AMINO-ACID-TRANSPORT SYSTEMS REQUIRED FOR DIAZOTROPHIC GROWTH IN THE CYANOBACTERIUM ANABAENA SP STRAIN PCC-7120

Uptake of 16 amino acids by the filamentous, heterocyst-forming cyanob acterium Anabaena sp. strain PCC 7120 was characterized with regard to kinetic parameters of transport, intracellular accumulation of the tr ansported amino acids, and sensitivity of the transport process to ene rgy metabolism inhibitors. Mutants resistant to certain toxic analogs of some amino acids were isolated that were impaired in amino acid tra nsport. Results obtained in this study together with those reported pr eviously (A. Herrero and E. Flores, J. Biol. Chem. 265:3931-3935, 1990 ), suggest that there are at least five amino acid transport systems i n strain PCC 7120: one high-affinity, active system for basic amino ac ids; one low-affinity, passive system for basic amino acids; two high- affinity, active systems with overlapping, but not identical, specific ities for neutral amino acids; and one putative system for acidic amin o acids. Some of the amino acid transport mutants were impaired in dia zotrophic growth. These mutants were unable to develop a normal percen tage of heterocysts and normal nitrogenase activity in response to nit rogen stepdown. Putative roles for the amino acid transport systems in uptake of extracellular amino acids, recapture of amino acids that ha ve leaked from the cells, and intercellular transfer of amino acids in the filaments of Anabaena sp. strain PCC 7120 are discussed.