GENES ENCODING ISOFORMS OF TRANSCRIPTION ELONGATION-FACTOR TFIIS IN XENOPUS AND THE USE OF MULTIPLE UNUSUAL RNA PROCESSING SIGNALS

We have identified cDNAs encoding three related forms of transcription elongation factor TFIIS (S-II) in Xenopus laevis ovary. Comparison of Xenopus and mammalian sequences identifies likely diagnostic amino ac ids that distinguish classes of vertebrate TFIIS. The diversity of TFI IS polypeptides in Xenopus is due partly to the presence of two diverg ed genes in this tetraploid genome. We isolated genomic clones contain ing one of the genes, xTFIIS.oA, and, unlike a previously described ve rtebrate TFIIS gene, found that it contains introns. Alternative splic ing at a CAG/CAG motif containing the 3' splice site of intron 4 produ ces the third form of xTFIIS, which differs from one of the others sim ply in lacking Ser109. Intron 6 of xTFIIS.oA contains splice and branc h site consensus sequences conforming to those of the minor class of A T-AC introns and this was confirmed for the homeologous xTFIIS.oB gene by genomic PCR. Other unusual but functional variants of RNA processi ng signals were found in xTFIIS genes at the 5' splice site of intron 8 and the polyadenylation hexanucleotides. Utilization of multiple unu sual processing signals may make the generation of mature xTFIIS.o mRN As inefficient and the possible regulatory consequences of this are di scussed.