Penicillin resistance development in enterococci has been associated with overproduction of a low-affinity penicillin-binding protein (PBP) that is a normal component of the PBP pattern of these bacteria and is apparently able to substitute the functions of the other PBPs. In resistant mutants of Enterococcus hirae ATCC 9790 the low-affinity PBP (PBP5) overproduction was associated with a deletion in a genetic element, located 1 kb upstream of the pbp5 gene, which negatively controlled PBP5 synthesis. Hypersusceptibility to penicillin was associated with a point mutation in the pbp5 gene, which causes premature termination of translation. Structural homologies between low-affinity PBPs of the different enterococcal species have been suggested by cross-reactivity of antibodies raised against E. hirae PBP5 with PBP5 of Enterococcus faecium and Enterococcus faecalis. Acquisition of a high-level ampicillin resistance in E. faecium was associated with overproduction of PBP5, which, compared with PBP5 of moderately resistant strains, appeared to be modified in its penicillin-binding capability. The modified phenotype of PBP5 was found to be associated to some amino acid substitutions in the region between the SDN and KTG motifs. In particular, the substitution converting a polar residue (T) in a nonpolar one (A or I) could play an important role in remodeling the penicillin-binding domain and determining the decrease in penicillin affinity.