XRPFI, AN AMPHIBIAN TRANSCRIPTION FACTOR COMPOSED OF MULTIPLE POLYPEPTIDES IMMUNOLOGICALLY RELATED TO THE GA-BINDING PROTEIN ALPHA-SUBUNITSAND 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 promo ter. Its target sequence, 5'-TAACCGGAAGTTTGT-3', is required to fully activate the promoter, and the two G's of the central motif are essent ial for factor binding and transcriptional activation; our data also s uggest 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-bin ding proteins both supershifted the retarded bands formed by XrpFI. Th e Xenopus polypeptides related to GA-binding protein alpha interact wi th DNA both as monomers and as heterodimers associated with beta-relat ed proteins. Oocyte nuclei contain multiple forms of alpha- and beta-r elated proteins: the alpha-like proteins remain throughout development , while the pattern of the beta species changes in the embryonic stage s 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 .