Tryptophan Is an Essential Amino Acid That Neurospora Crassa Needs

Multiple Choice

Tryptophan is an essential amino acid that Neurospora crassa needs in order to be able to grow. The tryptophan biosynthetic pathway has three main steps by which the amino acid tryptophan is produced from chorismate. In an experiment, three N. crassa strains that were suspected of having defects in tryptophan synthesis were cultured on minimal medium (without tryptophan) and on minimum medium (plus tryptophan) , with the following results. Which strain is defective in the tryptophan biosynthetic pathway?
$$\begin{array} { | l | c | c | } \hline & \text { Minimal medium } & \begin{array} { c } \text { Minimal medium } \\+ \text { tryptophan }\end{array} \\\hline \text { Strain 1 } & \text { no growth } & \text { growth } \\\hline \text { Strain 2 } & \text { no growth } & \text { no growth } \\\hline \text { Strain 3 } & \text { growth } & \text { growth } \\\hline\end{array}$$

A) Strain 1
B) Strain 2
C) Strain 3