Members of the OPN family have been found in all prokaryotic genera. Oligopeptides constitute an important source of nutrients and several systems are involved also in cell-cell communication. Oligopeptide transporters have been implicated in the uptake of a class of opines such as agrocinopines, agropinic and mannopinic acids ( Pubmed : 8366042 ) . Nickel is an essential cofactor for a number of enzymatic reactions. The Nik system of E. coli provides Ni2+ ion for the anaerobic biosynthesis of hydrogenases and is similar in its composition and in the primary sequence of its components to the oligopeptide ABC transporters ( Pubmed : 7934931 ) . Nik importers appear to be more restricted in their distribution than oligopeptide transporters since homologues could be identified only in the genomes of Staphylococcus aureus, Bacillus halodurans and Deinococcus radiodurans.