The Bacteroides mobilizable insertion element, NBU1, integrates into the 3' end of a Leu-tRNA gene and has an integrase that is a member of the lambda integrase family

NBU1 is a 10.3-kbp integrated Bacteroides element that ran he induced to excise from the chromosome and can be mobilized to a recipient by trans-acting functions provided bg certain Bacteroides conjugative transposons, The NBU1 transfer intermediate is a covalently closed circle, which is presumed to be tale form that integrates into the recipient genome, We report here that a 2.4-kbp segment of NBU1 was all that was required for site-specific integration into the chromosome of Bacteroides thetaiotaomicron 5482. This 2.4-kbp region included the joined ends of the NBU1 circular form (attN1) and a single open reading frame, intN1, which encoded the integrase. Previously, we had found that NBU1 integrates preferentially into a single site in B, thetaiotaomicron 5482. We have now shown that the NBU1 target site is located al the 3' end of a Leu-tRNA gene, The NBU1 integrase gene, intN1, was sequenced, The predicted protein had little overall amino acid sequence similarity to any proteins in the databases but had limited carboxy-terminal similarity to the integrases of lambdoid phages and to the integrases of the gram-positive conjugative transposons Tn916 and Tn1545. We also report that the intN1 gene is expressed constitutively.