The nuclear 5S RNAs from chicken, rat and man. U5 RNAs are encoded by multiple genes

Preparations of chicken, rat and human nuclear 5S RNA contain two sets of molecules. The set with the lowest electrophoretic mobility (5Sa) contains RNAs identical or closely related to ribosomal 5S RNA from the corresponding animal species. In HeLa cells and rat brain, we only detected an RNA identical to the ribosomal 5S RNA. In hen brain and liver, we found other species differing by a limited number of substitutions. The results suggest that mutated 5S genes may be expressed differently according to the cell type. The set with the highest mobility corresponds to U5 RNA. In both rat brain and HeLa cells, U5 RNA was found to be composed of 4 and 5 different molecules respectively (U5A, U5B1-4) differing by a small number of substitutions or insertions. In hen brain, no U5B was detected but U5A' differing from U5A by the absence of the 3'-terminal adenosine. All the U5 RNAs contain the same set of modified nucleotides. They also have the same secondary structure which consists of two hairpins joined together by a 17 nucleotide long single-stranded region. The 3' half of the molecule has a compact conformation. Together, the results suggest that U5 RNAs are transcribed from a multigene family and that mutated genes may be expressed as far as secondary structure is conserved. The conformation of U5 RNA is likely to be related to its function and it is of interest to mention that several similarities of structure are found between U5 and U1A RNA.