CONICET | Buscador de Institutos y Recursos Humanos

The incorporation of nitrogen from the atmosphere is energetically demanding, nitrate being the most oxidized formthat can be assimilated by some eukaryotes. We used phylogenetic inference and sequence-similarity networks to studythe origin, evolution and distribution of the gene families specifically involved in the incorporation of nitrate among anupdated sampling of eukaryotic genomes. We show that the main family of nitrate transporters and the two families ofnitrite reductases (NIR) described in eukaryotes are of bacterial origin, while the nitrate reductase (NR) was originatedthrough the fusion of three different genes. The resulting phylogenetic trees and the patchy distribution suggest thathorizontal gene transfer (HGT) played an important role in the evolution of these gene families. Among the recentlyavailable genomes analyzed, we detected the presence of nitrate-related gene families among three unicellular relativesof 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 speciesfrom the red algae group. Interestingly, both genomes are the only in our sampling containing an uncharacterizedputative molybdopterin oxidoreductase physically linked to nitrate-related genes, possibly acting as a NR. Finally, twoexperimental approaches were carried out: (i) we cultured the three teretosporean organisms on different media to checkif they are able to grow on nitrate as a sole nitrogen source and (ii) we quantified with RT-qPCR the expression ofnitrate-related genes to understand their regulation in response to different nitrogen sources. Results and implicationswill be discussed.