Heterogeneity in the Xenopus ribosomal transcription factor xUBF has a molecular basis distinct from that in mammals

The Xenopus polymerase I transcription factor xUBF is an HMG-box protein which has been purified as two polypeptides of approximately 82 and 85 kDa. Recently a cDNA sequence predicted an xUBF protein (xUBF1) of 677 amino acids (79 kDa) containing five tandem HMG-boxes. Here a second and distinct xUBF cDNA has been isolated and characterised. This cDNA codes an xUBF protein (xUBF2) of 701 amino acids (82 kDa), having 93% homology with xUBF1 but containing an insertion of 22 amino acids between HMG-boxes 3 and 4. In vitro translation of synthetic mRNAs derived from the xUBF1 and 2 cDNAs was used to show that the electrophoretic mobility of the gene products accounted for the major xUBF molecular weight heterogeneity noted in vivo. It is also shown that the Xenopus laevis genome contains 3 or 4 distinct loci which hybridise with xUBF coding sequences, leaving open the possibility of yet further unrecognised heterogeneity in xUBF.