A TolC-like protein is required for heterocyst development in Anabaena sp. strain PCC 7120

LMU, Dep. of Biology I, VW-Research Group, Menzinger Str. 67, 80638 München, Germany, Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, 93040 Regensburg, Germany, Instituto de Bioquímica Vegetal y Fotosíntesis, CSIC and Universidad de Sevilla, Américo Vespucio 49, E-41092 Seville, Spain

^* To whom correspondence should be addressed. Email: iris.maldener{at}uni-tuebingen.de.

	Abstract

The filamentous cyanobacterium Anabaena sp. strain PCC 7120 forms heterocysts in a semi-regular pattern when grown on N₂ as sole nitrogen source. The transition from vegetative cells to heterocysts requires marked metabolic and morphologic changes. We show that a trimeric pore-forming outer membrane ß-barrel protein of the TolC family, Alr2887, is upregulated in developing heterocysts and essential for diazotrophic growth. Mutants defective in alr2887 did not form the specific glycolipid layer of the heterocyst cell wall, which is necessary to protect nitrogenase from external oxygen. Comparison of the glycolipid content of wild-type and mutant cells indicated that the protein is not involved in the synthesis of glycolipids but might instead serve as an exporter for the glycolipid moieties or enzymes involved in glycolipid attachment. We propose that Alr2887 together with an ABC transporter like DevBCA is part of a protein export system essential for the assembly of the heterocyst glycolipid layer. We designate the alr2887 gene hgdD (from heterocyst glycolipid deposition protein).