A New Class of Cobalamin Transport Mutants (btuF) Provides Genetic Evidence for a Periplasmic Binding Protein in Salmonella typhimurium

No periplasmic binding protein has been demonstrated for the ATP-binding cassette (ABC)-type cobalamin transporter BtuCD. New mutations (btuF) are described that affect inner-membrane transport. The BtuF protein has a signal sequence and resembles the periplasmic binding proteins of several other ABC transporters.