EFFECT OF PH ON INTERACTIONS BETWEEN DNA AND HIGH-MOBILITY GROUP PROTEIN HMG1

Fluorescence quenching was used to test the effect of pH changes on th e binding of high-mobility group protein 1 (HMG1) to double-stranded a nd single-stranded DNA. At pH 7.5, the binding constant K for double-s tranded DNA was 3 X 10(6) M(-1), the binding site size n was 13, and t he cooperativity factor q was 78, while at pH 6 the corresponding valu es were K = 12 X 10(6) M(-1), n = 54, and q = 770. For the binding of HMG1 to single-stranded DNA at pH 7.5, the values were K = 2 X 10(6) M (-1), n = 7, and q = 60, whereas at pH 6 they were K = 3 X 10(6) M(-1) , n = 14, and q = 440. Denaturation of HMG1 by oxidation of its sulfhy dryl groups substantially affected the binding parameters. At pH 6, do uble-stranded DNA bound oxidized HMG1 with K = 6 X 10(6) M(-1), n = 16 , and q = 200, and single-stranded DNA bound with K = 3 X 10(6) M(-1), n = 7, and q = 180. The sensitivity of the double-stranded DNA-HMG1 i nteraction to pH, along with an earlier report of a sharp optimum of b inding at 140 mM NaCl, reveals a potential for in vivo regulation of t he strength and mode of HMG1 binding by DNA through the action of anal ogous factors in the cellular milieu.