You have isolated a number of E.coli strains with mutations in genes involved in
ID: 19656 • Letter: Y
Question
You have isolated a number of E.coli strains with mutations in genes involved in lactose utilization (lac operon genes). Depending on the particular mutation they carry, the different E.coli strains have different fitness values when grown in media containing lactose as the only carbon cource, as shown in the table below:
'Mutation Fitness (relative to the WT strain)
LacI_mut 0
LacZ_mut1 0.5
LacZ_mut2 1.2
LacY_mut1 0.6
LacY_mut2 1.3
LacZ_mut1'LacY_mut1 0.2
LacZ_mut1'LacY_mut2 0.8
LacZ_mut2'LacY_mut1 0.8
LacZ_mut2'LacY_mut2 1.5
Based on your knowledge about the function of the LacI, LacZ and LacY genes in the wild type strain, and the fitness values measured here, propose hypotheses that could explain the effect that each mutation has in the functions of LacI, LacZ, and LacY
Explanation / Answer
LacI_mut has inability to bind the lactose and repressor will constitutively bind to the operator region of the DNA and doesn't allow the transcription. Finally, lactose will not be metabolized and its fitness will be zero.
LacZ_mut1 leads to the decreased enzymatic activity of the B-galactosidase and it is not able to cleave the lactose into glucose and galactose with optimum efficiency. This is the reason the fitness decreases to 0.5.
LacZ_mut2 has hyper activity of the B-galactosidase and it is able to cleave the lactose more efficiently than wild type and leading to higher fitness value.
LacY_mut1 has diminished activity of B-galactoside permease and it pumps less lactose inside the cells compered to wild type and if lactose pumping is less fitness will ultimately decrease because of less energy available to cell.
LacY_mut2 has hyper activity of B-galactoside permease and pumps more number of lactose inside the cells compared to wild type and cells has more lactose to metabolize and gain more energy ultimately leading to increased fitness.
LacZ_mut1'LacY_mut1 will have decreased enzymatic activity of the B-galactosidase as well decresed activity of B-galactoside permease. in this case, less number of lactose is pumped inside the cells and futher decreased number of lactose is metabolized. this leads to twice dcreement in fitness and this why value is coming 0.2.
LacZ_mut1'LacY_mut2- LacZ_mut1 leads to the decreased enzymatic activity of the B-galactosidase LacY_mut2 has hyper activity of B-galactoside permease. both mutant has antagonizing effect and increased fitness.
LacZ_mut2'LacY_mut1- LacZ_mut2 has hyper activity of the B-galactosidase and LacY_mut1 has diminished activity of B-galactoside permease which antagonizes the each other effect and fitness is 0.8 (near to 1).
LacZ_mut2'LacY_mut2- LacZ_mut2 has hyper activity of the B-galactosidase and LacY_mut2 has hyper activity of B-galactoside permease. both leads to incresed pumping and metabolism of lactose and incresed fitness over wild type.
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