Cytochrome b5 occurrence in giant and other viruses belonging to the phylum Nucleocytoviricota

Cytochrome b5 is an electron transport protein found in eukaryotes and bacteria, and plays roles in energy production, lipid biosynthesis and cytochrome P450 biochemistry. Here we report that genes for cytochrome b5 occur broadly among viruses in the class Megaviricetes isolated from the deep ocean, freshwater and terrestrial sources, and human patients. Transcriptional analysis showed that Mimivirus bradfordmassiliense cytochrome b5 is expressed in the host and has characteristic spectral properties. Viral cytochrome b5s have either a unique N-terminal transmembrane anchor or are predicted to be soluble proteins. Virus cytochrome b5 proteins share 45-95% sequence identity with one another but no more than 25% identity with that in Acanthamoeba castellanii, a host for many giant viruses. Thus, the origin of cytochrome b5 genes in giant viruses remains unknown. Our findings raise questions regarding the evolution and diversity of cytochrome b5, and about the origin of viral haemoproteins in general.