Growth of Lactococcus lactis in milk depends on the utilization of extracellular peptides. Up to now, oligopeptide uptake was thought to be due only to the ABC transporter Opp. Nevertheless, analysis of several Opp-deficient L. lactis strains revealed the implication of a second oligopeptide ABC transporter, the so-called Opt system. Both transporters are expressed in wild-type strains such as L. lactis SK11 and Wg2, whereas the plasmid-free strains MG1363 and IL-1403 synthesize only Opp and Opt, respectively. The Opt system displays significant differences from the lactococcal Opp system, which made Opt much more closely related to the oligopeptide transporters of streptococci than to the lactococcal Opp system: (i) genetic organization, (ii) peptide uptake specificity, and (iii) presence of two oligopeptide-binding proteins, OptS and OptA. The fact that only OptA is required for nutrition calls into question the function of the second oligopeptide binding protein (Opts). Sequence analysis of oligopeptide-binding proteins from different bacteria prompted us to propose a classification of these proteins in three distinct groups, differentiated by the presence (or not) of precisely located extensions.The ABC transporter family is characterized by a high affinity for their substrates, and it uses ATP hydrolysis to drive a unidirectional accumulation of solutes into the bacterial cytoplasm. A structural similarity of five proteins is generally observed (10). The substrate-binding protein specifically captures the substrate and is therefore considered one of the determinants of transport specificity (5). The binding protein is located in the periplasm of gram-negative bacteria. In gram-positive bacteria, it is present as a lipoprotein anchored to the cell membrane via an N-terminal moiety. The structure of the protein consists of three distinct domains. Two distinct lobes are connected by a flexible hinge (29), which allows two different conformations: an open, unliganded form with a high affinity for the substrate, and a closed, liganded state, in which the substrate is entrapped into the binding protein (the socalled Venus flytrap model). The closed liganded state is characterized by a low affinity for the substrate. The closed liganded binding protein delivers the substrate to the translocon, which consists of two integral transmembrane proteins and two ATP-binding proteins located on the cytoplasmic side of the membrane. According to a recent model (38), a consequence of the substrate binding would be the transmission of a signal to the ATP subunits of the ABC system. This signal results in an increased affinity of the ATP-binding proteins for ATP, which in turn leads to the opening of the transmembrane channel and the simultaneous release of the solute from the binding protein to the opened pore (6).In gram-positive bacteria, uptake of peptides by ABC transport systems is involved in nitrogen nutrition of the organism, as has been well established for lactic acid bacteria, for instance (11,22), or in the regula...