The multi-copy single-stranded DNA (msDNA) is yielded by the action of reverse 
transcriptase of retro-element in a wide range of pathogenic bacteria. Upon this 
phenomenon, it has been shown that msDNA is only produced by Eubacteria because 
many Eubacteria species contained reverse transcriptase in their special 
retro-element. We have screened around 111 Archaea at KEGG (Kyoto Encyclopedia 
of Genes and Genomes) database available at genome net server and observed three 
Methanosarcina species (M.acetivorans, M.barkeri and M.mazei), which also 
contained reverse transcriptase in their genome sequences. This observation of 
reverse transcriptase in Archaea raises questions regarding the origin of this 
enzyme. The evolutionary relationship between these two domains of life 
(Eubacteria and Archaea) hinges upon the phenomenon of retrons. Interestingly, 
the evolutionary trees based on the reverse transcriptases (RTs) and 16S 
ribosomal RNAs point out that all the Eubacteria RTs were descended from Archaea 
RTs during their evolutionary times. In addition, we also have shown some 
significant structural features among the newly identified msDNA-Yf79 in 
Yersinia frederiksenii with other of its related msDNAs (msDNA-St85, msDNA-Vc95, 
msDNA-Vp96, msDNA-Ec78 and msDNA-Ec83) from pathogenic bacteria. Together the 
degree of sequence conservation among these msDNAs, the evolutionary trees and 
the distribution of these ret (reverse transcriptase) genes suggest a possible 
evolutionary scenario. The single common ancestor of the organisms of Eubacteria 
and Archaea subgroups probably achieved this ret gene during their evolution 
through the vertical descent rather than the horizontal transformations followed 
by integration into this organism genome by a mechanism related to phage 
recognition and/or transposition.