A polyketide synthase HglEA, but not HglE2, synthesizes heterocyst specific glycolipids in Anabaena sp. PCC 7120

Heterocysts are the specialized cells for nitrogen fixation in some filamentous cyanobacteria. To protect the oxygen labile nitrogen fixing enzyme, nitrogenase, heterocysts keep their inner environment microoxic by developing layers of barrier on the outside of their outer membranes. Heterocyst specific glycolipids (Hgls) are constituents of the layer of barrier and amphipathic compounds, synthesized from a very long chain fatty alcohol as a hydrophobic tail and a sugar as a polar head. In the model heterocystous cyanobacterium Anabaena sp. PCC 7120, Hgls are made of fatty alcohol with 26 carbons and a glucose, linked by an ether bond in alpha configuration. The fatty alcohol is synthesized via reactions of a polyketide synthase, HglEA. In Anabaena sp. PCC 7120, another polyketide synthase HglE2 shared more than 50% identity in an amino acid sequence with HglEA and is expected to be involved in Hgls synthesis. However, no direct evidence has been reported. Here, we experimentally show that HglEA is the contributor of Hgls synthesis, and that HglE2 is not involved in the development of the heterocyst specific glycolipid layer.