The present invention features tobacco nicotine demethylase nucleic acid and amino acid sequences, tobacco plants and plant components containing such sequences, including tobacco plants and plant components having reduced expression or altered enzymatic activity of nicotine demethylase, methods of use of nicotine demethylase sequences to create plants having altered levels of nornicotine or N'-nitrosonornicotine ("NNN") or both relative to a control plant, as well as tobacco articles having reduced levels of nornicotine or NNN.