Typically they encode one large open reading frame of 300 to 400 amino acids and by definition the protein encoded by this reading frame is involved in the transposition event. Although there are exceptions, insertion sequences tend to be present in a small number of copies in the genome. For example, IS1 is present in 6 to l0 copies in E. coli chromosome while IS2 and 3 are typically present in about five copies. This restoration is probably one of the few things that do not happen in transposition. This is to be contrasted with the replicative mode of transposition that involves the generation of a second copy without alteration of the initial copy of the transposition sequence.
Insertion sequences mediate a variety of DNA rearrangements.
One of the first recognitions of this fact was the involvement of insertion sequences in the integration of F and R plasmids into the host chromosome. This event gives rise to Hfr strains and will be discussed in the section on conjugation . IS1 generates an 8 or 9 base pair duplication while IS2 generates a 5 base pair duplication. As noted above, several elements cause no duplication at all, and integrate into target DNA by a completely different mechanism which is related to the site-specific system employed by many phage. Less common but in some ways more important to the genetic use of insertion sequences and transposons is their deletogenic properties.