Diversity and evolution of nitric oxide synthases (NOS) genes in photosynthetic eukaryotes: New insights from Klebsormidiophyceae (Charophytes).

Although nitric oxide synthase (NOS) proteins homologous to mammalian NOS are absent in terrestrial plants (embryophytes), they are present in various algal groups. Of the 367 algal species for which genome data is available on the NCBI database, 40 proteins belonging to 36 species in various algal classes have been identified. The sporadic distribution of NOS orthologues across the algae phylogenetic tree raises the question of why this enzyme is absent in most species. Genome and recent transcriptome sequencing in Klebsormidiophyceae provided an opportunity to study the evolution of NOS sequences in closely related organisms. Based on a dataset comprising 18 species, including 13 Klebsormidium species, we showed that only eight species possess complete or partial NOS sequences. Both in silico and experimental approaches suggest that the divergence observed in Klebsormidiophyceae NOS genes, compared to their human homologs, reflect specific modifications, including putative pseudogenization. Indeed, although copies of genes encoding NOS are expressed in certain species of algae, some are evolving into non-functional copies through the accumulation of mutations, which could affect the synthesis of a functional protein. This process may be considered relatively recent in Charophytes history since the presence or absence of a gene can be observed in very closely related species.