A directed alteration of ribonuclease specificity. Hydrolysis of polyribonucleotides containing modified cytosine bases.

The action of ribonucleases on poly and oligoribonucleotides containing cytosine bases modified by methoxyamine and bisulphite was examined. Resistance of phosphodiester bonds in (Cp)nXp (where n greater than or equal to 1 and X stands for A, G or U) to T2 RNase hydrolysis was observed if substrates were modified chemically. The phenomenon formed the basis for isolation of (Cp)nXp blocks as an additional tool in sequence investigations. After modification of cytosine pancreatic RNase was unable to hydrolyse (Cp)nUp blocks. Therefore the specificity of pyrimidyl RNase may be narrowed to 'uridyl RNase'.