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Infect. Immun., Oct 1995, 3994-4002, Vol 63, No. 10
S Pan and GT Cole
Results of earlier investigations have indicated that the saprobic phase of
Coccidioides immitis produces a heat-stable, 19-kDa antigen with serine
proteinase activity which has been suggested to be specific for this
pathogenic fungus. In the present study we have determined the N-terminal
and partial internal amino acid sequences of the purified, 19-kDa antigen,
cloned the gene which encodes this polypeptide, and confirmed that the
secreted proteinase is a Coccidioides-specific antigen (CS-Ag). Both the
genomic and cDNA sequences are reported and reveal that the csa gene which
encodes this antigen has no introns. A 543-bp open reading frame encodes a
181-amino-acid-containing protein with a predicted molecular mass of 19.8
kDa and an isoelectric point of 8.3. The csa gene was localized on
chromosome I of three representative C. immitis clinical isolates on the
basis of Southern hybridizations. Expression of the csa gene in Escherichia
coli using the pET21a plasmid vector yielded a recombinant protein that was
recognized in immunoblot assays by antibody raised to the purified 19-kDa
CS-Ag. Secretion of the native antigen is suggested to occur by cleavage of
a putative 23- residue signal peptide. The native CS-Ag showed a low degree
of glycosylation. Analysis of the carbohydrate composition of the CS-Ag
revealed xylose, mannose, galactose, and glucose. However, the purified
antigen showed no affinity for concanavalin A. A PCR method with
specificity and high sensitivity for detection of C. immitis genomic DNA,
using a pair of synthetic oligonucleotide primers whose sequences were
based on that of the csa gene, was developed. A 520-bp product was
amplified only when C. immitis genomic DNA was used as the template. The
lower limits of DNA detection using this PCR method were 1 pg of C. immitis
genomic DNA by ethidium bromide staining and 100 fg after Southern
hybridization. The csa gene-based PCR method for detection of C. immitis
DNA is useful for culture identification and may have clinical applications
for the diagnosis of coccidioidal infections.
Copyright © 1995, American Society for Microbiology
Molecular and biochemical characterization of a Coccidioides immitis- specific antigen
Department of Microbiology, Medical College of Ohio, Toledo 43699-0008, USA.
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