Both Arabidopsis TATA binding protein (TBP) isoforms are functionally identical in RNA polymerase II and III transcription in plant cells: evidence for gene-specific changes in DNA binding specificity of TBP.

Promoters of pol II and pol III transcribed U-snRNA genes in plants have identical sequence elements comprised of a -30 TATA box and an upstream sequence element (USE), located four or three helical turns upstream of the TATA box in pol II and pol III genes, respectively; it is this difference in element spacing that determines the RNA polymerase specificity of the gene. We are interested in identifying factors binding to U-snRNA gene promoters and their role in selection of RNA polymerase. In this work we have investigated possible differences in the activity of the two TATA binding proteins (TBPs) encoded by two different TBP genes of Arabidopsis. Using mutant TBPs with altered DNA binding specificity, similar to those described previously in yeast, we show that two Arabidopsis TBP isoforms are equally active with both pol II and pol III U-snRNA genes and with an mRNA gene transfected into plant protoplasts. In contrast to yeast, where modified TBP permits transcription only from promoters containing the TGTAAA mutant of the consensus (TATAAA) TATA element, altered Arabidopsis TBPs also suppress other TATA box mutants. Similar results were obtained with human and yeast TBP mutants expressed in plant cells. Interestingly, in several cases suppression of different TATA box mutants by altered TBPs was gene or RNA polymerase specific suggesting that assembly of TBP into specific complexes containing different TBP-associated factors may alter DNA binding specificity of the protein.