CONICET | Buscador de Institutos y Recursos Humanos

The incorporation of nitrogen from the atmosphere is energetically demanding, nitrate being the most oxidized form that can be assimilated by some eukaryotes. We used phylogenetic inference and sequence-similarity networks to study the origin, evolution and distribution of the gene families specifically involved in the incorporation of nitrate among an updated sampling of eukaryotic genomes. We show that the main family of nitrate transporters and the two families of nitrite reductases (NIR) described in eukaryotes are of bacterial origin, while the nitrate reductase (NR) was originated through the fusion of three different genes. The resulting phylogenetic trees and the patchy distribution suggest that horizontal gene transfer (HGT) played an important role in the evolution of these gene families. Among the recently available genomes analyzed, we detected the presence of nitrate-related gene families among three unicellular relatives of animals that belong to Teretosporea. In two of them, the canonical C-terminal region of the NR is replaced by the N-terminal duplicated domain of the NIR gene, which is found downstream to NR. In the third teretosporean investigated, we found the transporter and NIR genes but not the NR, an unexpected pattern present also in a distantly related species from the red algae group. Interestingly, both genomes are the only in our sampling containing an uncharacterized putative molybdopterin oxidoreductase physically linked to nitrate-related genes, possibly acting as a NR. Finally, two experimental approaches were carried out: (i) we cultured the three teretosporean organisms on different media to check if they are able to grow on nitrate as a sole nitrogen source and (ii) we quantified with RT-qPCR the expression of nitrate-related genes to understand their regulation in response to different nitrogen sources. Results and implications will be discussed.