Question

A series of mutations in a biosynthetic pathway of Salmonella result in the requirement of supplements to allow growth (+) as shown in the following table.

	R	X	M	K	Minimal Media
1	+	+	-	+	-
2	+	-	-	+	-
3	+	+	+	+	-
4	+	-	-	-	-

Which of the following steps is regulated by the enzyme that is defective in Mutant 2?

a. R → M

b. M → X

c. X → K

d. K → R

e. M → K

f. K → M

g. R → X

h. M → R

Answer 1

c. X → K

From this data, we can conclude that R is the last molecule in the pathway and is required for growth, as all Mutants can grow on media supplemented with R.

Now, since Mutant 2 can grow in the presence of K and R, therefore, K must be converted to R and this enzyme is functional in Mutant 2. Further, since Mutant 1 can grow on media containing R, K and X, therefore, x must be the precursor to K.

So, Mutant 2 is defective in the conversion of X to K.