SCHISTOSOME SATELLITE DNA ENCODES ACTIVE HAMMERHEAD RIBOZYMES

Using a computer program designed to search for RNA structural motifs in sequence databases, we have found a hammerhead ribozyme domain enco ded in the Sm alpha repetitive DNA of Schistosoma mansoni. Transcripts of these repeats are expressed as long multimeric precursor RNAs that cleave in vitro and in vivo into unit-length fragments. This RNA doma in is able to engage in both cis and trans cleavage typical of the ham merhead ribozyme. Further computer analysis of S. mansoni DNA identifi ed a potential trans cleavage site in the gene coding for a synaptobre vin-like protein, and RNA transcribed from this gene was efficiently c leaved by the Sma! ribozyme in vitro. Similar families of repeats cont aining the hammerhead domain were found in the closely related Schisto soma haematobium and Schistosomatium douthitti species but were not pr esent in Schistosoma japonicum or Heterobilharzia americana, suggestin g that the hammerhead domain was not acquired from a common schistosom e ancestor.