Identification and characterization of a strain-dependent cystathionine β/γ-lyase in Lactobacillus casei potentially involved in cysteine biosynthesis

The trans -sulfuration pathways allow the interconversion of cysteine and methionine with the intermediary formation of cystathionine and homocysteine. The genome database of Lactobacillus casei ATCC 334 provides evidence that this species cannot synthesize cysteine from methionine via the trans -sulfuration pathway. However, several L. casei strains use methionine as the sole sulfur source, which implies that these strains can convert methionine to cysteine. Cystathionine synthases and lyases play a crucial role in the trans -sulfuration pathway. By applying proteomic techniques, we have identified a protein in cell-free extracts of L. casei , which showed high homology to a gene product encoded in the genome of Lactobacillus delbrueckii ssp. bulgaricus, Streptococcus thermophilus and Lactobacillus helveticus but not in the genome of L. casei ATCC 334. The presence of the gene was only found in strains able to grow on methionine as the sole sulfur source. Moreover, two gene variants were identified. Both gene variants were cloned and expressed heterologously in Escherichia coli . The recombinant enzymes exhibited cystathionine lyase activity in vitro and also cleaved cysteine, homocysteine and methionine releasing volatile sulfur compounds.