Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that can fix N(2) in differentiated cells called heterocysts. Anabaena open reading frames alr4167 and alr3187 encode, respectively, an ATPase subunit, BgtA, and a composite protein bearing periplasmic substrate-binding and transmembrane domains, BgtB, of an ABC-type high-affinity basic amino acid uptake transporter (Bgt). Open reading frame alr4167 is clustered with open reading frames alr4164, alr4165 and alr4166 that encode a periplasmic substrate-binding protein, NatF, and transmembrane proteins NatG and NatH respectively. The NatF, NatG, NatH and BgtA proteins constitute an ABC-type uptake transporter for acidic and neutral polar amino acids (N-II). The Bgt and N-II transport systems thus share the ATPase subunit, BgtA. These transporters together with the previously characterized ABC-type uptake transporter for proline and hydrophobic amino acids (N-I) account for more than 98% of the amino acid transport activity exhibited by Anabaena sp. strain PCC 7120. In contrast to N-I that is expressed only in vegetative cells, the Bgt and N-II systems are present in both vegetative cells and heterocysts. Whereas Bgt is dispensable for diazotrophic growth, N-II appears to contribute together with N-I to the diazotrophic physiology of this cyanobacterium.