Plasmid-mediated regulation of chromosomal gene expression and virulence in Chlamydia psittaci.

UNLABELLED: Chlamydia psittaci (Cps) causes psittacosis, a zoonotic disease characterized by severe pneumonia and systemic infection, yet its pathogenic mechanisms remain poorly understood. While plasmids are established virulence determinants in C. trachomatis (Ct) and Chlamydia muridarum (Cm), their role in Cps pathogenesis is unknown. Here, we generated a panel of Cps transformants with individual deletions of plasmid genes (pgp1-pgp8) and characterized in vitro and in vivo. Genetic analysis revealed that Pgp1 and Pgp2 are required for plasmid maintenance, whereas Pgp3-Pgp8 are dispensable. Transcriptomic and qRT-PCR analyses identified a subset of chromosomal genes regulated by the Cps plasmid, distinct from those described in Ct and Cm. Pgp4 functions as a central regulator of chromosomal gene expression; however, unlike Ct or Cm, Pgp4 does not affect pgp3 transcript levels but is required for Pgp3 protein expression, indicating post-transcriptional regulation. We further show that Pgp7 positively regulates plasmid-dependent gene expression, whereas Pgp6 and Pgp8 negatively regulate this process. Functionally, the plasmid is a determinant of virulence in a murine lung infection model. Pgp3 is essential for host inflammatory responses and bacterial virulence in vivo. Collectively, these findings demonstrate plasmid-mediated regulation in Cps is distinct from that in other chlamydial species and highlight species-specific regulatory mechanisms underlying chlamydial pathogenesis.

IMPORTANCE: Chlamydia psittaci is a zoonotic pathogen that causes severe and fatal pneumonia in humans, yet its molecular pathogenesis remains poorly understood. Here, we generated a series of Cps transformants carrying individual ORF deletion and characterized their functions both in vitro and in vivo. We show that Pgp4 and Pgp7 positively regulate a subset of chromosomal genes, whereas Pgp6 and Pgp8 act as negative regulators. Unexpectedly, Pgp4 regulates the major virulence factor Pgp3 at a post-transcriptional level, revealing a previously unrecognized mode of plasmid control. These findings provide new insights into the roles of plasmid-encoded genes in Cps biology and pathogenesis.