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 species requires the intermolecular recombination of coding sequences expressed from transcriptional units localized on different chromosomes, in both chicken and human. We now report that a putative splicing factor (PR264), extremely well conserved in chicken and human, is encoded by the opposite strand of the c-myb trans-spliced exon. The PR264 polypeptide, which contains a typical ribonucleoprotein 80 and an arginine/serine-rich domain, is highly homologous to the Drosophila splicing regulators tra, tra-2, and su(wa) and to the human alternative splicing factor ASF/SF2. Furthermore, we show that PR264-specific mRNAs are expressed in normal hematopoietic cells of chicken and human origin and that the relative proportion of the PR264 transcripts is developmentally regulated in chicken.