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

Here, the pathway shows the synthesis of amino acid Arginine from Ornithine. Three enzymes ARG-E, ARG-F and ARG-H are required by Neurospora to synthesise Arginine from Ornithine.

The correct matches are,

Mutant A - ARG-F

Mutant B - ARG-E

Mutant C- ARG-H

Here, in mutant strain A, the mutant gene is ARG-F. It is because, for mutant A, the strain is not able to grow in minimal media, and the media in which ornithine and citrulline are given as supplements. That, in this situations they are not able to produce arginine and hence they cannot grow. But, when Argininosuccinate is given, they grows which shows the enzyme ARG-H is functional . So, it is clear from the pathway that it is ARG-F that is mutated. The reason is, it has already mentioned that only a single gene in pathway is mutated and when orniothine as well as citrulline is supplied, growth has not occured. This might probably because of a mutation in ARG-F so that, only when Argininosuccinate is given directly, Neurospora grows (obviously depicts that enzyme for synthesizing argininosuccinate is lacking).

Similarly, in mutatnt strain B, only when citrulline, argininosuccinate and arginine are provided additionaly the Neurospora grows. From the pathway, it is clear that, the enzyme required for the synthesize of citrulline is absent. Hence, when citrulline is provided, they are able to grow and when all the components prior to citrulline synthesis is provided,the B strain is not able to grow.

In case of C mutant strain, the final product arginine itself has to be supplied , only then the strain is able to grow. All substrates prior to this, when provided, the growth is absent, which shows the final enzyme for the arginine symnthesis is absent, ie, ARG-H is lacking.

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