An X1α plasmid from a Salmonella enterica serovar Ohio isolate carrying a novel IS26-bounded tet(C) pseudo-compound transposon

The sequence of a conjugative plasmid, pSRC22-2, found in a multiply antibiotic resistant Salmonella enterica serovar Ohio isolate SRC22 originally cultured from swine in 1999, was determined. Plasmid pSRC22-2 has a copy number of approximately 40 and transfers tetracycline resistance at very high frequency. It was typed as IncX1 using the three typing schemes proposed for X-type plasmids, which utilize the replication region, iteron region and taxC conjugation gene and pSRC22-2 belongs to the X1α subgroup. The plasmid backbone, derived by removing mobile elements, is shared with pOLA52, which was the first fully sequenced IncX1 plasmid, and five other X1α plasmids. The pSRC22-2 backbone is interrupted by a complete copy of an IS903 isoform, partial copies of IS1 and IS903 on either side of a 5930 bp IS26-bounded pseudo-compound transposon (PCT), and a novel 256 bp miniature inverted repeat transposable element (MITE). The MITE belongs to the Tn3 family and was named MITESen1. The PCT, which carries a tet(C) tetracycline resistance determinant, is bounded by copies of a novel IS26 variant, IS26-v4, and was designated PTn6184. Comparison of PTn6184 with other tet(C)-carrying PCTs revealed that it can be derived from the largest, PTntet(C), via a two-step process that re-orders the central fragment and involves both an IS26-mediated event and homologous recombination. IS26-v4, which encodes a variant transposase, Tnp26 G184D, has appeared in only 46 entries in the GenBank non-redundant database.