Determinants of Conjugative Transposition

Emory University, Atlanta GA

Investigators

Abstract

Movement of conjugative transposons from one bacterial host to another requires transposon excision from the donor chromosome, conjugal transfer of the excised transposon from donor to recipient and integration of the transposon into the recipient chromosome. The best-studied conjugative transposon is Tn916. Movement of Tn916 is catalyzed by the transposon-encoded bivalent Int protein, whose domains recognize and bind to two distinct DNA sequences. Tn916 does not mobilize adjacent chromosomal genetic markers and expression of the genes involved in conjugal transfer is dependent upon excision and circularization of the transposon. Thus, the first step in conjugative transposition of Tn916 is excision of the element, and conjugal transfer does not occur prior to excision. A second transposon encoded protein that is required for excision, Xis, plays a regulatory role in transposon excision and a model for Xis function has recently been suggested. This model will be tested. Transposon excision and transfer can be dissociated by expression of Int and Xis from a heterologous promoter. Transposon excision occurs at a greatly elevated frequency in the donor cell population but the frequency of conjugative transfer and integration remain unchanged under these conditions. This implies that the step of conjugal transfer may also be regulated. The Int recombinase, in addition to its binding and cleavage of the transposon ends, has been shown to bind specifically to the origin of conjugal DNA transfer, oriT. Whether Int is required for conjugal transfer of Tn916, or if Int plays a regulatory role in this process will be determined. Examinion of Int binding to oriT has suggested that the N-terminal DNA binding domain of Int may exert an effect on the activities of the C-terminal catalytic domain. The possible role of such interactions between the two DNA binding domains of Int will be examined. Conjugative transposons such as Tn916 have the remarkable property of being not only able to move from place to place in the genome of a single bacterial species, but to transfer themselves between an amazingly large number of diverse bacterial species. They also have the ability to mobilize other genetic elements that normally are not self-transmissible. These properties mean that conjugative transposons are likely to be of major importance in promoting genetic exchange in the complex microbial communities that exist in natural environments. These complex communities have a great impact on human activities. The study of agents of genetic exchange in these communities will increase our understanding of how microbial populations adapt to changing conditions and prosper.