We previously showed that reduced intracellular levels of the TATA binding protein (TBP), brought about by tbp heterozygosity in DT40 cells, resulted in a mitotic delay reflecting reduced expression of the mitotic regulator cdc25B but did not significantly affect overall transcription. Here we extend these findings in several ways. We first provide evidence that the decrease in cdc25B expression reflects reduced activity of the cdc25B core promoter in the heterozygous (TBP-het) cells. Strikingly, mutations in a previously described repressor element that overlaps the TATA box restored promoter activity in TBP-het cells, supporting the idea that the sensitivity of this promoter to TBP levels reflects a competition between TBP and the repressor for DNA binding. To determine whether cells might have mechanisms to compensate for fluctuations in TBP levels, we next examined expression of the two known vertebrate TBP homologues, TLP and TBP2. Significantly, mRNAs encoding both were significantly overexpressed relative to levels observed in wild-type cells. In the case of TLP, this was shown to reflect regulation of the core promoter by both TBP and TLP. Together, our results indicate that variations in TBP levels can affect the transcription of specific promoters in distinct ways, but overall transcription may be buffered by corresponding alterations in the expression of TBP homologues.Perhaps the most widely studied general transcription factor has been the TATA binding protein (TBP). TBP was initially thought to be a universal transcription factor, essential for transcription of genes transcribed by all three RNA polymerases (17). However, the isolation of several TBP homologues (5, 10, 12) has provided evidence that this is not the case and that transcriptional regulation, even at the level of this basal factor, is more complex than provided for by a universal transcription factor. TBP consists of two domains, an amino (N)-terminal domain that is species specific and a conserved carboxy C-terminal domain. The conserved core domain directly contacts the TATA box of promoters and nucleates the assembly of other factors required for initiation of transcription (17). The core domain is largely conserved in the more recently described TBP homologues, while the species specific N-terminal domain is either absent or entirely dissimilar (11,24).One such homologue is TBP-like protein (TLP; also known as TRF2 or TLF; for a review, see reference 5). For some time it was speculated that TLP regulated transcription by sequestering essential factors (47), preventing their utilization in initiation complexes containing TBP. Indeed, TLP sequences necessary for binding the general transcription factors TFIIA and TFIIB are identical to those found in TBP (5, 11), and TLP, when overexpressed, does repress transcription from the TATA-containing adenovirus major late promoter (39). However, in vitro analysis demonstrated that artificial recruitment of TLP to a promoter by fusion to a Gal-4 DNA-binding domain activated transcription (38...

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