An ABC Exporter is Essential for the Localisation of Envelope Material in Heterocysts of Cyanobacteria

Abstract

To perform oxygenic photosynthesis and fix dinitrogen simultaneously the filamentous cyanobacterium Anabaena sp. protects the extremely oxygen-sensitive nitrogenase by spatial separation of the two processes in two different cell types, the oxygen evolving vegetative cell and the N₂-f?xing heterocyst. A thick envelope, consisting of heterocyst-specifíc glycolipids and polysaccharides, forms outside the gram-negative cell wall to reduce the diffusion of gases into the heterocyst and to establish a microaerobic environment tolerated by the nitrogenase [1]. Transposon mutagenesis of Anabaena7120 was used to create mutants that are arrested in different stages of heterocyst development [2]. One of these mutants, M7, is able to fix dinitrogen under anaerobic conditions (Fix⁺), but not under aerobic conditions (Fox^-). This defect is due to an aberrant heterocyst envelope (Hen^-) and an arrest in protoplast maturation, visible by lack of heterocyst-specifíc oxidation of diamino benzidine (Dab^-) [3]. Maldener et al. (1994) [4] showed that the phenotype of mutant M7 was caused by transposition of Tn 1063a into the devAgene. Expression studies using luxABas reporter genes showed that about four hours after nitrogen stepdown devAexpression increases ca. 8 fold in whole filaments [4]. The deduced amino acid sequence of DevA shows striking similarity to the ATP-binding subunit of ABC transporters [5]. These are export and import systems common in bacteria and eukaryotes, catalysing an ATP-dependent transport of a great variety of substrates. Prokaryotic ABC transporters consist of several subunits that are organized in an operon [6].