Catalyzes the alpha,gamma-elimination of L-methionine to produce methanethiol, 2-oxobutanoate and ammonia. H2O + L-methionine = 2-oxobutanoate + methanethiol + NH4(+) Homotetramer; dimer of active dimers. Mutant cannot make methanethiol (MeSH) or dimethylsulfide (DMS) from either minimal medium or medium supplemented with methionine. However, it can make DMS when exogenous MeSH is added. Belongs to the trans-sulfuration enzymes family. L-methionine gamma-lyase subfamily.