Horizontal gene transfer of two cytoskeletal elements from a eukaryote to a cyanobacterium

The concept of horizontal gene transfer (HGT) as a potent evolutionary force has prompted the re-evaluation of prokaryotic genome shaping and speciation. Horizontal gene transfer enables prokaryotes to rearrange their genomes dynamically, facilitating responses to changing environmental conditions and invasions of new ecological niches. Here we report that the genome of the cyanobacterium Microcystis aeruginosa contains a genomic island encoding proteins with extensive amino-acid sequence identity to two components of the eukaryotic actin cytoskeleton: actin itself; and profilin, an actin binding protein hitherto only known in eukaryotes. Our data indicate that a rare eukaryote-to-prokaryote HGT has introduced both sequences into the Microcystis lineage. We found both genes to be actively expressed and propose a unique role in Microcystis cell stabilization for actin, differing substantially from what is observed for bacterial actin homologs. Because we detected both eukaryote-like genes only in one strain in culture and in recent samples collected from its original habitat we suggest that both proteins may contribute to the adaptation of this strain to its specific ecological niche.