Question

6. A tetracycline repressor (TetR) protein, which is present in E. coli at 10^–8 M concentration, binds to the tetO site with a KD of 10^–10M in the absence of the drug tetracycline and a KD of 10^–6M in the presence of tetracycline. There is one tetO site in the E. coli genome, giving a concentration of 10^–9M per cell. There are 2×10^5 nonmatching sites per cell (a concentration of 5 × 10^–2M), to which TetR binds nonspecifically with a KD of 10^-5M.

a. What is specificity for the tetO site over the nonmatching sites in the presence of tetracycline? (4 pts)

b. What is specificity for the tetO site over the nonmatching sites without tetracycline present? (4 pts)

Answer 1

According to the question the concentration per cell of the Non-matching site is maximum and that of tetO site is minimum. Also the concentration of TetR is greater than that of tetO site in a cell and smaller than the concentration of Non matching site.

a. The specificity for the tetO site in the presence of tetracycline will be more over the non-matching site as the concentration of tetO site is much less, that can easily bind.

Also the binding in the presence of tetracycline with tetO is high(as Kd value is high) as compared when tetracycline is absent.

b. The specificity for the tetO site over the non-matching site in the absence of tetracycline will be less. This is because in the absence of tetracycline the KD value is lesser that means binding is lesser with the tetO and here the higher KD value of the non-matching site dominates.

As a result it will bind more with the Non- matching site in the absence of tetracycline.