Origin and evolution of eukaryotic nitrate assimilation: its occurrence in Holozoa

IÑAKI RUIZ-TRILLO; CLAUDIO SCAZZOCHIO; EDUARD OCAÑA-PALLARÉS; SEBASTIÁN R. NAJLE

Sant Feliu de Guixols

Workshop; EMBO Workshop - Comparative genomics of eukaryotic microbes: Dissecting the sources of evolutionary diversity; 2017

European Molecular Biology Organization (EMBO)

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 proteins. 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 some Teretosporea taxa (the sister clade of Filasterea, Choanoflagellata and Metazoa), namely in: Creolimax fragrantissima, Sphaeroforma arctica and Corallochytrium limacisporum. In the first two, we found the three genes in a cluster that was most likely acquired by HGT from an oomycete donor. Surprisingly, the canonical C-terminal region of NR is replaced by the N-terminal duplicated domain of NIR. In C. limacisporum, we found transporter and NIR genes but not NR, an unexpected pattern present also in the red alga Galdieria sulphuraria. 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 above mentioned 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.