Abstract: The disclosure provides methods and related kits, reagents, and systems for selectively deaminating unmethylated cytosine residues in nucleic acid molecules. In some embodiments, the methods and related kits, reagents, and systems are applied for methods of detecting and/or mapping methylated cytosine residues in nucleic acids. The nucleic can be RNA or DNA. Some embodiments include contacting the polynucleic acid with a bacterial cytosine deaminase, for example DddA or SsdA, or functional fragments or derivatives thereof. Representative DddA and SsdA have sequences set forth in SEQ ID NOS:1 and 2, respectively. The bacterial cytosine deaminases of the disclosure are sensitive to methylation and, thus, deaminate only unmethylated cytosines to provide a cytosine to uracil conversion. The conversion can be detected as a C•G-to-T•A transitions in subsequent sequencing analysis.