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Journal of Bacteriology, November 1999, p. 6844-6849, Vol. 181, No. 21
Departamento de Biología Molecular,
Facultad de Medicina, Universidad de Cantabria, Santander, Spain
Received 4 June 1999/Accepted 11 August 1999
IntI1 integrase is a member of the prokaryotic DNA integrase
superfamily. It is responsible for mobility of antibiotic resistance cassettes found in integrons. IntI1 protein, as well as IntI1-COOH, a
truncated form containing its carboxy-terminal domain, has been purified. Electrophoretic mobility shift assays were carried out to
study the ability of IntI1 to bind the integrase primary target sites
attI and aadA1 attC. When using double-stranded
DNA as a substrate, we observed IntI1 binding to attI but
not to attC. IntI1-COOH did not bind either
attI or attC, indicating that the N-terminal
domain of IntI1 was required for binding to double-stranded attI. On the other hand, when we used single-stranded (ss)
DNA substrates, IntI1 bound strongly and specifically to ss
attC DNA. Binding was strand specific, since only the
bottom DNA strand was bound. Protein IntI1-COOH bound ss
attC as well as did the complete integrase, indicating that
the ability of the protein to bind ss aadA1 attC was
contained in the region between amino acids 109 and 337 of IntI1.
Binding to ss attI DNA by the integrase, but not by
IntI1-COOH, was also observed and was specific for the attI
bottom strand, indicating similar capabilities of IntI1 for binding
attI DNA in either double-stranded or ss conformation. Footprinting analysis showed that IntI1 protected at least 40 bases of
aadA1 attC against DNase I attack. The protected sequence contained two of the four previously proposed IntI1 DNA binding sites,
including the crossover site. Preferential ssDNA binding can be a
significant activity of IntI1 integrase, which suggests the utilization
of extruded cruciforms in the reaction mechanisms leading to cassette
excision and integration.
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
The IntI1 Integron Integrase Preferentially
Binds Single-Stranded DNA of the attC Site

*
Corresponding author. Mailing address: Departamento de
Biología Molecular, Facultad de Medicina, Universidad de
Cantabria, Cardenal Herrera Oria s/n, 39011-Santander, Spain. Phone: 34 942 201948. Fax: 34 942 201945. E-mail:
jmglobo{at}medi.unican.es.
Present address: Department of Biologic and Material Sciences,
School of Dentistry, University of Michigan, Ann Arbor, MI 48109.
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