A double-stranded RNA unwinding activity introduces structural alterations by means of adenosine to inosine conversions in mammalian cells and Xenopus eggs.

Amphibian eggs and embryos as well as mammalian cells have been reported to contain an activity that unwinds double-stranded RNA. We have now found that adenosine residues have been modified in the RNA products of this unwinding activity. Although the modified RNA remains double-stranded, the modification causes the RNA to be susceptible to single-strand-specific RNase and to migrate as a retarded smear on a native polyacrylamide electrophoresis gel. The modification is specific for double-stranded RNA. At least 40% of the adenosine residues can be modified in vitro in a given random sequence RNA molecule. By using standard two-dimensional TLC and HPLC analyses, the modified base has been identified as inosine. Mismatched base-pairing between inosine and uridine appears to be responsible for the observed characteristics of the unwound RNA. The biological significance of this modifying activity and also of the modified double-stranded RNA is discussed.