On the flexible interaction of yeast factor tau with the bipartite promoter of tRNA genes.

Yeast transcription factor tau (analogous to vertebrate TFIIIC) interacts specifically with the internal split promoter of tRNA genes. Binding to the two promoter elements (A block and B block) occurs within 30 seconds even when they are separated by a long intervening sequence. Dimethylsulfate protection analysis of contact points between tau and the noncoding strand of a series of internally deleted tRNA3(Leu) genes shows that the specificity of the interaction is not affected by changes in the distance or in the relative helical orientation of the promoter elements. This result is consistent with the results of previous footprinting experiments (Baker, R.E., Camier, S., Sentenac, A. and Hall, B.D., 1987, Proc. Natl. Acad. Sci. USA, 84, 8768-8772). To test if any physical constraint is imposed on the DNA molecule upon tau binding, we analyzed the effect of introducing random single-strand breaks in the noncoding strand of the tRNA gene. Whereas some nicks located in the A block were found to prevent tau binding, no single-strand break in the B block region or in the DNA between the A and B blocks were observed to inhibit or facilitate the binding of tau. We therefore propose that the great flexibility of the tau-tDNA interaction is mostly due to the tau protein itself.