Modulation of MccB synthesis in the presence of homocysteine or cystine in various mutants. The homocysteine γ-lyase activity of MccB was detected on zymogram. A total of 100 μ g of crude extracts were charged on a native polyacrylamide gel (12%). The release of sulfide from homocysteine due to homocysteine γ-lyase activity was detected by the precipitation of insoluble PbS. The proteins were extracted from strain 13 (l and 2), TS140 (vrr::erm) (3 and 4), TS186 (virX ::erm) (5 and 6) and TS133 (virR::tet) (7 and 8) after growth in the presence of 0.5 mM cystine (1, 3, 5 and 7) or 1 mM homocysteine (2, 4, 6 and 8).