Evidence is presented that a homogeneous cytoplasmic species known as
7S RNA is the only abundant
RNA in uninfected HeLa cells which can form strong hybrids with the dominant family of middle repetitive DNA sequences in the human genome. These DNA sequences are known collectively as the Alu family, because most of them share a common Alu I restriction site. When purified
7S RNA was hybridized to three different genomic clones containing Alu family DNA sequences, a specific region (or regions) comprising at most half the RNA sequence was protected from mild digestion with T1
ribonuclease; moreover, the hybrids between
7S RNA and cloned Alu family
DNA wer imperfect, since T1 RNAase was able to nick the protected
7S RNA sequences under conditions where a true
RNA:
DNA duplex would have been resistant. This suggests that
7S RNA is encoded either by a small subset of the 300,000 Alu family sequences in the human genome or by an entirely different family of genes. The sequence of
7S RNA has been highly conserved through recent evolution, and in both avian and murine cells the
RNA is selectively incorporated into oncornavirus particles during productive
infection. The cellular function of
7S RNA is unknown.