Objective       To analyze the interactions among the proteins encoded in the cryptic plasmid of Chlamydia trachomatis (C.trachomatis).       Methods       The DNA fragments of C.trachomatis plasmid genes

pgp1~4 and pgp8

were cloned into the vectors (pRAC and pRBR)

and the target genes were in-frame fused to α subunit N-terminal domain (α-NTD) and DNA binding protein

CI protein of bacteriophage λ (λCI)

respectively. The expression of cloned genes in Escherichia coli（E.coli） were determined by Western blotting. The protein-protein interactions were analyzed by a transcription-based bacterial two-hybrid system.       Results       Western blotting indicated that isopropyl β-D-1-thiogalactopyranoside (IPTG) induced the expression of fusion proteins

α-Pgp1

α-Pgp3

CI-Pgp1

CI-Pgp3 and CI-Pgp4 in E.coli. In the presence of Pgp3 (encoded by plasmid pBRα-Pgp3 )

the expression of α-Pgp3 and CI-Pgp3 activated the transcription from a reporter gene β-galactosidase in the bacterial two-hybrid assay strain

but Pgp3 had no interaction with other test proteins.       Conclusion       Pgp3 encoded by pgp3 interacts with Pgp3 in C.trachomatis.