An underlying repeat in some transcriptional control sequences corresponding to half a double helical turn of DNA.

Transcription factor IIIA, which binds to the internal control region of the Xenopus 5S RNA gene has a novel structure consisting of nine tandemly repeated structural units. It was proposed by us that each unit interacts with about 5 bp of DNA. We show here that there is a periodicity on this scale in the DNA sequence and, by fine scale probing with nucleases, a corresponding structural repeat. Similar sequence periodicities are found in the internal control regions of other genes transcribed by RNA polymerase III, and also in the SV40 promoter and a monkey gene region to which the transcription factor Sp1 binds. We propose that transcription factor IIIA is the type of a novel class of transcription factors offering combinatorial possibilities for the specific recognition of DNA.