Genes for two small cytoplasmic Ro RNAs are adjacent and appear to be single-copy in the human genome.

Anti-Ro autoantibodies precipitate several small cytoplasmic ribonucleoproteins from mammalian cells. The RNA components of these particles, designated hY1-hY5 in human cells and mY1 and mY2 in mouse cells, are about 100 nucleotides long. We have analyzed a genomic clone that appears to contain true RNA-coding regions for two of the human Ro RNAs, hY1 and hY3. These RNAs exhibit many sequence and secondary structure homologies, both with each other and with the recently sequenced hY5 RNA. The hY2 RNA is a slightly truncated form of hY1; several shorter versions of hY3 are also detected in cell extracts and immunoprecipitates. The human hY1 and hY3 genes cross-hybridize with the mouse Ro RNAs, mY1 and mY2, respectively; we show that the mouse Ro RNAs are exclusively contained in Ro particles. The genes for hY1 and hY3 are transcribed in vitro by RNA polymerase III. In contrast with all other mammalian class III genes described, they appear to be present as single copies in the human genome.