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Complementary change in cis determinants and trans factors in the evolution of an mRNP stability complex.

RNA‐protein (RNP) complexes play significant roles in the fate and expression of mRNAs. The prolonged half‐life of human alpha‐globin mRNA, a major determinant of normal erythroid differentiation, is dependent on the assembly of a sequence‐specific 3′‐untranslated region (3′UTR) RNP (alpha‐complex). We demonstrate that the stability of murine alpha‐globin mRNA is controlled by a parallel mechanism. Unexpectedly, however, the respective 3′UTR RNP complexes that stabilize the h(alpha)‐ and m(alpha)‐globin mRNAs differ in structure. While the cis determinants in both species are encoded in polypyrimidine tracks, the human determinant is C‐rich (CCUCC motif) while the mouse alpha‐3′UTR consists of an equal distribution of Cs and Us (CCUUCU motif). The protein components of the corresponding human and murine alpha‐complexes differ in a complementary manner: the previously described 39 kDa poly(C) binding protein (PCBP) present in the human alpha‐complex is replaced in the mouse alpha‐complex by a 48 kDa cytoplasmic poly(CU) binding protein (CUBP). These results reveal that drift in the primary sequences of the alpha‐globin mRNA 3′UTR polypyrimidine tracks in a comparison between mouse and human is paralleled by an alteration in the composition of the corresponding trans‐acting components. Surprisingly, these structurally distinct complexes appear to perform the identical function of stabilizing the corresponding alpha‐globin mRNAs.