IS Families/IS6 family

General

There are at present nearly 160 family members in ISfinder (https://www-is.biotoul.fr/scripts/search-db.php) from nearly 80 bacterial and archaeal species but this represents only a fraction of those present in the public databases. The family was named ��[1]� after the directly repeated insertion sequences in transposon Tn6 ��[2]� to standardize the various names that had been attributed to identical elements (e.g. IS15, IS26, IS46, IS140, IS160, IS176) ��[3–15]�, including one isolate, IS15, corresponding to an insertion of one iso-IS6 (IS15D) into another ��[4,5]�. More recently there has been some attempt to rename the family as the IS26 family (see ��[16]�), presumably because of accumulating experimental data from IS26 itself and the importance of this IS in accumulation and transmission of multiple antibiotic resistance, although this might potentially introduce confusion in the literature. IS6 family members have a simple organization (Fig. IS6.1) and generate 8bp direct target repeats on insertion. This family is very homogenous with an average length of about 800 bp for the majority (between 700 and 890 bp) and highly conserved short, generally perfect, IRs (Fig. IS6.1 and IS6.2). There are two examples of MITES (Miniature Inverted repeat Transposable Elements composed of both IS ends and no intervening orfs; ��[17]� of 227 and 336 bp), 7 members between 1230 and 1460 bp and three members between 1710 and 1760 bp. One member, IS15, of 1648 bp represents and insertion of one IS into another ��[3,5]�. Many are found as part of compound transposons (called pseudo-compound transposons ��[1]� described below ��[16]�) invariably as flanking direct repeats (Fig. IS6.1) a consequence of their transposition mechanism ��[7,9,13,14,18–30]�.

Acknowledgements

We would like to thank Susu He (Nanjing University) for stimulating discussions concerning the transposition models and for supplying the IS26 transposase secondary structure predictions in Fig. IS6.2C).