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: 3+5 and 1+6. Many recombinants form colonies when strains 2 and 5 are mixed and when strains 1 and 4 are mixed. All other combinations produce no surviving recombinants on basal media.

What are the mating types (F+, F-, Hfr) of all 6 strains?

Can you explain a little bit further on how the answer was found?

Explanation / Answer

Strain 1-3 are (a-b+), auxotrophs. This means that a should be added to the media for their growth. When they are mixed with 4-6 (a+b-), the recombinants become (a+b+). For growth of these recombinants (which have now become prototrophs) does not require anything to be added to the basal media for growth. Therefore, the recombinants from, 3+5, 1+6, 2+5 and 1+4 form colonies.

All other combination do not produce colonies because they have become a-b-, or a-b+, or a+b-. In all cases, one of the ingredients, either a or b or both has to be added to the basal media for growth.

All mating types of the 6 strains are Hfr.

If they had been F- conjugation would not have had occured at all.

If they were F+, then complete conjugation would have occured making the a-b+ cell to be a+b- and vice-versa. Then all strains would still remain auxotrophic even after conjugation and no recombinant colonies could have grown. The above recombinant colonies are results of interrupted mating.

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