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Mol Biochem Parasitol. 1997 Nov;89(2):271-82.

Identification and characterization of SRS1, a Toxoplasma gondii surface antigen upstream of and related to SAG1.

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Department of Microbiology and Immunology, Stanford University School of Medicine, CA 94305, USA.


Previous investigations of the major surface antigen (SAG1) promoter of Toxoplasma gondii indicated an ability to function bi-directionally in transient transformation assays at least. This suggests there might be another tachyzoite-specific gene being divergently transcribed from the SAG1 promoter in its normal chromosomal location. To investigate this possibility we have characterized the region upstream of SAG1 and report here a co-directional transcription unit coding for a probable GPI-anchored surface protein with homology to SAG1 and SAG3. This antigen, which had not previously been identified in surface iodination experiments is given the acronym SRS1, for SAG1-related sequence 1. Genomic organization and sequence of a full-length cDNA of SRS1 are presented. Antisera against a recombinant SRS1 protein produced in Escherichia coli, recognize a specific band of 46 kDa in parasite lysates which corresponds to the largest of the GPI-anchored proteins by Western blot. The possible role of this previously unidentified surface antigen is discussed.

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