BROAD-HOST-RANGE GENE-TRANSFER - PLASMIDS AND CONJUGATIVE TRANSPOSONS

Conjugation is the primary route of broad host range DNA transfer betw een different genera of bacteria. Plasmids are the most familiar conju gative elements, but there are also self-transmissible integrated elem ents called conjugative transposons. Conjugative transposons have been found in many genera of gram-positive bacteria, in mycoplasmas and in gram negative bacteria such as Bacteroides spp. and Moraxella spp., a nd they have a very broad host range. The best-studied conjugative tra nsposons are: the ones related to Tn916, a 16 kb conjugative transposo n found originally in Gram-positive bacteria; Tn5276, a 70 kb conjugat ive transposon from Lactococcus lactis; and a group of large (> 70 kb) conjugative transposons found in Bacteroides spp. Transfer of conjuga tive transposons takes place in three steps: excision to form a circul ar intermediate, transfer of one strand of the circular intermediate t o a recipient, and integration into the recipient genome. Some conjuga tive transposons integrate almost randomly, whereas others integrate s ite-specifically. Conjugative transposons not only transfer themselves but also mobilize co-resident plasmids, either by providing transfer functions in trans or by inserting themselves into the plasmid. In add ition, the conjugative transposons found in Bacteroides spp. can excis e and mobilize unlinked integrated elements, called NBUs. Transfer of many of the Bacteroides conjugative transposons is regulated by tetrac ycline, whereas transfer of Tn916 and other conjugative transposons ap pears to be constitutive. The conjugative transposons are clearly wide spread in clinical isolates, but their distribution in environmental i solates remains to be determined.