MORE KINGDOMS USING TEMPLATES. “Viruses likely originated from ancient cells through encapsulating cellular plasmids or genome fragments by cellular protein. Indeed, cellular complexes resembling the structure and topology of HK97 gp5 have been found in both bacteria and archaea cells. Similarly, it is natural to expect that cellular proteins with the BPP-1 topology of the HK97-like fold also exist elsewhere.”
http://www.aimspress.com/article/10.3934/biophy.2015.2.200/fulltext.html