BOTH ARABIDOPSIS TATA-BINDING PROTEIN (TBP) ISOFORMS ARE FUNCTIONALLYIDENTICAL IN RNA POLYMERASE-II AND POLYMERASE-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 ha ve identical sequence elements comprised of a -30 TATA box and an upst ream sequence element (USE), located four or three helical turns upstr eam of the TATA box in pol II and pol III genes, respectively; it is t his difference in element spacing that determines the RNA polymerase s pecificity of the gene. We are interested in identifying factors bindi ng to U-snRNA gene promoters and their role in selection of RNA polyme rase. In this work we have investigated possible differences in the ac tivity of the two TATA binding proteins (TBPs) encoded by two differen t 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 transcr iption only from promoters containing the TGTAAA mutant of the consens us (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 bi nding specificity of the protein.