This invention provides polypeptides having lyase activity, polynucleotides encoding these polypeptides, and methods of making and using these polynucleotides and polypeptides. In one aspect, the invention is directed to polypeptides having ammonia lyase activity, e.g., phenylalanine ammonia lyase, tyrosine ammonia lyase and/or histidine ammonia lyase activity, including thermostable and thermotolerant activity, and polynucleotides encoding these enzymes, and making and using these polynucleotides and polypeptides. The polypeptides of the invention can be used in a variety of pharmaceutical, agricultural and industrial contexts. ##STR00001## X.dbd.NO.sub.2, Cl, Br, F, NH.sub.2, OH, H, alkyl at one or several o, m, and p positions R.dbd.H or alkyl