TABLE 1-3.

Virion-Associated Enzymes and Their Functions

Family	Virus	Function
Enzymes affecting interaction of virions with the host cell surface
Neuraminidase	Orthomyxovirus, paramyxovirus	Cleaves N-acetylneuraminic acid from surface polysaccharides
Enzymes transcribing the viral genome into mRNAa
DNA-dependent RNA polymerase	Poxvirus, African swine fever virus	Transcribes RNA
dsRNA-dependent RNA polymerase	Viruses with dsRNA	Transcribes RNA
ssRNA-dependent RNA polymerase	Viruses with (–) sense ssRNA	Transcribes RNA
Enzymes adding specific terminal groups to viral mRNA
Nucleotide phosphohydrolase	Viruses synthesizing mRNA in virions (e.g., poxvirus, reovirus)	Converts terminal 5’ triphosphate to diphosphate as prelude to guanylylation
Guanylyltransferase	Viruses synthesizing mRNA in virions (e.g., poxvirus, reovirus)	Adds guanylyl residue to 5’ end diphoshpate in mRNA
RNA methylases	Viruses synthesizing mRNA in virions (e.g., poxvirus, reovirus)	Methylate guanylyl residue at 5’ end of mRNA and some riboses in 2’ position
Poly(A) polymerase	Viruses synthesizing mRNA in virions (e.g., poxvirus, reovirus)	Synthesizes poly(A) tail at 3’ end of mRNA
Enzymes involved in copying virion RNA into DNA
RNA-dependent DNA polymerase (reverse transcriptase)	Retrovirus	Makes DNA–RNA hybrids
RNase H (an activity of the reverse transcriptase)	Retrovirus	Breaks down RNA strand in RNA–DNA hybrid
Polynucleotide ligase	Retrovirus	Closes ss breaks in dsDNA
Enzymes for nucleic acid replication or processing
DNA-dependent DNA polymerase	Hepadnavirus	Synthesizes dsDNA
Deoxyribonuclease (exo- and endo-)	Poxvirus, retrovirus, adenovirus	Break DNA chains and cross-links
Endoribonuclease	Poxvirus	Processing of mRNA
Other enzymes
Protein kinases	Retrovirus, orthomyxovirus, paramyxovirus, herpesvirus, adenovirus	Phosphorylate proteins
tRNA aminoacylases	Retrovirus	Aminoacylate tRNA

^aAlso called transcriptases.