POLY(A) METABOLISM IN XENOPUS-LAEVIS EMBRYOS - SUBSTRATE-SPECIFIC ANDDEFAULT POLY(A) NUCLEASE ACTIVITIES ARE MEDIATED BY 2 DISTINCT COMPLEXES

The metabolism of the poly(A) tail is a process important for the tran slational regulation of maternal mRNAs in Xenopus laevis oocytes and e arly embryos. Two poly(A) nuclease (PAN) activities have been describe d in Xenopus embryo or activated egg extracts (Legagneux et al (1995) RNA I, 1001-1008). These activities (default PAN and EgPAN) are distin guishable by their deadenylation kinetics and their substrate specific ities. In this report, we show that these activities display different sensitivities to biochemical treatments. Urea and, to a lesser extent , spermidine, inhibit EgPAN at concentrations which have no effect on default PAN. Heparin activates default PAN but inhibits EgPAN. When ex tracts are fractionated by ultracentrifugation, the default activity i s recovered in one unique fraction, whereas two fractions must be comb ined to reconstitute the EgPAN activity. Moreover, these two deadenyla tion activities are separable by size exclusion chromatography under n ative conditions. We conclude that these two deadenylation activities are mediated by two protein complexes.