NpmA methyltransferase, molecular model. Methyltransferase enzymes act to add methyl groups to nucleic acids such as DNA, a process called DNA methylation. This can silence and regulate genes without changing the genetic sequence. DNA methylation is being studied in relation to cancer. These enzymes can also act on RNA (ribonucleic acid), with NpmA methylating 16S rRNA. NpmA is being studied for its ability to confer antibiotic resistance (specifically resistance to aminoglycosides) in bacteria such as Escherichia coli. This enzyme has two protein chains (red and blue).