Inter-RNA homology and possible roles of small RNAs

The nucleotide sequence of a segment of U1 and U3b small RNAs (sRNAs) is shown to have a high complementarity with the nucleotide sequence of a part of the leader region of almost all eukaryotic genes studied so far. The complementary region of U3b is located in the unpaired segment of the secondary structure of U3b constructed by Reddy et al. (1979). A similar complementarity is also observed between these RNAs and the leader regions of eukaryotic viruses, but the complementary region is not always identical with that for eukaryotic genes. Complementarity is also observed between the 3′ end of 18S rRNA and a segment of U1 or U3b which is almost contiguous to the region complementary with mRNA. These observations suggest that U1 and U3b may be involved in mRNA processing and transport in the nucleus or in translation in the cytoplasm. In addition to U1 and U3b, another sRNA, i.e., 4.5S RNA_I, is shown to have segments which are homologous to the Hogness box of the flanking region of gene and the Proudfoot-Brownlee (PB) box of mRNA near the poly(A) attachment site. The two segments which are complementary with these boxes are located almost contiguously on a co-joined loop of the secondary structure of 4.5S RNA_I constructed by Ro-Choi et al. (1972). Since the Hogness box and PB box are both considered as a recognition site by the RNA polymerase, it is possible that 4.5S RNA_I is involved in mediating gene transcription.