title: A mobile genetic element with unknown function found in distantly related viruses.
abstract: 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.
authors.affiliation: National Veterinary Institute, Oslo, Norway. torstein.tengs@vetinst.no
authors.forename: Torstein
authors.initials: T
authors.lastname: Tengs
authors.forename: Anja Bråthen
authors.initials: AB
authors.lastname: Kristoffersen
authors.forename: Tsvetan R
authors.initials: TR
authors.lastname: Bachvaroff
authors.forename: Christine Monceyron
authors.initials: CM
authors.lastname: Jonassen
mesh_terms: D016000:Cluster Analysis; D019295:Computational Biology; D020071:Interspersed Repetitive Sequences; D009690:Nucleic Acid Conformation; D010802:Phylogeny; D012328:RNA Viruses / Q000235:genetics*