Affiliation:
1. Department of Internal Medicine, Virginia Commonwealth University School of Medicine, McGuire Hall Room 103, 1112 East Clay Street, Richmond, Virginia 23298
Abstract
ABSTRACT
The gene encoding resistance to methicillin and other β-lactam antibiotics in staphylococci,
mecA
, is carried on a genomic island, SCC
mec
(for staphylococcal cassette chromosome
mec
). The chromosomal excision and integration of types I to IV SCC
mec
are catalyzed by the site-specific recombinases CcrA and CcrB, the genes for which are encoded on each element. We sought to identify the relative contributions of CcrA and CcrB in the excision and integration of SCC
mec
. Purified CcrB but not CcrA was shown to mediate the gel shift of chromosomal target integration sequences (
attB
) in electrophoretic mobility shift assays. However, preincubation of CcrB-DNA complexes with increasing concentrations of CcrA blocked gel shift. The interaction of CcrB and CcrA was confirmed by
Escherichia coli
two-hybrid analysis. SCC
mec
excision mediated by plasmid-encoded and inducible
ccrA
,
ccrB
, or both genes was assessed by PCR in
Staphylococcus aureus
. CcrB alone could mediate excision but excision was at an alternate
att
site (
attR2
) within the right extremity of SCC
mec
. In contrast, both CcrB and CcrA were required to mediate excision at the chromosomal
attB
site (called
attR
when SCC
mec
is integrated). Insertion of a plasmid containing the SCC
mec att
site (
attS
) into the chromosome required both CcrA and CcrB, but CcrA overexpression lowered integration frequency. Thus, while CcrB binds DNA, interaction between CcrA and CcrB, in a precise ratio, is required for
attB
site-specific excision and SCC
mec
chromosomal insertion.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
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