E. coli SSB tetramer binds the first and second molecules of (dT)35 with heat capacities of opposite sign Original Research Article

We have previously shown that formation of a 1:1 fully wrapped complex of Escherichia coli SSB tetramer with (dT)70 displays a temperature-dependent sign reversal of the binding heat capacity (ΔCP). Here we examine SSB binding to shorter oligodeoxynucleotides ((dX)35) to probe whether this effect requires binding of one or two (dX)35 molecules per SSB tetramer. We find that the ΔCP for the first molecule of (dX)35 is always negative. However, a sign reversal of ΔCP from negative to positive occurs with increasing temperature for binding of the second (dX)35. This striking behavior of ΔCP for the second (dX)35 appears linked to conformational changes within the ssDNA–SSB complex that are required to form a fully wrapped (SSB)65 binding mode. These results also underscore that binding heat capacities of macromolecular interactions have multiple origins that cannot be understood simply on the basis of examining static structures.