Regulation of TATA-binding protein dynamics in living yeast cells.

Although pathways for assembly of RNA polymerase (Pol) II transcription preinitiation complexes (PICs) have been well established in vitro, relatively little is known about the dynamic behavior of Pol II general transcription factors in vivo. In vitro, a subset of Pol II factors facilitates reinitiation by remaining very stably bound to the promoter. This behavior contrasts markedly with the highly dynamic behavior of RNA Pol I transcription complexes in vivo, which undergo cycles of disassembly/reassembly at the promoter for each round of transcription. To determine whether the dynamic behavior of the Pol II machinery in vivo is fundamentally different from that of Pol I and whether the static behavior of Pol II factors in vitro fully recapitulates their behavior in vivo, we used fluorescence recovery after photobleaching (FRAP). Surprisingly, we found that all or nearly all of the TATA-binding protein (TBP) population is highly mobile in vivo, displaying FRAP recovery rates of <15 s. These high rates require the activity of the TBP-associated factor Mot1, suggesting that TBP/chromatin interactions are destabilized by active cellular processes. Furthermore, the distinguishable FRAP behavior of TBP and TBP-associated factor 1 indicates that there are populations of these molecules that are independent of one another. The distinct FRAP behavior of most Pol II factors that we tested suggests that transcription complexes assemble via stochastic multistep pathways. Our data indicate that active Pol II PICs can be much more dynamic than previously considered.