Abstract
We report the discovery and characterization of the mercury resistance transposon, Tn 5044, from a Xanthomonas strain from the Kamchatka peninsula. In addition to the standard set of merRTPCAD genes, the mer operon of Tn 5044 contains a gene named sigY that encodes the RNA polymerase sigma factor-like protein. Mercury resistance determined by Tn 5044 is expressed at low (30 °C) but not at elevated temperatures (37 °C). None of the mer operon genes downstream of merA is responsible for the temperature-sensitive mercury resistance. The transposition module of Tn 5044 is closely related to those of Tn 1412 isolated from medical sources and to Tn 5563 and IS Xc5 from environmental sources. However, Tn 5044 differs from these transposons in that it has unusually long terminal inverted repeats. Sequence analysis of the transposase ( tnpA) genes places Tn 5044 and its close relatives into the Tn 3 subgroup of the Tn 3 family. However, the orientation of their resolvase and transposase genes is unusual for the Tn 3 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 Tn 5044 and its close relatives possesses a C-terminal extension showing no homology to known proteins. Despite this peculiarity, Tn 5044 resolvase can resolve cointegrates formed during Tn 5044 transposition controlled by tnpA. Genetic data suggest that the extension is essential for TnpR functioning.
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