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Infection and Immunity, January 2002, p. 163-170, Vol. 70, No. 1
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.70.1.163-170.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
RS1 Element of Vibrio cholerae Can Propagate Horizontally as a Filamentous Phage Exploiting the Morphogenesis Genes of CTX
Shah M. Faruque,1* Asadulghani,1 M. Kamruzzaman,1 Ranjan K. Nandi,2 A. N. Ghosh,3 G. Balakrish Nair,1,3 John J. Mekalanos,4 and David A. Sack1
Molecular Genetics Laboratory, International Centre for Diarrhoeal Disease Research, Bangladesh, Dhaka 1212, Bangladesh,1
ICMR Virus Unit,2
National Institute of Cholera and Enteric Diseases, Beliaghata Calcutta 700010, India,3
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 021154
Received 1 June 2001/
Returned for modification 31 August 2001/
Accepted 1 October 2001
In toxigenic Vibrio cholerae, cholera toxin is encoded by the CTX prophage, which consists of a core region carrying ctxAB genes and genes required for CTX
morphogenesis, and an RS2 region encoding regulation, replication, and integration functions. Integrated CTX
is often flanked by another genetic element known as RS1 which carries all open reading frames (ORFs) found in RS2 and an additional ORF designated rstC. We identified a single-stranded circularized form of the RS1 element, in addition to the CTX
genome, in nucleic acids extracted from phage preparations of 32 out of 83 (38.5%) RS1-positive toxigenic V. cholerae strains analyzed. Subsequently, the corresponding double-stranded replicative form (RF) of the RS1 element was isolated from a representative strain and marked with a kanamycin resistance (Kmr) marker in an intergenic site to construct pRS1-Km. Restriction and PCR analysis of pRS1-Km and sequencing of a 300-bp region confirmed that this RF DNA was the excised RS1 element which formed a novel junction between ig1 and rstC. Introduction of pRS1-Km into a V. cholerae O1 classical biotype strain, O395, led to the production of extracellular Kmr transducing particles, which carried a single-stranded form of pRS1-Km, thus resembling the genome of a filamentous phage (RS1-Km
). Analysis of V. cholerae strains for susceptibility to RS1-Km
showed that classical biotype strains were more susceptible to the phage compared to El Tor and O139 strains. Nontoxigenic (CTX-) O1 and O139 strains which carried genes encoding the CTX
receptor toxin-coregulated pilus (TCP) were also more susceptible (>1,000-fold) to the phage compared to toxigenic El Tor or O139 strains. Like CTX
, the RS1
genome also integrated into the host chromosomes by using the attRS sequence. However, only transductants of RS1-Km
which also harbored the CTX
genome produced a detectable level of extracellular RS1-Km
. This suggested that the core genes of CTX
are also required for the morphogenesis of RS1
. The results of this study showed for the first time that RS1 element, which encodes a site-specific recombination system in V. cholerae, can propagate horizontally as a filamentous phage, exploiting the morphogenesis genes of CTX
.
* Corresponding author. Mailing address: Molecular Genetics Laboratory, Laboratory Sciences Division, ICDDR,B, GPO Box 128, Dhaka 1000, Bangladesh. Phone: 880-2-8811751. Fax: 880-2-8812529. E-mail:
faruque{at}icddrb.org.
Editor: J. T. Barbieri
Infection and Immunity, January 2002, p. 163-170, Vol. 70, No. 1
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.70.1.163-170.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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