Abstract: | When MS2 RNA is heated at low pH in the presence of formaldehyde, a fast-sedimenting conformation is irreversibly formed. This species is homogeneous and stable at neutral pH. Its formation further requires Mg++ ions and low ionic strength. The most compact form sediments at 46S and is obtained after short reaction times at high temperature or after long reaction times at 35°C. Melting curves suggest that the specific acid conformation is not destabilized by the formaldehyde addition reaction. The pH at which this acid conformation is formed depends on the MgC12 concentration. At 10?2M MgCl2 the midpoint is pH 5.3. Removal of more than half of the bound formaldehyde has no effect on the compactness of the molecule, although most of the original secondary structure has not yet re-formed. |