The mobilization regions of two integrated Bacteroides elements, NBU1 and NBU2, have only a single mobilization protein and may be on a cassette.

Bacteroides conjugative transposons can act in trans to excise, circularize, and transfer unlinked integrated elements called NBUs (for nonreplicating Bacteroides units). Previously, we localized and sequenced the mobilization region of one NBU, NBU1, and showed that this mobilization region was recognized by the IncP plasmids RP4 and R751, as well as by the Bacteroides conjugative transposons. We report here that the single mobilization protein carried by NBU1 appears to be a bifunctional protein that binds to the oriT region and catalyzes the nicking reaction that initiates the transfer process. We have also localized and sequenced the mobilization region of a second NBU, NBU2. The NBU2 mobilization region was 86 to 90% identical at the DNA sequence to the oriT-mob region of NBU1. The high sequence similarity between NBU1 and NBU2 ended abruptly after the stop codon of the mob gene and about 1 kbp upstream of the oriT region, indicating that the oriT-mob regions of NBU1 and NBU2 may be on some sort of cassette. A region on NBU1 and NBU2 which lies immediately upstream of the oriT region had 66% sequence identity to a region upstream of the oriT region on a mobilizable transposon, Tn4399, an element that had previously appeared to be completely unrelated to the NBUs.