Abstract
Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that can use inorganic compounds such as nitrate or ammonium as nitrogen sources. In the absence of combined nitrogen, it can fix N2 in differentiated cells called heterocysts. Anabaena also shows substantial activities of amino acid uptake, and three ABC-type transporters for amino acids have been previously characterized. Seven new loci encoding predicted amino acid transporters were identified in the Anabaena genomic sequence and inactivated. Two of them were involved in amino acid uptake. Locus alr2535-alr2541 encodes the elements of a hydrophobic amino acid ABC-type transporter that is mainly involved in the uptake of glycine. ORF all0342 encodes a putative transporter from the dicarboxylate/amino acid:cation symporter (DAACS) family whose inactivation resulted in an increased uptake of a broad range of amino acids. An assay to study amino acid release from Anabaena filaments to the external medium was set up. Net release of the alanine analogue α-aminoisobutyric acid (AIB) was observed when transport system N-I (a hydrophobic amino acid ABC-type transporter) was engaged in the uptake of a specific substrate. The rate of AIB release was directly proportional to the intracellular AIB concentration, suggesting leakage from the cells by diffusion.
Highlights
Cyanobacteria are a group of prokaryotic microorganisms characterized by their ability to fix CO2 at the expense of oxygenic photosynthesis
Sucrose is a universal vehicle of reduced carbon in plants [8], and it seems to be important in cyanobacteria [9]; alanine can be an immediate source of reducing power in the heterocyst, where it is metabolized by alanine dehydrogenase [6]; and glutamate is mainly synthesized in vegetative cells by glutamate synthase and used in the heterocysts by glutamine synthetase to produce glutamine [2,10]
Open Reading Frames (ORF) alr2535 would encode a protein of 268 amino acid residues with homology to the periplasmic substrate-binding protein BraC of the branched-chain amino acid transport system of Pseudomonas aeruginosa [42]
Summary
Cyanobacteria are a group of prokaryotic microorganisms characterized by their ability to fix CO2 at the expense of oxygenic photosynthesis. Heterocyst-forming cyanobacteria present two cell types: vegetative cells that perform oxygenic photosynthesis and heterocysts that carry out N2 fixation [1]. These specialized cells rely on each other: heterocysts require photosynthate that is provided by vegetative cells, and heterocysts provide vegetative cells with fixed nitrogen [2,3]. Heterocysts accumulate cyanophycin, a non-ribosomically synthesized peptide made of aspartate and arginine (multi-L-arginyl-poly [L-aspartic acid]) [11]
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