The 6.3 kb Clostridium perfringens transposon Tn4451 encodes a 50 kDa protein, TnpZ, which has amino acid sequence similarity to a group of plasmid mobilization and recombination proteins that comprise the Mob/Pre family. Members of this family interact with an upstream palindromic sequence called an RSA site, and an RSA-like sequence has been identified upstream of the tnpZ gene. In Escherichia coli, in the presence of a chromosomally integrated derivative of the broad-host-range IncP plasmid, RP4, TnpZ was able to promote plasmid mobilization in cis and was able to function in trans to enable the mobilization of a co-resident plasmid carrying an RSA site. It was also able to mediate the conjugative transfer of plasmids from E. coli to C. perfringens. Site-directed mutagenesis of two bases within the RSA site resulted in a significant reduction in mobilization frequency, demonstrating that the RSA site is required for efficient plasmid mobilization. TnpZ is the only Mob/Pre protein known to be associated with a transposable genetic element, and Tn4451 is the first mobilizable but non-self-transmissible transposon to be identified from a gram-positive bacterium.