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 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?

Please help me understand why, thank you.

Explanation / Answer

Generalized transduction is the process of gene transfer in which transducing phage transfers any gene or fragment of DNA randomly from a donor cell to recipient cell. As the gene is transfer from a his(+)met(+)phe(+) donor strain of E. coli to a his(-)met(-)phe(-) recipient strain. The transducing phage randomly can transfers the different set of genes from donor to recipient cell. When transductants selected on a medium containing histidine and methionine it indicates that both genes are transfered from donor to recipient randomly. But when the selected transductants are tested for growth on media supplemented only with histidine has more colonies than on supplemented only with methionine, it clearly indicates that the gene for his(+) and gene for met(+) are not closely linked and randomly phage tranfer them to different bacterial cell by generalized tranduction. Hence the number of cells varies on histidine suplemented media and methionine suplemented media separately.

The correct gene order can be his(+)phe(+)met(+)

The 2 genes most closely linked may be his(+)phe(+) or phe(+)met(+) but not the his(+)met(+) becuase if his(+)met(+) are closely limked, they transfer together and produce the approx same number of colonies on histidine suplemented media and methionine suplemented media.

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