Structural analysis of the L-methionine γ-lyase gene from Pseudomonas putida

The gene encoding L-methionine γ-lyase from Pseudomonas putida was cloned and the primary structure of the enzyme was deduced from its nucleotide sequence. The L-methionine γ-lyase gene was expressed in Escherichia coli. The amino acid sequences of BrCN-digested peptides agreed with the corresponding parts of the L-methionine γ-lyase sequence determined from the gene structure. The polypeptide is composed of 398 amino acid residues with a calculated molecular weight of 42,626, corresponding to the subunit of the homotetrameric enzyme. The deduced amino acid sequence of L-methionine γ-lyase only showed extensive homology with other well known α,γ-elimination and/or γ-replacement pyridoxal 5'-phosphate-dependent enzymes, such as cystathionine γ-lyase, cystathionine γ-synthase, and O-acetylhomoserine O-acetylserine sulfhydrylase, that participate in the biosynthesis of sulfur amino acids. However, the deduced essential cysteine residue of L-methionine γ-lyase was not conserved in these enzymes. We confirmed the presence of a part of an open reading frame in the 3'-flanking region of the L-methionine γ-lyase gene, which showed high homology with the N-tenninal region of pyruvate dehydrogenase (lipoamide) from E. coli, suggesting that it participates in the degradative pathway for L-methionine together with L-methionine γ-lyase.