XrpFI, an amphibian transcription factor composed of multiple polypeptides immunologically related to the GA-binding protein alpha and beta subunits, is differentially expressed during Xenopus laevis development.

XrpFI, first identified in the extract of Xenopus laevis oocyte nuclei, binds to a proximal sequence of the L14 ribosomal protein gene promoter. Its target sequence, 5'-TAACCGGAAGTTTGT-3', is required to fully activate the promoter, and the two G's of the central motif are essential for factor binding and transcriptional activation; our data also suggest that XrpFI may play a role in cap site positioning. The binding site of XrpFI is homologous to the sequence recognized by the family of ets genes. Antibodies specific for Ets-1 and Ets-2 proteins did not react with XrpFI, but those raised against the rat alpha and beta GA-binding proteins both supershifted the retarded bands formed by XrpFI. The Xenopus polypeptides related to GA-binding protein alpha interact with DNA both as monomers and as heterodimers associated with beta-related proteins. Oocyte nuclei contain multiple forms of alpha- and beta-related proteins: the alpha-like proteins remain throughout development, while the pattern of the beta species changes in the embryonic stages examined. beta-like proteins are undetectable in the cleavage period up to the neurula stage, but at later stages, when ribosomal protein genes are actively transcribed, two beta-related polypeptides reappear.