A POTENTIAL SPLICING FACTOR IS ENCODED BY THE OPPOSITE STRAND OF THE TRANS-SPLICED C-MYB EXON

We previously established that the expression of a thymic c-myb mRNA s pecies requires the intermolecular recombination of coding sequences e xpressed from transcriptional units localized on different chromosomes , in both chicken and human. We now report that a putative splicing fa ctor (PR264), extremely well conserved in chicken and human, is encode d by the opposite strand of the c-myb transpliced exon. The PR264 poly peptide, which contains a typical ribonucleoprotein 80 and an arginine /serine-rich domain, is highly homologous to the Drosophila splicing r egulators tra, tra-2, and su(w(a)) and to the human alternative splici ng factor ASF/SF2. Furthermore, we show that PR264-specific mRNAs are expressed in normal hematopoietic cells of chicken and human origin an d that the relative proportion of the PR264 transcripts is development ally regulated in chicken.