Synthesis of the amino acid histidine is a multistep anabolic pathway that uses

Question

Synthesis of the amino acid histidine is a multistep anabolic pathway that uses the products of 13 genes (hisA to hisM) in E. coli. Two independently isolated his E. coli mutants, designated his1 and his2, are studied in a conjugation experiment. A his+ F donor strain that carries a copy of the hisJ gene on the plasmid is mated with a his1 recipient strain in experiment 1 and with a his2 recipient in experiment 2. The exconjugants are grown on plates lacking histidine. Growth is observed among the exconjugants of experiment 2 but not among those of experiment 1.

a) Why is the growth observed in experiment 2 but not in experiment 1?

The textbook solution seems incomplete and I do not understand.

Explanation / Answer

Let us first analyze the given data

1. There are two E.coli mutants designated as his1- and his2- .
2. his+ F donor strain that carries a copy of the hisJ gene on the plasmid

Now, the mutants are lacking in a particluar gene required for the synthesis of histidine. Therefore, in a plate lacking histidine, we would expect that the mutants would not grown, unless the missing gene is provided to the mutants.

In both the experiments, the gene that is provided to the mutants is the hisJ gene. Therefore, the experimental conditions are same upto this point. The expected outcome is that the mutants should not grow.

However, the mutants manage to grow in experiment 2, but not in experiment 1.

We can draw the following interpretations based on the results-

1. Mutant 2 was lacking in the gene hisJ. Therefore, when hisJ was supplied by the expternal plasmid, it began to synthesize histidine and grew on the plate.

2. There is insufficient information regarding mutant 1. It can be possible that mutant 1 is deficient in more than one gene (apart from hisJ), or it already has a copy of hisJ and is lacking in some other gene.

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