NBU1, a mobilizable site-specific integrated element from Bacteroides spp., can integrate nonspecifically in Escherichia coli.

NBU1 is an integrated Bacteroides element that can he mobilized from Bacteroides donors to Bacteroides recipients. Previous studies have shown that a plasmid carrying the internal mobilization region of NBU1 could be transferred by conjugation from Bacteroides thetaiotaomicron to Escherichia coli. In this report, we show that NBU1 can integrate in E. coli. Whereas integration of NBU1 in B. thetaiotaomicron is site specific, integration of NBU1 in E. coli was relatively random, and the insertion frequency of NBU1 into the E. coli chromosome was 100 to 1,000 times lower than the frequency of integration in B. thetaiotaomicron. The frequency of NBU1 integration in E. coli could be increased about 10- to 70-fold, to a value close to that seen with B. thetaiotaomicron, if the primary integration site from B. thetaiotaomicron, BT1-1, was provided on a plasmid in the E. coli recipient or the NBU1 integrase gene, intN1, was provided on a high-copy-number plasmid to increase the amount of integrase available in the recipient. When the primary integration site was available in the recipient, NBU1 integrated site specifically in E. coli. Our results show that NBUs have a very broad host range and are capable of moving from Bacteroides spp. to distantly related species such as E. coli. Moreover, sequence analysis of NBU1 integration sites provided by integration events in E. coli has helped to identify some regions of the NBU1 attachment site that may play a role in the integration process.