A general L-amino acid permease (Aap) from the ABC transporter family, encoded by four genes (aapJ, Q, M, P), has been cloned and characterized in Rhizobium leguminosarum. It transports a wide range of L-amino acids but has a preference for those with polar side-chains. A single binding protein of broad specificity (AapJ) is required for transport of all solutes. Unusually for an ABC transporter, Aap has both high affinity for and supports high rates of solute uptake. Genes for putative amino acid permeases with broad specificity for amino acids also exist in Escherichia coli and probably in Pseudomonas fluorescens, although the permease from E. coli does not appear to be expressed. Aap is an active uptake system that also affects the efflux of a broad range of amino acids. Efflux can be measured both as the loss of an intracellular amino acid after the addition of an excess of a homologous or heterologous amino acid, and as excretion of intracellularly synthesized glutamate. Mutation of Aap prevented efflux of intracellular amino acids caused by the addition of an extracellular heterologous amino acid, while overexpression increased the rates of such efflux. Furthermore, excretion of glutamate synthesized inside the cell was reduced by 76% in an aap strain. All four gene products, including the binding protein (AapJ), appear to be needed for efflux. Aap from R. leguminosarum expressed in E. coli also promoted efflux on addition of an extracellular heterologous amino acid. These results indicate either that Aap regulates an efflux channel/transporter or that solute has access to the translocation pathway of Aap from both sides of the membrane.