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Genetic basis for the β‐haemolytic/cytolytic activity of group B Streptococcus
Author(s) -
Pritzlaff Craig A.,
Chang Jennifer C. W.,
Kuo Shrin P.,
Tamura Glen S.,
Rubens Craig E.,
Nizet Victor
Publication year - 2001
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1046/j.1365-2958.2001.02211.x
Subject(s) - biology , cytolysin , plasmid , microbiology and biotechnology , genetics , open reading frame , gene , virulence , peptide sequence
Group B streptococci (GBS) express a β‐haemolysin/cytolysin that contributes to disease pathogenesis. We report an independent discovery and extension of a genetic locus encoding the GBS β‐haemolysin/cytolysin activity. A plasmid library of GBS chromosomal DNA was cloned into Escherichia coli , and a transformant was identified as β‐haemolytic on blood agar. The purified plasmid contained a 4046 bp insert of GBS DNA encoding two complete open reading frames (ORFs). A partial upstream ORF ( cyl B) and the first complete ORF ( cyl E) represent the 3′ end of a newly reported genetic locus ( cyl ) required for GBS haemolysin/cytolysin activity. ORF cyl E is predicted to encode a 78.3 kDa protein without GenBank homologies. The GBS DNA fragment also includes a previously unreported ORF, cyl F, with homology to bacterial aminomethyltransferases, and the 5′ end of cyl H, with homology to 3‐ketoacyl‐ACP synthases. Southern analysis demonstrated that the cyl locus was conserved among GBS of all common serotypes. Targeted plasmid integrational mutagenesis was used to disrupt cyl B, cyl E, cyl F and cyl H in three wild‐type GBS strains representing serotypes Ia, III and V. Targeted integrations in cyl B, cyl F and cyl H retaining wild‐type haemolytic activity were identified in all strains. In contrast, targeted integrations in cyl E were invariably non‐haemolytic and non‐cytolytic, a finding confirmed by in frame allelic exchange of the cyl E gene. The haemolytic/cytolytic activity of the cyl E allelic exchange mutants could be restored by reintroduction of cyl E on a plasmid vector. Inducible expression of cyl E, cyl F and cyl EF demonstrated that it is CylE that confers haemolytic activity in E. coli . We conclude that cyl E probably represents the structural gene for the GBS haemolysin/cytolysin, a novel bacterial toxin.