A transcription factor, TFIS, interacts with both the promoter and enhancer of the Xenopus rRNA genes.

An activity that binds sequence specifically to the enhancer of the Xenopus laevis rRNA genes has been highly purified. This activity stimulates transcription of coinjected rRNA templates in Xenopus oocytes and has been named TFIS, as it binds to the enhancer sequences within the intergenic spacer. In addition to its enhancer binding activity, TFIS binds to the promoter of the Xenopus rRNA genes, as predicted by models for enhancer action. DNase I footprinting on promoter mutants suggests that there are three TFIS-binding sites between -70 and -240 and that TFIS binding is unusually tolerant of mutations. The large region of protein-DNA interaction and the occurrence of DNase I enhancements at integral multiples of the helical repeat are consistent with the promoter and enhancer DNA wrapping around TFIS.