Fine structure mapping and complementation studies of the metD methionine transport system in Salmonella typhimurium

A fine structure deletion map of the metD region of the chromosome of Salmonella typhimurium responsible for a high-affinity methionine transport system has been constructed. Complementation tests involving the introduction of metD+DNA contained in a pUC8 vector into metD strains indicated the presence of four complementation groups in the metD region. This suggested that the methionine system belongs to the osmotic shock-sensitive class of transport system, and therefore should possess a periplasmic methionine-binding protein and several membrane proteins. But a deletion mutation covering all known metD point mutations did not affect the level of a methionine binding activity in osmotic shock fluids, suggesting either that the deletion did not extend into the gene encoding the binding protein, or that the binding activity is not associated with the metD system. Possible reasons for the failure to isolate mutations in the gene for the binding protein are discussed.