A non-natural methanogenic archaea, wherein the non-natural methanogenic archaea metabolizes a CO x substrate, optionally in the presence of H 2, to produce methionine at a higher level than a parent methanogenic archaea and wherein the non-natural methanogenic archaea expresses at least one genetically engineered endogenous polypeptide or at least one exogenous polypeptide chosen from: (a) a polypeptide having an amino acid sequence as set forth in at least one of SEQ ID NOS.:4, 8, or 32; (b) a MMP1359 protein of SEQ ID NO: 3 that is mutated, wherein the protein has a mutation at residue D439, wherein the residue numbering corresponds to residue positions of MMP1359 protein from Methanococcus maripaludis S2 DSM14266; or (c) an ortholog of SEQ ID NO:3 that is mutated, wherein the ortholog has a mutation at residue D439, wherein the residue numbering corresponds to residue positions of MMP1359 protein from Methanococcus maripaludis S2 DSM 14266; or (d) a MMP1358 protein of SEQ ID NO: 7 that is mutated, wherein the protein has a mutation at residue G114, wherein the residue numbering corresponds to residue positions of MMP1358 protein from Methanococcus maripaludis S2 DSM14266; or (e) an ortholog of SEQ ID NO:7 that is mutated, wherein the ortholog has a mutation at residue G114, wherein the residue numbering corresponds to residue positions of MMP1358 protein from Methanococcus maripaludis S2 DSM14266; or (f) a C2A1821 protein of SEQ ID NO: 30 that is mutated, wherein the protein has a mutation at residue S489, wherein the residue numbering corresponds to residue positions of C2A1821 protein from Methanosarcina acetivorans C2A; or (g) an ortholog of SEQ ID NO:30 that is mutated, wherein the ortholog has a mutation at residue S489, wherein the residue numbering corresponds to residue positions of C2A1821 protein from Methanosarcina acetivorans C2A. (a) a polypeptide having an amino acid sequence as set forth in at least one of SEQ ID NOS.:4, 8, or 32; (b) a MMP1359 protein of SEQ ID NO: 3 that is mutated, wherein the protein has a mutation at residue D439, wherein the residue numbering corresponds to residue positions of MMP1359 protein from Methanococcus maripaludis S2 DSM14266; or (c) an ortholog of SEQ ID NO:3 that is mutated, wherein the ortholog has a mutation at residue D439, wherein the residue numbering corresponds to residue positions of MMP1359 protein from Methanococcus maripaludis S2 DSM 14266; or (d) a MMP1358 protein of SEQ ID NO: 7 that is mutated, wherein the protein has a mutation at residue G114, wherein the residue numbering corresponds to residue positions of MMP1358 protein from Methanococcus maripaludis S2 DSM14266; or (e) an ortholog of SEQ ID NO:7 that is mutated, wherein the ortholog has a mutation at residue G114, wherein the residue numbering corresponds to residue positions of MMP1358 protein from Methanococcus maripaludis S2 DSM14266; or (f) a C2A1821 protein of SEQ ID NO: 30 that is mutated, wherein the protein has a mutation at residue S489, wherein the residue numbering corresponds to residue positions of C2A1821 protein from Methanosarcina acetivorans C2A; or (g) an ortholog of SEQ ID NO:30 that is mutated, wherein the ortholog has a mutation at residue S489, wherein the residue numbering corresponds to residue positions of C2A1821 protein from Methanosarcina acetivorans C2A.