Two aberrant mercury resistance transposons in the Pseudomonas stutzeri plasmid pPB

The two mer operons of the Pseudomonas stutzeri OX plasmid pPB and their flanking regions have been sequenced and found to be part of two aberrant transposons. The narrow spectrum mer operon is almost identical to that of Tn501, but is associated with the remnants of Tn5053 tni genes rather than the Tn501 transposition module. The broad spectrum mer operon shows an overall homology with that of Tn5053, but differs from it in the presence of a merB gene, absent in Tn5053, and a merC gene instead of a merF. The pPB broad spectrum mer operon is associated with an incomplete Tn5053-like transposition module and with the Tn501 tnp genes, which are proximal, respectively, to the end and to the beginning of the mer operon. A hypothesis about pPB evolution is presented.