ABSTRACT

Nucleic acids and proteins may be modified during or after synthesis by the addition of specific chemical groups. RNAs and proteins are frequently modified (see RNA Processing, Proteins), but DNA modification is limited. Some viruses alter specific residues in their genome, a process which may protect the DNA from nucleases or may facilitate packaging into the capsid ( Table 7.1 ). The major exception is DNA methylation, which is frequently observed in both prokaryotes and higher eukaryotes, and has many roles concerning the recognition and function of DNA. Modified DNA nucleosides found in viral genomes Virus Modified nucleoside FV3 ~20% 5-Methylcytidine for cytidine ϕW-14 α-Putrescinylthymidine for thymidine PBS1 Deoxyuridine for thymidine SPO1 5-Hydroxyuridine for thymidine T-even family Hydroxymethylcytidine for cytidine FV3 is a eukaryotic virus which encodes its own DNA cytosine methyltransferase. The other viruses are bacteriophage.