A new chemical probe for single-stranded RNA

The reaction of cytidine with sodium bisulfite and methylamine gives transamination to N⁴-methylcytidine and deamination to uridine. At pH 7.4, however, transamination takes place exclusively. This reaction was applied to yeast RNA (7 days, 37°) and afforded the specific conversion of 54% of the cytidine residues to N⁴-methylcytidine residues. The same procedure converted up to 95% of the cytidines of polycytidylic acid to N⁴-methylcytidines. A complex of polycytidylic acid with polyinosinic acid was, however, totally resistant to the reaction. Because of its cytosine specificity, single-strand specificity, possibility of use of radioactive methylamine, mild conditions, and ease of analysis, this reaction seems ideally suited as a chemical probe for single-stranded regions in RNA.