Evolutionary conservation of the U7 small nuclear ribonucleoprotein in Drosophila melanogaster.

The U7 snRNP involved in histone RNA 3' end processing is related to but biochemically distinct from spliceosomal snRNPs. In vertebrates, the Sm core structure assembling around the noncanonical Sm-binding sequence of U7 snRNA contains only five of the seven standard Sm proteins. The missing Sm D1 and D2 subunits are replaced by U7-specific Sm-like proteins Lsm10 and Lsm11, at least the latter of which is important for histone RNA processing. So far, it was unknown if this special U7 snRNP composition is conserved in invertebrates. Here we describe several putative invertebrate Lsm10 and Lsm11 orthologs that display low but clear sequence similarity to their vertebrate counterparts. Immunoprecipitation studies in Drosophila S2 cells indicate that the Drosophila Lsm10 and Lsm11 orthologs (dLsm10 and dLsm11) associate with each other and with Sm B, but not with Sm D1 and D2. Moreover, dLsm11 associates with the recently characterized Drosophila U7 snRNA and, indirectly, with histone H3 pre-mRNA. Furthermore, dLsm10 and dLsm11 can assemble into U7 snRNPs in mammalian cells. These experiments demonstrate a strong evolutionary conservation of the unique U7 snRNP composition, despite a high degree of primary sequence divergence of its constituents. Therefore, Drosophila appears to be a suitable system for further genetic studies of the cell biology of U7 snRNPs.