Genome streamlining in a cosmopolitan oceanic bacterium.
Giovannoni S.J., Tripp H.J., Givan S., Podar M., Vergin K.L., Baptista D., Bibbs L., Eads J., Richardson T.H., Noordewier M., Rappe M.S., Short J.M., Carrington J.C., Mathur E.J.
The SAR11 clade consists of very small, heterotrophic marine alpha-proteobacteria that are found throughout the oceans, where they account for about 25% of all microbial cells. Pelagibacter ubique, the first cultured member of this clade, has the smallest genome and encodes the smallest number of predicted open reading frames known for a free-living microorganism. In contrast to parasitic bacteria and archaea with small genomes, P. ubique has complete biosynthetic pathways for all 20 amino acids and all but a few cofactors. P. ubique has no pseudogenes, introns, transposons, extrachromosomal elements, or inteins; few paralogs; and the shortest intergenic spacers yet observed for any cell.