant $\text{2}$ also grows on min $\text{+ 3-PHP}$ and min $\text{+ 3-PS}$. Mutant $\text{ 3}$ grows on min $\text{+ 3-PS}$ and min $\text{+ Ser}$. Identify the step of the serine biosynthesis pathway at which each mutant is defective.