Apparent heat capacity change accompanying a nonspecific protein-DNA interaction. Escherichia coli SSB tetramer binding to oligodeoxyadenylates

ABSTRACT: We have examined the effects of temperature on the equilibrium constant, Kobs, for Escherichia coli SSB tetramer binding to a series of single-stranded (ss) oligodeoxyribonucleotides, dT(pT),, dC(pC),, and dA(pA), ( n = 34,55, and 69) in order to investigate the thermodynamic basis for the strong preference of E. coli SSB (and other SSB proteins) for binding polypyrimidine stretches of ss-DNA. In addition to the expected base-dependent differences in the magnitude of Kobs, we also observe qualitatively different temperature dependencies for the binding of the SSB tetramer to oligodeoxyadenylates. Linear van? Hoff plots are obtained for SSB tetramer binding to dT(pT), and dC(pC),, with AHo,b ranging from-50 to-100 kcal/mol depending on the oligodeoxynucleotide length and salt concentration. In contrast, all van't Hoff plots for SSB tetramer binding to dA(pA)N are distinctly nonlinear with maxima in Kobs occurring near 25 "C, indicative of an apparent large negative change in molar heat capacity (AC0p,,bs < 0). Thus for the SSB-dA(pA), interaction, AHo,b and Mo0b are both highly temperature dependent, but compensate such that AGoobs is relatively insensitive to temperature. These nonlinear van't Hoff plots are not due to coupling of SSB assembly to dA(pA), binding or to temperature-dependent shifts in the formation of other SSB-DNA binding modes. The nonlinear van't Hoff plots for SSB tetramer binding to dA(pA), appear to result from the coupling of two processes: ( 1) the unstacking of the dA(pA), bases (occurring with AHo> 0 and ACop = 0) and (2) the binding of SSB to the unstacked D N A (occurring with AHo < 0 and ACop = 0). Therefore, although each isolated equilibrium occurs with ACop = 0, the overall equilibrium display