Marck c. Nucleic Acids Res. 5 Doh-ura K. Biochem. We have characterized a candidate marsupial PrP gene. We propose that antisense ORFs found in other species are artefactual. No significant homology to any known sequence was found, leaving the origin of the prion gene family as mysterious as ever.
Chickens have a hexapeptide repeat instead of an octapeptide but align fairly well later.
Going back farther, nothing homologous was found in nematode, fruit fly, or yeast, despite some false alarms. Taken at face value, this leaves prions emerging more or less with the appearance of the vertebrates and not being part of any protein super-family. Normally, proteins appear from diverging gene duplications or domain assembly, but if so here, the orginal gene cannot yet be found. So we are left without the usual clues as to the biological function of normal prion protein or its domain.