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.Many Bacteroides clinical isolates carry large (Ͼ70-kbp) integrated self-transmissible elements, called conjugative transposons (2, 11). These elements have also been called Tc r elements because most of them carry a tetracycline resistance (Tc r ) gene, tetQ. Bacteroides Tc r elements have an unusual activity not reported for any other known conjugative transposon. They act in trans to excise and mobilize smaller unlinked DNA elements called NBUs (nonreplicating Bacteroides units) ( Fig. 1) (1,12,13,16,17). Two NBUs, NBU1 and NBU2, have been characterized (12, 13). These NBUs are 10 to 12 kbp in size and appear to be unrelated to the conjugative transposons that excise and mobilize them (12). During transfer, the NBUs are excised from the chromosome to form a double-stranded circle, which is the transfer intermediate ( Fig. 1) (7, 13). The circular form of the NBU then uses the transfer pore provided by the conjugative transposon to move into a recipient cell (7). The Bacteroides conjugative transposons also enhance, by at least 1000-fold, the transfer frequency of another integrated element, the mobilizable transposon Tn4399 (5, 8). Results of previous studies had indicated that Tn4399 was not related to the NBUs. For example, Tn4399 has a different mode of integration than that of the NBUs, and the mobilization region of Tn4399 had no significant sequence similarity to the mobilization region of the NBUs (6, 7, 13). In this report, we provide the first evidence that Tn4399 and the NBUs do in fact share some sequence similarity.The oriT-mob region of NBU1 allows NBU1 to be mobilized not only by Bacteroides conjugative transposons but also by the IncP plasmids R751 and RP4 (7). Thus, mobilization of the NBU1 circle form by IncP plasmids presumably ...

Galactose is an abundant and essential sugar used for the biosynthesis of many macromolecules in different organisms, including plants. Galactose metabolism is tightly and finely controlled since excess galactose and derivatives are inhibitory. In Arabidopsis, root growth and pollen germination were strongly inhibited upon excess galactose. However, the mechanism of galactose induced inhibition during pollen germination remains obscure. In this study, we characterized AtSWEET5 as a glucose and galactose plasma-membrane transporter localized in the pollen. SWEET5 protein level start to accumulate since tricellular stage of pollen development and peaked in mature pollen before rapidly declining after pollen was germinated. SWEET5 levels are responsible for the dosage-dependent sensitivity of galactose and GALK is essential for the inhibitory effects of galactose during pollen germination. The unexpected observation that GALK is required for efficient galactose uptake in pollen may reveal an unknown regulatory mechanism for galactose transporters. Overall, SWEET5 and GALK contribute to the maintenance of galactose metabolic homeostasis during pollen germination, and galactose transport is positively regulated by GALK. The study of SWEET5 upon galactose condition also suggests SWEET5 is a major low-affinity hexose transporter at the early stage of pollen germination.