Crystallographic Fragment Screening on the Shigella Type III Secretion System Chaperone IpgC.

The Shigella pathogenicity factor IpgC belongs to the class II of type III secretion system chaperones, whose members are characterized by a tetratricopeptide repeat (TPR) domain consisting of three and a half TPR motifs. Since IpgC is essential for Shigella virulence, we determined a high-resolution crystal structure of this chaperone to facilitate its use as a target for the structure-based design of anti-shigellosis compounds. The crystal structure revealed two possible homodimer assemblies, which strongly differ from the homodimer architectures so far known for IpgC and orthologues thereof. Through crystallographic fragment screening, we identified 10 small molecules that bind to IpgC and, therefore, are available for expansion to generate larger, more potent binders. A follow-up compound, based on one of our fragment hits, binds to a strictly conserved site, which overlaps with the binding site of the chaperone's substrates, IpaB and IpaC. Therefore, it constitutes a promising starting point for the design of functional IpgC inhibitors.

Interaction of two photoreceptors in the regulation of bacterial photosynthesis genes.

The expression of photosynthesis genes in the facultatively photosynthetic bacterium Rhodobacter sphaeroides is controlled by the oxygen tension and by light quantity. Two photoreceptor proteins, AppA and CryB, have been identified in the past, which are involved in this regulation. AppA senses light by its N-terminal BLUF domain, its C-terminal part binds heme and is redox-responsive. Through its interaction to the transcriptional repressor PpsR the AppA photoreceptor controls expression of photosynthesis genes. The cryptochrome-like protein CryB was shown to affect regulation of photosynthesis genes, but the underlying signal chain remained unknown. Here we show that CryB interacts with the C-terminal domain of AppA and modulates the binding of AppA to the transcriptional repressor PpsR in a light-dependent manner. Consequently, binding of the transcription factor PpsR to its DNA target is affected by CryB. In agreement with this, all genes of the PpsR regulon showed altered expression levels in a CryB deletion strain after blue-light illumination. These results elucidate for the first time how a bacterial cryptochrome affects gene expression.