A mobile genetic element with unknown function found in distantly related viruses

BACKGROUND: The genetic element s2m seems to represent one of very few examples of mobile genetic elements in viruses. The function remains obscure and a scattered taxonomical distribution has been reported by numerous groups. METHODS: We have searched GenBank in order to identify all viral accessions that have s2m(−like) sequence motifs. Rigorous phylogenetic analyses and constrained tree topology testing were also performed in order to investigate the apparently mobile nature of s2m. RESULTS: The stem-loop s2m structure can be found in four families of + ssRNA viruses; Astroviridae, Caliciviridae, Picornaviridae and Coronaviridae. In all of these virus families, with the possible exception of Caliciviridae, multiple gains and/or losses of s2m would have to be postulated in order to explain the distribution of this character. CONCLUSIONS: s2m appears to be a mobile genetic element with a unique evolutionary history in all of the four virus families where it can be found. Based on our findings and a review of the current literature on s2m, a hypothesis implying an RNAi-like function for the s2m element is also outlined.