Evidence for functional binding and stable sliding of the TATA binding protein on nonspecific DNA

The TATA binding protein (TBP) is required at RNA polymerase I, II, and III promoters that either contain or lack a TATA box. In an effort to understand how TBP might function at such a wide variety of promoters, we have investigated the specific and nonspecific DNA binding properties of human TBP. We show that TBP has less than a 10(3)-fold preference for binding a TATA box (TATAAAAG) than for an average nonspecific site. In contrast to TBP, which binds to the minor groove of DNA, major groove binding proteins typically display binding specificities in the range of 10(6). Once TBP is bound to DNA, whether it be a TATA box or nonspecific DNA, binding is quite stable with a t1/2 of dissociation in the range of 20-60 min for a 300-base pair DNA fragment. In this binding state, TBP appears to be capable of stable one-dimensional sliding along the DNA. Sequence-specific binding can be accounted for, in part, by different rates of sliding. Additional findings demonstrate that specific and nonspecific DNA impart upon TBP an enormous and equivalent degree of thermal stability, suggesting that the TBP-DNA interface on non-specific DNA is not radically different from that on TATA. Consistent with this notion, we find that nonspecifically bound TBP is competent in establishing pol II transcription complexes on DNA. Together, these finding provide a plausible mechanistic explanation for the ability of TBP to function at TATA-containing and TATA-less promoters.