Question

The lacY gene of E. coli codes for a protein that transports the sugar lactose from outside the cell to inside, where it is broken down and used for fuel. An E. coli that has a disabling mutation in the lacY gene therefore cannot grow on medium in which lactose is the only source of fuel. If 109 lacY mutant E. coli are plated on agar that contains lactose as the only fuel, approximately 50 colonies are formed.

How are these colonies able to form? If this experiment is repeated using a lacY mutant E. coli that has in addition a mutation that disables the mismatch repair system, 3,000 colonies are formed. Suggest an explanation for this increase in the number of colonies.

Answer 1

Ans -- There is always a chance of some leaky transport of lactose to the bacteria that can be utilised as a fuel to grow. Under nutrient starvation there is a positive mutation selection pressure that might revert the mutation on lacY to functional genes that might be the possible explanation how 50 colonies are formed on plate. Further disabling the mismatch repair system accelerates the random mutation process and that's why around 3000 functional colonies are formed.