Genetic and lipidomic analyses suggest that Nostoc punctiforme , a plant-symbiotic cyanobacterium, does not produce sphingolipids.

Sphingolipids, a class of amino-alcohol-based lipids, are well characterized in eukaryotes and in some anaerobic bacteria. However, the only sphingolipids so far identified in cyanobacteria are two ceramides (i.e. an acetylsphingomyelin and a cerebroside), both based on unbranched, long-chain base (LCB) sphingolipids in and , respectively. The first step in sphingolipid biosynthesis is the condensation of l-serine with palmitoyl-CoA to produce 3-keto-diyhydrosphingosine (KDS). This reaction is catalyzed by serine palmitoyltransferase (SPT), which belongs to a small family of pyridoxal phosphate-dependent α-oxoamine synthase (AOS) enzymes. Based on sequence similarity to molecularly characterized bacterial SPT peptides, we identified a putative SPT (Npun_R3567) from the model nitrogen-fixing, plant-symbiotic cyanobacterium, strain PCC 73102 (ATCC 29133). Gene expression analysis revealed that is induced during late-stage diazotrophic growth in . However, Npun_R3567 could not produce the SPT reaction product, 3-keto-diyhydrosphingosine (KDS), when heterologously expressed in . This agreed with a sphingolipidomic analysis of cells, which revealed that no LCBs or ceramides were present. To gain a better understanding of Npun_R3567, we inferred the phylogenetic position of Npun_R3567 relative to other bacterial AOS peptides. Rather than clustering with other bacterial SPTs, Npun_R3567 and the other cyanobacterial BioF homologues formed a separate, monophyletic group. Given that does not appear to possess any other gene encoding an AOS enzyme, it is altogether unlikely that is capable of synthesizing sphingolipids. In the context of cross-kingdom symbiosis signalling in which sphingolipids are emerging as important regulators, it appears unlikely that sphingolipids from play a regulatory role during its symbiotic association with plants.