Expression of a Plasmid Gene of Chlamydia Trachomatis Encoding a Novel 28 kDa Antigen

J Gen Microbiol. 1993 May;139(5):1083-92. doi: 10.1099/00221287-139-5-1083.


Plasmid pCT is present in essentially all isolates of Chlamydia trachomatis and may encode factors important for survival in the natural environment. However, no pCT-associated phenotype has been described so far. With the purpose of investigating the possibility of a role of pCT in C. trachomatis pathogenicity we examined the expression of an ORF (ORF3), potentially encoding a 28 kDa polypeptide (pgp3). Analysis of RNA extracted from chlamydia-infected Vero cells detected ORF3-specific transcripts, from 20 h post-infection onwards, mainly as discrete RNA species of 1390 nucleotides comprising the downstream ORF4 sequence. ORF3 DNA was cloned and expressed in Escherichia coli as a 39 kDa fusion protein (MS2/pgp3). Antibodies raised against purified MS2/pgp3, specifically recognized a 28 kDa protein on Western blots of protein from purified chlamydial elementary bodies (EBs). The same antibodies detected chlamydial inclusions in methanol-fixed infected cells by immunofluorescence. Western blot analysis of EBs extracted with 2% Sarkosyl, showed that a large proportion of the 28 kDa antigen is associated with the detergent-insoluble ('membrane') fraction. Antibodies recognizing pgp3 epitopes were detected in sera from patients with chlamydial infections, but not in sero-negative control sera. The finding support the hypothesis that pCT may provide a function related to chlamydial cell physiology.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Antibodies, Bacterial / blood
  • Antigens, Bacterial*
  • Bacterial Proteins / genetics*
  • Bacterial Proteins / immunology
  • Bacterial Proteins / isolation & purification
  • Base Sequence
  • Chlamydia Infections / immunology
  • Chlamydia trachomatis / genetics*
  • Genes, Bacterial / genetics*
  • Humans
  • Membrane Proteins / isolation & purification
  • Molecular Sequence Data
  • Open Reading Frames / genetics
  • Plasmids / genetics*
  • RNA, Messenger / genetics*
  • Vero Cells


  • Antibodies, Bacterial
  • Antigens, Bacterial
  • Bacterial Proteins
  • Membrane Proteins
  • RNA, Messenger
  • pgp3 protein, Chlamydia

Associated data

  • GENBANK/J03321