Dual role for the RNA-binding domain of Xenopus laevis SLBP1 in histone pre-mRNA processing

The replication-dependent histone mRNAs end in a conserved 26-nt sequence t hat forms a stem-loop structure, This sequence is required for histone pre- mRNA processing and plays a role in multiple aspects of histone mRNA metabo lism, Two proteins that bind the 3' end of histone mRNA are found in Xenopu s oocytes, xSLBP1 is found in the nucleus, where it functions in histone pr e-mRNA processing, and in the cytoplasm, where it may control histone mRNA translation and stability. xSLBP2 is a cytoplasmic protein, inactive in his tone pre-mRNA processing, whose expression is restricted to oogenesis and e arly development, These proteins are similar only in their RNA-binding doma ins (RBD), A chimeric protein (1-2-1) in which the RED of xSLBP1 has been r eplaced with the RED of xSLBP2 binds the stem-loop with an affinity similar to the original protein. The 1-2-1 protein efficiently localizes to the nu cleus of the frog oocyte, but is not active in processing of histone pre-mR NA in vivo, This protein does not support processing in a nuclear extract, but inhibits processing by competing with the active SLBP by binding to the substrate, The 1-2-1 protein also inhibits processing of synthetic histone pre-mRNA injected into frog oocytes, but has no effect on processing of hi stone pre-mRNA transcribed from an injected histone gene, This result sugge sts that sequences in the RED of xSLBP1 give it preferential access to hist one pre-mRNA transcribed in vivo.