THE JOINT OF TN916 CIRCULAR INTERMEDIATES IS A HOMODUPLEX IN ENTEROCOCCUS-FAECALIS

Tn916 is a 18-kb conjugative transposon originally identified in Enter ococcus faecalis. The first step for Tn916 movement is its excision fr om the donor replicon with rha formation of a nonreplicating covalentl y closed circular intermediate. Studies on formation of circular inter mediates in Escherichia coli have shown that the joint between the Tn9 16, termini is a 6-bp heteroduplex formed by the two regions flanking the transposon before its excision (coupling sequences), Ln this work we studied the joint of Tn916 termini in circular intermediates formed in both E. coli and E. faecalis. Our strategy was to use direct seque ncing of amplification products obtained from the joint region of sing le target molecules. In E. coli, 50% of circular intermediates contain ed a heteroduplex joint, while the remaining 50% displayed a homoduple x joint formed by one of the hua coupling sequences, In E. faecalis, w e could not demonstrate the presence of any heteroduplex joint. In thi s case 77.7% of the analyzed joints were formed by the left coupling s equence. (C) 1997 Academic Press.