Arg-
Strain #4 can grow with the addition of any of the
three
amino acids. Therefore, it must affect the production of enzyme A,
the first step in the pathway. Absence of a functional enzyme A
prevents synthesis from some precursor substance of X, which
therefore
prevents subsequent synthesis of Y or Z. Addition of any
of X, or Y, or Z will permit growth, since
enzymes B
&
C are functional, and can convert these substances to arginine.
However, analysis of this strain provides no information
about the relative order of arg, cit, and orn in
the pathway.
