Chlamydia trachomatis protein GrgA activates transcription by contacting the nonconserved region of σ66

Abstract

The bacterial RNA polymerase holoenzyme consists of a catalytic core enzyme in complex with a σ factor that is required for promoter-specific transcription initiation. Primary, or housekeeping, σ factors are responsible for most of the gene expression that occurs during the exponential phase of growth. Primary σ factors share four regions of conserved sequence, regions 1-4, which have been further subdivided. Many primary σ factors also contain a nonconserved region (NCR) located between subregions 1.2 and 2.1, which can vary widely in length. Interactions between the NCR of the primary σ factor of Escherichia coli, σ70, and the β′ subunit of the E. coli core enzyme have been shown to influence gene expression, suggesting that the NCR of primary σ factors represents a potential target for transcription regulation. Here, we report the identification and characterization of a previously undocumented Chlamydia trachomatis transcription factor, designated GrgA (general regulator of genes A). We demonstrate in vitro that GrgA is a DNA-binding protein that can stimulate transcription from a range of σ66-dependent promoters. We further show that GrgA activates transcription by contacting the NCR of the primary σ factor of C. trachomatis, σ66. Our findings suggest GrgA serves as an important regulator of σ66- dependent transcription in C. trachomatis. Furthermore, because GrgA is present only in chlamydiae, our findings highlight how non-conserved regions of the bacterial RNA polymerase can be targets of regulatory factors that are unique to particular organisms.