With some plasmids, recombination can occur in a site specific manner that is independent of RecA. In such cases, the recombination event requires another protein called Pre. Pre is a plasmid recombination enzyme. This protein is also known as Mob (conjugative mobilisation). This family is also known as Mob-V. One of the family members, MobM, is encoded by a promiscuous plasmid actively involved in the spread of antibiotic resistance. Homologs of MobM are found in many plasmids and other mobile genetic elements of pathogenic bacteria, including S. aureus. MobM is a metal-dependent nuclease that uses histidine nitrogen for the nucleophilic attack on the scissile phosphate. Furthermore, in contrast to other DNA-processing enzymes, MobM is a histidine relaxase, a DNA-breaking and -joining enzyme, that operates through a phosphorus-nitrogen protein-DNA adduct for cell-to-cell DNA transfer. Mutational analysis indicate that the H(N/D)(Q/E)R N-terminal motif of MobM plays a crucial role in the cleavage and generation of stable DNA-protein adducts [2].

1: Region of the streptococcal plasmid pMV158 required for conjugative mobilization. Priebe SD, Lacks SA; J Bacteriol 1989;171:4778-4784. PMID:2768188

2: Structural basis of a histidine-DNA nicking/joining mechanism for gene transfer and promiscuous spread of antibiotic resistance. Pluta R, Boer DR, Lorenzo-Diaz F, Russi S, Gomez H, Fernandez-Lopez C, Perez-Luque R, Orozco M, Espinosa M, Coll M; Proc Natl Acad Sci U S A. 2017;114:E6526. PMID:28739894