Cloning and characterisation of the Proteus mirabilis xerD gene

The Xer site-specific recombination system is involved in the stable maintenance of replicons (certain plasmids and chromosomes) in Escherichia coli and other bacteria by converting multimers into monomers. This system requires a cis-acting DNA sequence (the chromosomal dif site or the ColE1 cer site) and two trans-acting factors: the XerC and XerD recombinases, which belong to the lambda integrase family of tyrosine site-specific recombinases. In addition, in order to resolve plasmid multimers into monomers, two additional factors are required: the ArgR and PepA proteins. We have previously shown the presence of xerC and xerD genes (and their function) by Southern hybridisation and by in vivo recombination in a wide variety of Enterobacteriaceac. We have now cloned and sequenced the xerD gene of Proteus mirabilis using degenerate and inverse PCR methods. This gene encodes a tyrosine recombinase which is highly similar to the E. coli XerD recombinase, is capable of complementing an E. coli xerD mutant, and displays sequence-specific DNA binding activity.