T4 gene 32 protein trypsin-generated fragments. Fluorescence measurement of DNA-binding parameters

The intrinsic fluorescence of the T4 helix-destabilizing protein specified by gene 32 (32P) is not altered by the proteolytic removal of either the 6200-dalton COOH-terminal "A" region (32P*-A) or both the A and the 2300-dalton NH2-terminal "B" region (32P*-(A + B)). The intrinsic fluorescence of 32P, 32P*-A, and 32P*-(A + B) is decreased 23% by the addition of d(pT)8 and 34% by the addition of poly(dT). Saturation binding curves of the percentage of change in protein fluorescence as a function of nucleotide concentration show that the intact 32P as well as the two proteolysis-generated fragments all have association constants of approximately 10(6) M-1 for d(pT)8. This demonstrates that the DNA binding site is not contained within either the A or B regions of 32P. Both 32P and 32P*-A bind cooperatively to poly(dT) as evidenced by a 400- to 1000-fold increase in association constant for poly(dT) compared to d(pT)8. Since within the limits of our measurements 32P and 32P*-A bind equally well to poly(dT) (Kassoc approximately 5 . 10(8) M-1), the enhanced helix-destabilizing properties previously reported for 32P*-A cannot be accounted for by a significant increase in binding affinity of 32P*-A for single-stranded DNA. The binding constant for the 32P*-(A + B):poly(dT) complex is only 3-fold higher than that for the 32P*-(A + B):d(pT)8 complex, which confirms our proposal that the B region is essential for cooperative 32P:32P protein interactions.