Specific interaction of a purified transcription factor with an internal control region of 5S RNA genes.

A factor necessary for the accurate transcription of cloned Xenopus 5S genes in vitro has been isolated from soluble extracts of X. laevis ovaries. The activity of the factor was monitored by its ability to facilitate transcription of exogenous 5S genes in unfertilized egg extracts which are otherwise incompetent for 5S gene transcription. The factor was purified via ion exchange chromatography, and apparently consists of a 37,000 dalton polypeptide. This factor is necessary for the transcription of both the oocyte-type and somatic-type 5S genes of Xenopus, but is not required for, and has no detectable effect upon, the transcription of a cloned Xenopus tRNA1Met gene. The site of action of the factor has been investigated using the "footprinting" method of Galas and Schmitz (1978). The factor binds specifically to intragenic regions extending, approximately, from nucleotide positions 45 to 96 on both somatic and oocyte-type 5S genes. Additionally, this binding occurs independently of, and is not altered by, the presence of purified RNA polymerase III or unfertilized egg extracts. The probable role of this factor in transcription initiation is discussed.