A new subgroup of the IS3 family and properties of its representative member ISPpy1.

Recently, we described a novel insertion element, ISPpy1, isolated from a permafrost strain of Psychrobacter maritimus. In this work, we demonstrated that ISPpy1 is a member of a novel subgroup of the IS3 family of insertion sequences (ISs) that was not identified and characterized previously. IS elements of this subgroup termed the ISPpy1 subgroup are broadly distributed among different taxa of Eubacteria, including Geobacteraceae, Chlorobiaceae, Desulfobacteraceae, Methylobacteriaceae, Nitrosomonadaceae and Cyanobacteria. While displaying characteristic features of the IS3-family elements, ISPpy1 subgroup elements exhibit some unusual features. In particular, most of them have longer terminal repeats with unconventional ends and frameshifting box with an atypical organization, and, unlike many other IS3-family elements, do not exhibit any distinct IS specificity. We studied the transposition and mutagenic properties of a representative member of this subgroup, ISPpy1 and showed that in contrast to the original P. maritimus host, in a heterologous host, Escherichia coli K-12, it is able to translocate with extremely high efficiency into the chromosome, either by itself or as a part of a composite transposon containing two ISPpy1 copies. The majority of transposants carry multiple chromosomal copies (up to 12) of ISPpy1. It was discovered that ISPpy1 is characterized by a marked mutagenic activity in E. coli: its chromosomal insertions generate various types of mutations, including auxotrophic, pleiotropic and rifampicin-resistance mutations. The distribution of IS elements of the novel subgroup among different bacteria, their role in the formation of composite transposons and the horizontal transfer of genes are examined and discussed.