Iridovirus homologues of cellular genes - Implications for the molecular evolution of large DNA viruses

Iridoviruses belong to the group of large cytoplasmic deoxyriboviruses and infect either insects or vertebrates. In analogy to other large DNA viruses of eucaryotes it was found that iridoviruses encode a number of cellular p rotein homologues, The majority of these proteins represent orthologues of cellular enzymes involved in transcription, replication, and nucleotide met abolism. Others may have the potential to interfere with cell cycle regulat ion or immune defence mechanisms of the host.. This raises the question abo ut the phylogenetic origin of the corresponding viral genes. During the evo lution of large cytoplasmic DNA viruses such as iridoviruses, poxviruses, a nd African swine fever vints the acquirement of cellular genes appears to b e a crucial event. Each member of this group of viruses encodes a DNA polym erase, two subunits of the DNA-dependent RNA polymerase, and two subunits o f the ribonucleotide reductase. It is important to note that all of these v iral proteins show a high level of multidomain structure conservation as co mpared to their cellular orthologues. As a consequence the large cytoplasmi c DNA viruses have the ability to replicate independently of the cellular n ucleus in the cytoplasm of the infected cell. Assuming a common cellular or igin of viral DNA polymerase genes the corresponding amino acid sequences w ere chosen to construct a phylogenetic tree showing the relatedness among l arge DNA viruses of eucaryotes.