Thirty-eight ATP-binding cassette (ABC) protein constituents of bacterial extracytoplasmic receptor-dependent nutrient uptake systems, including one homologous chloroplast protein were analysed for sequence conservation and phylogenetic relatedness. The proteins were generally found to cluster in accordance with the clustering patterns previously observed for the extracytoplasmic receptors and the transmembrane channel-forming constituents of these permeases. The results suggest that these transport systems evolved from a single primordial system with minimal shuffling of the three dissimilar protein constituents of the systems.