Expression of a plasmid gene of Chlamydia trachomatis encoding a novel 28 kDa antigen

The expression of an open reading frame (ORF3) of plasmid pCT of Chlamydia trachomatis was examined. ORF3-specific transcripts were detected on Northern blots of RNA extracted from Vero cells infected with C. trachomatis. The ORF3-encoded product was then identified as a 28 kDa chlamydial antigen. Western blot analysis of elementary bodies treated with Sarkosyl showed the membrane association of the 28 kDa antigen. Moreover, recognition of the protein by human immune sera suggests that pCT has some function in the chlamydial cell.