Publications

4767

Synechococcusis among the most important contributors to global primary productivity. The genomes of several strains of this taxon have been previously sequenced in an effort to understand the physiology and ecology of these highly diverse microorganisms. Here we present a comparative study ofSynechococcusgenomes. For that end, we developed GenTaxo, a program written in Perl to perform genomic taxonomy based on average nucleotide identity, average amino acid identity and dinucleotide signatures, which revealed that the analyzed strains are drastically distinct regarding their genomic content. Phylogenomic reconstruction indicated a division ofSynechococcusin two clades (i.e. Synechococcus and the new genus Parasynechococcus), corroborating evidences that this is in fact a polyphyletic group. By clustering protein encoding genes into homologue groups we were able to trace the Pangenome and core genome of both marine and freshwaterSynechococcusand determine the genotypic traits that differentiate these lineages.

Summary The bacterial tree contains many deep‐rooting clades without any cultured representatives. One such clade is ‘ E ndomicrobia’, a class‐level lineage in the phylum E lusimicrobia represented so far only by intracellular symbionts of termite gut flagellates. Here, we report the isolation and characterization of the first free‐living member of this clade from sterile‐filtered gut homogenate of defaunated (starch‐fed) R eticulitermes santonensis . Strain R sa215 is a strictly anaerobic ultramicrobacterium that grows exclusively on glucose, which is fermented to lactate, acetate, hydrogen and CO 2 . Ultrastructural analysis revealed a G ram‐negative cell envelope and a peculiar cell cycle. The genome contains a single set of nif genes that encode homologues of G roup IV nitrogenases, which were so far considered to have functions other than nitrogen fixation. We documented nitrogenase activity and diazotrophic growth by measuring acetylene reduction activity and 15 N 2 incorporation into cell mass, and demonstrated that transcription of nifH and nitrogenase activity occur only in the absence of ammonium. Based on the ancestral relationship to ‘ C andidatus   E ndomicrobium trichonymphae’ and other obligate endosymbionts, we propose the name ‘ E ndomicrobium proavitum ’ gen. nov., sp. nov. for the first isolate of this lineage and the name ‘ E ndomicrobia ’ class. nov. for the entire clade.