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The Bacteroides mobilizable transposon Tn4555 integrates by a site-specific recombination mechanism similar to that of the gram-positive bacterial element Tn916.

ABSTRACT: The Bacteroides mobilizable transposon Tn4555 is a 12.2-kb molecule that encodes resistance to cefoxitin. Conjugal transposition is hypothesized to occur via a circular intermediate and is stimulated by coresident tetracycline resistance elements and low levels of tetracycline. In this work, the ends of the transposon were identified and found to consist of 12-bp imperfect inverted repeats, with an extra base at one end. In the circular form, the ends were separated by a 6-bp "coupling sequence" which was associated with either the left or the right transposon terminus when the transposon was inserted into the chromosome. Tn4555 does not duplicate its target site upon insertion. Using a conjugation-based transposition assay, we showed that the coupling sequence originated from 6 bases of genomic DNA flanking either side of the transposon prior to excision. Tn4555 preferentially transposed into a 589-bp genomic locus containing a 207-bp direct repeat. Integration occurred before or after the repeated sequence, with one integration site between the two repeats. These observations are consistent with a transposition model based on site-specific recombination. In the bacteriophage lambda model for site-specific recombination, the bacteriophage recombines with the Escherichia coli chromosome via a 7-bp "crossover" region. We propose that the coupling sequence of Tn4555 is analogous in function to the crossover region of lambda but that unlike the situation in lambda, recombination occurs between regions of nonhomologous DNA. This ability to recombine into divergent target sites is also a feature of the gram-positive bacterial transposon Tn916.

SUBMITTER: Tribble GD

PROVIDER: S-EPMC179024 | biostudies-other | 1997 Apr

REPOSITORIES: biostudies-other

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The Bacteroides mobilizable transposon Tn4555 integrates by a site-specific recombination mechanism similar to that of the gram-positive bacterial element Tn916.

Tribble G D GD Parker A C AC Smith C J CJ

Journal of bacteriology 19970401 8

The Bacteroides mobilizable transposon Tn4555 is a 12.2-kb molecule that encodes resistance to cefoxitin. Conjugal transposition is hypothesized to occur via a circular intermediate and is stimulated by coresident tetracycline resistance elements and low levels of tetracycline. In this work, the ends of the transposon were identified and found to consist of 12-bp imperfect inverted repeats, with an extra base at one end. In the circular form, the ends were separated by a 6-bp "coupling sequence" ...[more]

PMID: 9098073

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Project description:Results of a recent study of antibiotic resistance genes in human colonic Bacteroides strains suggested that gene transfer events between members of this genus are fairly common. The identification of Bacteroides isolates that carried an erythromycin resistance gene, ermG, whose DNA sequence was 99% identical to that of an ermG gene found previously only in gram-positive bacteria raised the further possibility that conjugal elements were moving into Bacteroides species from other genera. Six of seven ermG-containing Bacteroides strains tested were able to transfer ermG by conjugation. One of these strains was chosen for further investigation. Results of pulsed-field gel electrophoresis experiments showed that the conjugal element carrying ermG in this strain is an integrated element about 75 kb in size. Thus, the element appears to be a conjugative transposon (CTn) and was designated CTnGERM1. CTnGERM1 proved to be unrelated to the predominant type of CTn found in Bacteroides isolates-CTns of the CTnERL/CTnDOT family-which sometimes carry another type of erm gene, ermF. A 19-kbp segment of DNA from CTnGERM1 was cloned and sequenced. A 10-kbp portion of this segment hybridized not only to DNA from all the ermG-containing strains but also to DNA from strains that did not carry ermG. Thus, CTnGERM1 seems to be part of a family of CTns, some of which have acquired ermG. The percentage of G+C content of the ermG region was significantly lower than that of the chromosome of Bacteroides species-an indication that CTnGERM1 may have entered Bacteroides strains from some other bacterial genus. A survey of strains isolated before 1970 and after 1990 suggests that the CTnGERM1 type of CTn entered Bacteroides species relatively recently. One of the genes located upstream of ermG encoded a protein that had 85% amino acid sequence identity with a macrolide efflux pump, MefA, from Streptococcus pyogenes. Our having found >90% sequence identity of two upstream genes, including mefA, and the remnants of two transposon-carried genes downstream of ermG with genes found previously only in gram-positive bacteria raises the possibility that gram-positive bacteria could have been the origin of CTnGERM1.

Dataset Information

The Bacteroides mobilizable transposon Tn4555 integrates by a site-specific recombination mechanism similar to that of the gram-positive bacterial element Tn916.

Publications

The Bacteroides mobilizable transposon Tn4555 integrates by a site-specific recombination mechanism similar to that of the gram-positive bacterial element Tn916.

Similar Datasets

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