NUCLEAR POLYADENYLATION FACTORS RECOGNIZE CYTOPLASMIC POLYADENYLATIONELEMENTS

In the cytoplasm of oocytes and early embryos, addition of poly(A) to mRNAs can activate their translation. We demonstrate that despite many differences between poly(A) addition in the cytoplasm and nucleus, th ese two forms of polyadenylation may involve identical trans-acting fa ctors. Nuclear polyadenylation requires the sequence AAUAAA, the AAUAA A-binding cleavage and polyadenylation specificity factor (CPSF), and a poly(A) polymerase (PAP). We show that CPSF and PAP, purified from c alf thymus, exhibit the same sequence specificity observed in the cyto plasm during frog oocyte maturation, requiring both AAUAAA and a proxi mal U-rich sequence. The enhanced polyadenylation of RNAs containing U -rich sequences is caused by their increased affinity for CPSF. Frog n uclear polyadenylation factors display cytoplasmic sequence specificit y when dilute, suggesting that a difference in their concentrations in the nucleus and cytoplasm underlies the different sequence specificit ies in the two compartments. Because polyadenylation in extracts prepa red from oocytes before maturation is stimulated by addition of CPSF, the onset of polyadenylation during early development may be attributa ble to the activation or synthesis of a CPSF-like factor. We suggest t hat sequences upstream of AAUAAA that are required for cleavage and po lyadenylation of certain pre-mRNAs in the nucleus may be functionally equivalent to the upstream, U-rich sequences that function in the cyto plasm, enhancing CPSF binding. We propose that CPSF and PAP comprise a core polyadenylation apparatus in the cytoplasm of oocytes and early embryos.