Variations in intracellular levels of TATA binding protein can affect specific genes by different mechanisms

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.