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Genetic diversity of the C protein -antigen gene and its upstream regions within clonally related groups of type Ia and Ib group B streptococci

¹ Medical Microbiology Laboratory, Funabashi Medical Center, 1-21-1 Kanasugi, Funabashi, Chiba 273-8588, Japan
² Department of Microbiology, School of Medicine and Environmental Infectious Disease, Graduate School of Medical Science, Kitasato University, Sagamihara, Kanagawa, Japan

Correspondence
Noriyuki Nagano
naganoyn{at}d3.dion.ne.jp

C protein antigen (Bac), a surface protein of group B streptococci (GBS), is known to concurrently bind the Fc portion of IgA and factor H (FH). The authors' previous work has demonstrated that mRNA expression levels show diversity among clonally related strains containing genes (bac) encoding Bac, with high expression noted in invasive strains. In this study, the bac gene and upstream regions containing putative promoters, three ORFs and an IS1381 insertion sequence were characterized. Three invasive strains showed high bac expression levels and did not show any notable mutations except one strain producing Bac that was able to bind FH but not IgA. A deletion of 51 amino acid residues, including part of the Bac IgA-binding region, was identified and hypothesized to contribute to the loss of the IgA-binding ability of this strain. A vaginal strain that showed somewhat higher bac expression levels and produced Bac lacking immunoreactivity contained an 11 bp deletion, which generated a premature termination codon, in the region preceding the IgA-binding region. In another vaginal strain that did not express bac, disruption of the upstream ORFs of the sensor histidine kinase and DNA-binding response regulator, due to frameshift mutations, was noted although it is not known whether these proteins directly affect bac expression levels. An IS1381 insertion into the promoter region was found in another vaginal strain that showed low expression levels and produced Bac with a significantly larger proline-rich repeat region. These results demonstrate considerable genetic diversity of the bac and upstream regions of invasive and noninvasive GBS, which may contribute to the variability of bac expression levels among those strains.

The GenBank/EMBL/DDBJ accession numbers for the sequences reported in this paper are AB121739 (for strain 5) and AB221536 (for strain 12).

A supplementary figure showing the amino acid sequence alignment of Bac from strains 22 and 5 and the sequences in GenBank accession numbers X59771 and X58470 is available with the online version of this paper.

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