Consider the pathway for the synthesis of the amino acid arginine in Neurospora

Question

Consider the pathway for the synthesis of the amino acid arginine in Neurospora:

                        ARG-E         ARG-F                      ARG-H

            ornithine --> citrulline --> argininosuccinate --> arginine

Mutant strains of Neurospora are grown in minimal media supplements as follows. Each mutant strain carries only a single mutation. Growth is shown by (+) and no growth is shown by (-).

                                                    Supplements

Mutant strain     minimal     ornithine     citrulline     argininosuccinate     arginine

         A                   -                 -                  -                         +                      +

         B                   -                 -                  +                        +                      +

         C                   -                 -                  -                         -                       +

Pair each mutant strain with the correct gene in the pathway.

Explanation / Answer

Mutant A - Arg-H

This is due to the expression of enzyme responsible for conversion of argininosuccinate to ornithine and utilizing it for their growth. This is the reason you observe growth only when you supplement arginiosuccinate and ornithine.

Mutant B - Arg-F

This is due to the expression of enzyme responsible for conversion of citrulline to argininosuccinate and downstream of that. This is the reason you observe growth only when you supplement citrulline, argininosuccinate and arginine.

Mutant C- None

This mutant cannot be assigned to any of the enzyme in the pathway because supplementing either citrulline and argininosuccinate is not able to get the growth but only arginine supplement is able to see the growth.

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