A generalized transducing phage is used to transfer genes from a his(+)met(+)phe

Question

A generalized transducing phage is used to transfer genes from a his(+)met(+)phe(+) donor strain of E. coli to a his(-)met(-)phe(-) recipient strain. When transductants selected on a medium containing histidine and methionine are tested for growth on other media, many more colonies are found on media supplemented only with histidine than on media supplemented only with methionine. No transductants survive on a basal medium. What is the correct gene order and which 2 genes are most closely linked??

Explanation / Answer

His(+) means the organism can synthesize histidine. His(-) means the organisms cannot synthesize histidine , and it needs to be supplemented in the nutritional media.

The same is the case with met (+), and met(-) ; and phe(+), and phe(-).

Now, in the first step, the transuctancts are grown in a media containing only histidine and methionine. Phenylalanine was not present.

Therefore, it can be concluded that the transductants which grew in this media had Phe(+) gene , and hence they could synthesize phe and survive in the media. Therefore, phe is the first gene to be transferred.

Now, the transductants could be either His(+), or met(+). This is not directly evident from the first experiment.

In the second set of experiments, the media was supplemented with either histidine, or methionine. Now, if an organism is met(+) and his (-), it would grow on histidine supplemented media. And if an organism is met(-) and his (+), it would grow on methionine supplemented media. The number of colonies in each type of media would give an idea about which gene was transferred first, and would be more likely to be present in the colonies.

Since more organisms grew on histidine supplemented media, as compared to methionie supplemented, it implies that methionine was transferred first, and histidine was transferred later.

Hence the correct gene order would be -

Phe - Met - His

Since more colonies are found in histidine supplemented media, it can be assumed that the phe and met genes are more closely related.

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