RhsF has been identified as the fourth member of the RhsABCF subfamily
of genetic elements. This new element is found in Escherichia coli EC
OR-50 and several other strains but not in strain K-12. A novel featur
e of RhsF is that it represents a new arrangement of components previo
usly uniquely associated with RhsA and RhsC of strain K-12.