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

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.