Tn5044, a novel Tn3 family transposon coding for temperature-sensitive mercury resistance

We report the discovery and characterization of the mercury resistance transposon, Tn5044, from a Xanthomonas strain from the Kamchatka peninsula. In addition to the standard set of merRTPCAD genes, the mer operon of Tn5044 contains a gene named sigY that encodes the RNA polymerase sigma factor-like protein. Mercury resistance determined by Tn5044 is expressed at low (30 degrees C) but not at elevated temperatures (37 degrees C). None of the mer operon genes downstream of merA is responsible for the temperature-sensitive mercury resistance. The transposition module of Tn5044 is closely related to those of Tn1412 isolated from medical sources and to Tn5563 and ISXc5 from environmental sources. However, Tn5044 differs from these transposons in that it has unusually long terminal inverted repeats. Sequence analysis of the transposase (tnpA) genes places Tn5044 and its close relatives into the Tn3 subgroup of the Tn3 family. However, the orientation of their resolvase and transposase genes is unusual for the Tn3 family: tnpR is proximal to the end of the transposon, while divergently transcribed tnpA is oriented inwardly. The region between tnpA and tnpR genes is unusually large and contains two short conserved open reading frames. In addition to the complete set of sequence motifs common to true resolvases, the resolvase of Tn5044 and its close relatives possesses a C-terminal extension showing no homology to known proteins. Despite this peculiarity, Tn5044 resolvase can resolve cointegrates formed during Tn5044 transposition controlled by tnpA. Genetic data suggest that the extension is essential for TnpR functioning.