Structural analysis of a triple complex between the histone octamer, a Xenopus gene for 5S RNA and transcription...

This paper reports three experiments concerning the structural relationship between the Xenopus transcription factor IIIA (TFIIIA), the histone octamer and the Xenopus somatic gene for 5S RNA. Quantitative footprinting methods have been used in order to discover where and how TFIIIA and the histone octamer bind to the same gene independently and also in a triple complex. First, DNaseI and DNaseII protection experiments show that TFIIIA binds to positions 45-97 within the gene, in agreement with other workers. Second, the histone octamer takes up a unique, well-defined position with respect to DNA sequence. The nucleosome core extends to position 78 of the gene and therefore overlaps the TFIIIA binding region by approximately 35 bp. Third, it is shown that a triple complex can be formed between TFIIIA, the histone octamer and the 5S RNA gene. TFIIIA displaces the DNA from the histone surface in the 35-bp region of overlap. This has led to a three-dimensional model which explains how RNA polymerase III could interact simultaneously with transcription factors bound at the internal control region of the 5S RNA gene and the start point of transcription. The model also explains how histone H1 could repress transcription of 5S RNA genes.