Glycolipid composition of the heterocyst envelope of Anabaena sp. PCC 7120 is crucial for diazotrophic growth and relies on the UDP-galactose 4-epimerase HgdA

Abstract

The nitrogenase complex in the heterocysts of the filamentous freshwater cyanobacterium Anabaena sp. PCC 7120 fixes atmospheric nitrogen to allow diazotrophic growth. The heterocyst cell envelope protects the nitrogenase from oxygen and consists of a polysaccharide and a glycolipid layer that are formed by a complex process involving the recruitment of different proteins. Here we studied the function of the putative nucleoside-diphosphate-sugar epimerase HgdA, which along with HgdB and HgdC is essential for deposition of the glycolipid layer and growth without a combined nitrogen source. Using site-directed mutagenesis and single homologous recombination approach, we performed a thoroughly functional characterization of HgdA and confirmed that the glycolipid layer of the hgdA mutant heterocyst is aberrant as shown by transmission electron microscopy and chemical analysis. The hgdA gene was expressed during late stages of the heterocyst differentiation. GFP-tagged HgdA protein localized inside the heterocysts. The purified HgdA protein had UDP-galactose 4-epimerase activity in vitro. This enzyme could be responsible for synthesis of heterocyst-specific glycolipid precursors, which could be transported over the cell wall by the ABC transporter components HgdB/HgdC.\n\nImportanceThe phototrophic multicellular bacterium Anabaena sp. PCC 7120 is an important model organism for investigations of critical processes such as photosynthesis, nitrogen fixation, and cell differentiation in prokaryotes. The cyanobacterium utilizes specific cells, called heterocysts, to fix atmospheric nitrogen. Heterocyst function requires a specific cell envelope. We focused on unstudied aspects of heterocyst envelope formation and revealed the role of the UDP-galactose 4-epimerase HgdA in this process.