Characterization of an active partition system for the Enterococcus faecalis pheromone-responding plasmid pAD1.

Enterococcus faecalis plasmid pAD1 is a 60-kb conjugative, low-copy-number plasmid that encodes a mating response to the peptide sex pheromone cAD1 and a cytolytic exotoxin that contributes to virulence. Although aspects of conjugation have been studied extensively, relatively little is known about the control of pAD1 maintenance. Previous work on pAD1 identified a 5-kb region of DNA sufficient to support replication, copy control, and stable inheritance (K. E. Weaver, D. B. Clewell, and F. An, J. Bacteriol. 175:1900-1909, 1993), and recently, the pAD1 replication initiator (RepA) and the origin of vegetative replication (oriV) were characterized (M. V. Francia, S. Fujimoto, P. Tille, K. E. Weaver, and D. B. Clewell, J. Bacteriol. 186:5003-5016, 2004). The present study focuses on the adjacent determinants repB and repC, as well as a group of 25 8-bp direct repeats (iterons with the consensus sequence TAGTARRR) located between the divergently transcribed repA and repB. Through mutagenesis and trans-complementation experiments, RepB (a 33-kDa protein, a member of the ParA superfamily of ATPases) and RepC (a protein of 14.4 kDa) were shown to be required for maximal stabilization. Both were active in trans. The iteron region was shown to act as the pAD1 centromere-like site. Purified RepC was shown by DNA mobility shift and DNase I footprinting analyses to interact in a sequence-specific manner with the iteron repeats upstream of the repBC locus. The binding of RepC to the iteron region was shown to be modified by RepB in the presence of ATP via a possible interaction with the RepC-iteron complex. RepB did not bind to the iteron region in the absence of RepC.