Expression of hcp in freshwater Synechococcus spp., a gene encoding a hyperconserved protein in picocyanobacteria

Marine picoplankton of the genus Synechococcus and Prochlorococcus spp. are widely studied members of the picocyanobacterial clade, composed of unicellular cyanobacteria that dominate pelagic regions of the ocean. Less studied are the related freshwater Synechococcus spp. that similarly dominate the euphotic zone of oligotrophic lakes. Previous work has shown that marine picocyanobacteria harbor a small gene, hcp, that encodes a 62 amino acid protein 100% conserved among all strains examined. The gene is restricted exclusively to the picocyanobacterial lineage. The current study reveals that hcp is also 100% conserved in four freshwater Synechococcus spp. strains isolated from the Laurentian Great Lakes, and that the gene constitutively expressed with genes encoding a ribosomal protein and two tRNA genes. The synteny of the hcp region is also conserved between the marine and freshwater strains. Last, the hcp gene and the organization of the surrounding genetic region has been retained in the reduced genome of a picocyanobacterial endosymbiont of the amoeba Paulinella sp.