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Journal of Bacteriology, July 2000, p. 4035-4043, Vol. 182, No. 14
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Integration and Excision of a
Bacteroides Conjugative Transposon, CTnDOT
Qi
Cheng,
Brian J.
Paszkiet,
Nadja B.
Shoemaker,
Jeffrey F.
Gardner, and
Abigail A.
Salyers*
Department of Microbiology, University of
Illinois, Urbana, Illinois 61801
Received 20 January 2000/Accepted 25 April 2000
Bacteroides conjugative transposons (CTns) are thought
to transfer by first excising themselves from the chromosome to form a
nonreplicating circle, which is then transferred by conjugation to a
recipient. Earlier studies showed that transfer of most
Bacteroides CTns is stimulated by tetracycline, but it was
not known which step in transfer is regulated. We have cloned and
sequenced both ends of the Bacteroides CTn, CTnDOT, and
have used this information to examine excision and integration events.
A segment of DNA that contains the joined ends of CTnDOT and an
adjacent open reading frame (ORF), intDOT, was necessary
and sufficient for integration into the Bacteroides
chromosome. Integration of this miniature form of the CTn was not
regulated by tetracycline. Excision of CTnDOT and formation of the
circular intermediate were detected by PCR, using primers designed from
the end sequences. Sequence analysis of the PCR products revealed that
excision and integration involve a 5-bp coupling sequence-type
mechanism possibly similar to that used by CTn Tn916, a CTn
found originally in enterococci. PCR analysis also demonstrated that
excision is a tetracycline-regulated step in transfer. The integrated
minielement containing intDOT and the ends of CTnDOT did
not excise, nor did a larger minielement that also contained an ORF
located immediately downstream of intDOT designated
orf2. Thus, excision involves other genes besides
intDOT and orf2. Both intDOT and
orf2 were disrupted by single-crossover insertions.
Analysis of the disruption mutants showed that intDOT was
essential for excision but orf2 was not. Despite its
proximity to the integrase gene, orf2 appears not to be
essential for excision.
*
Corresponding author. Mailing address: Department of
Microbiology, 601 S. Goodwin Ave., University of Illinois, Urbana, IL 61801. Phone: (217) 333-7378. Fax: (217) 244-8485. E-mail:
abigails{at}uiuc.edu.
Journal of Bacteriology, July 2000, p. 4035-4043, Vol. 182, No. 14
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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