INDUCTION OF CYTOPLASMIC FACTORS THAT BIND TO THE 3' AU-RICH REGION OF HUMAN INTERFERON-BETA MESSENGER-RNA DURING EARLY DEVELOPMENT OF XENOPUS-LAEVIS

Certain endogenous Xenopus mRNAs, carrying a destabilizing 3' AU-rich sequence, are unusually very stable in oocytes and become unstable onl y after fertilization. In addition, heterologous short lived mRNA, con taining 3' AU-rich sequences, appear to be very stable when injected i nto Xenopus oocytes. In the present study, a human interferon beta (hu -IFN beta) mRNA, carrying the destabilizing 3' AU-rich element, was us ed as a probe to identify Xenopus proteins that specifically bind to t he 3' AU-rich element as well as to study their relative levels during early embryonic development. While three major proteins that specific ally bind to the 3' AU-rich element were detected in human SV80 cells, that naturally express hu-IFN beta (proteins termed AU-F1, F2 and F3) , only two proteins, migrating similarly to the SV80 AU-F1 and AU-F3, were detected in cytoplasmic extracts from Xenopus oocytes or eggs. Fo llowing fertilization, the intensity of the Xenopus AU-F1 and AU-F3 pr oteins increased considerably and a new protein, corresponding to SV80 AU-F2, was also detected. Cyclohexamide applied either at the morula or at the early blastula stages reduced the intensity of the AU-bindin g factors, while actinomycin D did not, indicating that the levels of these factors during these stages are regulated posttranscriptionally. In contrast, application of each of these metabolic inhibitors at the late blastula stage increased the intensity of the AU-binding protein s. The possible function of these AU-binding factors in regulating the expression and half life of AU-rich mRNAs is discussed.