The Xenopus B1 factor is closely related to the mammalian activator USF and is implicated in the developmental regulation of TFIIIA gene expression.

The Xenopus laevis TFIIIA promoter contains a motif that has been implicated in promoter activation in late-stage oocytes and contains the sequence (-269) CACGTG (-264). A cDNA encoding a protein (B1) that binds to this element has been cloned from X. laevis and Xenopus borealis ovarian cDNA libraries. We show that this protein is a member of the helix-loop-helix family of regulatory proteins and contains 80% sequence identity with the human adenovirus major late transcription factor (MLTF or USF). A survey of B1 protein expression during oogenesis and embryogenesis revealed both oocyte-specific and somatic cell-specific B1 protein-DNA complexes. Immunological data, RNA blot analysis, and proteolytic clipping band shift assays indicated that these complexes most likely represent altered forms of a single B1 polypeptide. Implications for TFIIIA gene regulation during development are discussed.