A subclass of the IS1202 family of bacterial insertion sequences targets XerCD recombination sites

We describe here a new family of IS which are related to IS1202, originally isolated from Streptococcus pneumoniae in the mid-1990s and previously tagged as an emerging IS family in the ISfinder database. Members of this family have impacted some important properties of their hosts. We describe here another potentially important property of certain family members: specific targeting of xrs recombination sites.

The family could be divided into three subgroups based on their transposase sequences and the length on the target repeats (DR) they generate on insertion: subgroup IS1202 (24–29 bp); ISTde1 (15–18 bp); and ISAba32 (5–6 bp). Members of the ISAba32 subgroup were repeatedly found abutting Xer recombinase recombination sites (xrs), separated by an intervening copy of a DR. These xrs sites, present in multiple copies in a number of Acinetobacter plasmids flanking antibiotic resistance genes, were proposed to form a new type of mobile genetic element using the chromosomally-encoded XerCD recombinase for mobility. Transposase alignments identified subgroup-specific indels which may be responsible for the differences in the transposition properties of the three subgroups (i.e. DR length and target specificity). We propose that this collection of IS be classed as a new insertion sequence family: the IS1202 family composed of three subgroups, only one of which specifically targets plasmid-borne xrs. We discuss the implications of xrs targeting for gene mobility.