It is known that Escherichia coli methionine mutants can grow on both enantiomers of methionine sulfoxide (met(o)), i.e., met-R-(o) or met-S-(o). indicating the presence of enzymes in E. coli that can reduce each of these enantiomers to methionine (met). Previous studies have identified two members of the methionine sulfoxide reductase (Msr) family of enzymes, MsrA and fSMsr, that could reduce free met-S-(o), but the reduction of free met-R-(o) to met has not been elucidated. One possible candidate is MsrB which is known to reduce met-R-(o) in proteins to met. However, free met-R-(o) is a very poor substrate for MsrB and the level of MsrB activity in E. coli extracts is very low. A new member of the Msr family (fRMsr) has been identified in E. coli extracts that reduces free met-R-(o) to met. Partial purification of FRMsr has been obtained using extracts from an MsrA/MsrB double mutant of E. coli. (C) 2002 Elsevier Science (USA). All rights reserved.