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Title:Interactions between the Bacteroides conjugative transposons and the plasmids and insertion elements (NBUs) they mobilize
Author(s):Li, Lhing-Yew
Doctoral Committee Chair(s):Salyers, Abigail A.
Department / Program:Biology, Molecular
Biology, Microbiology
Discipline:Biology, Molecular
Biology, Microbiology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Biology, Molecular
Biology, Microbiology
Abstract:Bacteroides spp. are Gram-negative obligative anaerobes that are the predominant components of the normal microflora in the human colon. Bacteroides spp. are clinically important because they are opportunistic pathogens. At present, most clinical isolates of Bacteroides are resistant to tetracycline and many have acquired resistance to other antibiotics, such as erythromycin and cefoxitin. A family of chromosomally located conjugative transposons is responsible for the spread of tetracycline resistance in the Bacteroides group and is also contributing to the spread of resistance to other antibiotics. Bacteroides conjugative transposons not only transfer themselves but also mobilize co-resident plasmids. In addition, they mediate a novel activity: excision and circularization of unlinked chromosomal insertion elements (NBUs: nonreplicative Bacteroides units). One NBU (Tn4555) carries a cefoxitin resistance determinant (cfxA). Others are cryptic at present but could acquire resistance genes later. Bacteroides conjugative transposons are thus capable of spreading antibiotic resistance genes in three different ways.
In elements that are transferred by conjugation, the location of the origin of transfer (oriT or nic site) usually indicates the physical structure of the transfer intermediate. In the case of NBU1 and NBU2, I have shown that the oriT is internal, suggesting that the transfer intermediate is a covalently closed circle. Data from sequence analysis and complementation experiments indicates that although the two NBUs studied (NBU1 and NBU2) differ from each other in many ways, their mob/oriT regions are quite similar. Moreover, NBUs encode only one multifunctional Mob protein which appears to recognize and initiate mobilization of the NBU by nicking at the oriT. Mobilizable plasmids generally require at least two proteins to carry out this reaction. I also have localized the oriT of the Bacteroides conjugative transposon, $\rm Tc\sp{r}Em\sp{r}$DOT, which transfers itself as well as NBUs. This oriT is also internal, indicating that the transfer intermediate of the conjugative transposon, like that of NBUs, is also a circle. The mob/oriT region of the Bacteroides conjugative transposon and that of the NBUs do not share significant sequence similarity either to each other or to the mob/oriT regions of Gram-negative conjugative plasmids. Thus, the Bacteroides conjugative transposons and NBUs appear to represent new groups of recognition sequences for mob/oriT systems.
Conjugative transfer mediated by the Bacteroides conjugative transposons is regulated by tetracycline. Two sets of positive regulatory genes were identified previously. The first set is rteA-rteB and the second is rteC. RteA-RteB is essential for all phenotypes associated with Bacteroides conjugative transposons, whereas RteC appears to be essential only for self-transfer. Nonetheless, I showed that RteC has some effect on mobilization of plasmids and NBUs. Sequence analysis showed previously that RteA and RteB comprise a two-component regulatory system. RteA is the putative environment sensor and RteB is the transcriptional activator. Nonetheless, RteA does not sense tetracycline. Instead, the inducer antibiotic, tetracycline, activates transcription of an operon containing rteA and rteB. Expression of rteC, which lies in a different transcription unit, requires the action of RteB. In the process of localizing the transfer (Tra) region of the Bacteroides conjugative transposon, $\rm Tc\sp{r}Em\sp{r}$DOT, I obtained evidence that a repressor, which probably interacts with RteC, also is involved in regulating expression of transfer genes.
Issue Date:1995
Rights Information:Copyright 1995 Li, Lhing-Yew
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9522138
OCLC Identifier:(UMI)AAI9522138

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