Eukaryotic nirK genes encoding copper-containing nitrite reductase: originating from the protomitochondrion?

Area of Science:

• Microbiology
• Evolutionary Biology
• Biochemistry

Background:

• Denitrification, or nitrate respiration, is a microbial process crucial for nitrogen cycling.
• While observed in some eukaryotes, the evolutionary link between eukaryotic and bacterial denitrification pathways has remained unclear.
• The absence of identified orthologous genes in eukaryotes hindered understanding of this relationship.

Purpose of the Study:

• To investigate the phylogenetic origins of eukaryotic denitrification.
• To identify and characterize eukaryotic genes homologous to bacterial denitrification enzymes.
• To determine if eukaryotic denitrification systems share evolutionary roots with bacterial systems.

Main Methods:

• Isolation and characterization of a gene from Fusarium oxysporum homologous to the bacterial nirK gene.
• Recombinant protein expression and functional analysis of the isolated fungal gene.
• Phylogenetic analysis of the fungal nirK gene and its homologs across various taxa.

Main Results:

• The first eukaryotic ortholog of the bacterial nirK gene, encoding copper-containing nitrite reductase (NirK), was identified in Fusarium oxysporum.
• Functional characterization confirmed the role of the fungal nirK gene in denitrification.
• Phylogenetic analysis revealed that eukaryotic nirK homologs cluster with bacterial and archaeal nirK genes, indicating a single common ancestor.

Conclusions:

• The fungal nirK gene represents the first identified eukaryotic ortholog involved in denitrification, sharing homology with bacterial NirK.
• The presence of nirK homologs in diverse eukaryotes like fungi and protozoa suggests a broader role in eukaryotic denitrification.
• The evolutionary analysis strongly implies that eukaryotic and bacterial denitrification systems originated from a single ancestral source, likely the protomitochondrion.