The repressor function of the Chlamydia late regulator EUO is enhanced by the plasmid-encoded protein Pgp4

A critical step in the intracellular Chlamydia infection is the production of infectious progeny through the expression of late genes. This differentiation step involves conversion from a reticulate body (RB), which is the replicating form of the bacterium, into an elementary body (EB), which is the developmental form that spreads the infection to a new host cell. EUO is an important chlamydial transcription factor that controls the expression of late genes, but the mechanisms that regulate EUO are not known. We report that a plasmid-encoded protein Pgp4 enhanced the repressor activity of EUO. Pgp4 did not function as a transcription factor because it did not bind or directly modulate transcription of its target promoters. Instead, Pgp4 increased the ability of EUO to bind and repress EUO-regulated promoters in vitro and physically interacted with EUO in pulldown assays with recombinant proteins. We detected earlier onset of EUO-dependent late gene expression by immunofluorescence microscopy in Pgp4-deficient C. trachomatis and C. muridarum strains. In addition, the absence of Pgp4 led to earlier onset of RB-to-EB conversion in C. muridarum. These data support a role for Pgp4 as a negative regulator of chlamydial transcription that delays late gene expression. Our studies revealed that Pgp4 also has an EUO-independent function as a positive regulator of chlamydial transcription. IMPORTANCEChlamydia trachomatis is an important human pathogen that causes more than 150 million active cases of genital and eye infection in the world. This obligate intracellular bacterium produces infectious progeny within an infected human cell through the expression of late chlamydial genes. We showed that the ability of a key chlamydial transcription factor EUO to repress late genes was enhanced by a plasmid-encoded protein Pgp4. In addition, studies with Chlamydia Pgp4-deficient strains provide evidence that Pgp4 delays late gene expression in infected cells. Thus, Pgp4 is a novel regulator of late gene expression in Chlamydia through its ability to enhance the repressor function of EUO.