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Comparative genomics has revealed many examples in which the same function is performed by unrelated or distantly related proteins in different cellular lineages. In some cases, this has been explained by the replacement of the original gene by a paralogue or non-homologue, a phenomenon known as non-orthologous gene displacement. Such gene displacement probably occurred early on in the history of proteins involved in DNA replication, repair, recombination and transcription (DNA informational proteins), i.e. just after the divergence of archaea, bacteria and eukarya from the last universal cellular ancestor (LUCA). This would explain why many DNA informational proteins are not orthologues between the three domains of life. However, in many cases, the origin of the displacing genes is obscure, as they do not even have detectable homologues in another domain. I suggest here that the original cellular DNA informational proteins have often been replaced by proteins of viral or plasmid origin. As viral and plasmid-encoded proteins are usually very divergent from their cellular counterparts, this would explain the puzzling phylogenies and distribution of many DNA informational proteins between the three domains of life.
Patrick Forterre (Sun,) studied this question.