IS91Rolling‐Circle Transposition

This chapter reviews a class of insertion sequence (IS) elements, the IS 91 family, that occupies a definite genetic niche, probably as a consequence of a rather peculiar transposition mechanism. It encompasses and summarizes the results obtained in our study of rolling circle (RC) transposition. The analysis is mainly based on IS 91 , the family prototype and the element studied in greatest detail. The fact that different family members, such as IS 91 and IS 1294 , appeared close to each other in association with virulence genes suggests that they are particularly adapted to move about this type of sequence. In summary, two conclusions can be drawn from analysis of the reported BLAST hits. Most if not all of the observed hits correspond to partial IS 91 -like elements, starting precisely at one or the other terminus ( ori or ter ) and ending up at widely variable positions within the element, and most of the hits are adjacent to functional bacterial virulence genes harbored in known virulence plasmids (and not in antibiotic resistance plasmids). The search was complicated by the fact that the conserved motifs are also present in Rep proteins of rolling-circle (RC)-replication plasmids and phages, and these are ubiquitous. Hairpin structures remind one of RC phage and plasmid replication origins. The authors say that a more detailed analysis is in progress that will define the commonalities and specificities of each of the family elements.