Several mRNAs with variable 3' untranslated regions and different stability encode the human PR264/SC35 splicing factor.

We have recently established that several mRNAs encode the PR264/SC35 splicing factor and that their expression is related to that of c-Myb in human hematopoietic cells. We now report that the various PR264 mRNAs whose expression is transactivated by c-Myb proteins encode an identical PR264/SC35 polypeptide. These mRNAs differ only in their 3' untranslated sequences and exhibit significantly different half-lives. We also show that there is a direct correlation between the length of the 3' untranslated region and the stability of the mRNA species. Given that 5' and 3' splice site selection mediated by the PR264/SC35 splicing factor is concentration-dependent, we postulate that alternative splicing of the 3' untranslated sequences might represent another key element in regulating the levels of PR264/SC35 expression during hematopoietic differentiation.