Three auxotrophic strains of E. coli (#1-3) of genotype a(-)b(+) are mixed in al

Question

Three auxotrophic strains of E. coli (#1-3) of genotype a(-)b(+) are mixed in all possible combinations with three auxotrophic strains (#4-6) of genotype a(+)b(-) and subsequently plated on basal media. A few recombinants form colonies when the following strains are mixed: 2+4 and 3+6. Many recombinants form colonies when the following strains are mixed: 1+6 and 2+5. All other combinations produce no surviving recombinants on basal media. What are the mating types (F+, F-, Hfr) of all six strains?

Explanation / Answer

E.coli has the mating types F+ and F-, with F+ having an F factor, a plasmid on the same.The F factor has genes for conjugation ,genetic material transfer, including pilus formation for cell to cell contact. Only when F factor integrates into E.coli choromosome, an HFr strain type forms, indicating high frequency recombination.Only HFr cells can pass a copy of the chromosome genes to the F- recipient cell.

E.coli strain # 1-3 (auxotroph for a) as well as #4-6 are auxotrophs for b. They cant grow on the minimal media as such without change in genotype or recombination.

Strains 2 and 4 are likely to be HFr cells, having an F factor which integrated into their genome and resulted in genetic material exchange. Strain 2 could be 2 F+ and 4 could be 4 F+. They could grow on minimal media only if post recombination their genotype is a+b+ , which can only be if both are HFr strains with F factor integrated into their genome.

Same is the case with cells 3 and 6,they are HFr cells, and so are 1 and 5. This because via genetic exchange due the presence of integrated F factor , resulting in HFr strain can enable a+b+ genotype to grow on basal media as with 1 and 6 , 2 and 5, 3 and 6.

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