Activating mutations in quorum-sensing regulator Rgg2 and its conformational flexibility in the absence of an intermolecular disulfide bond

In silico characterisation of stand-alone response regulators of Streptococcus pyogenes

Bacterial “stand-alone” response regulators (RRs) are pivotal to the control of gene transcription in response to changing cytosolic and extracellular microenvironments during infection. The genome of group A Streptococcus (GAS) encodes more than 30 stand-alone RRs that orchestrate the expression of virulence factors involved in infecting multiple tissues, so causing an array of potentially lethal human diseases. Here, we analysed the molecular epidemiology and biological associations in the coding sequences (CDSs) and upstream intergenic regions (IGRs) of 35 stand-alone RRs from a collection of global GAS genomes. Of the 944 genomes analysed, 97% encoded 32 or more of the 35 tested RRs. The length of RR CDSs ranged from 297 to 1587 nucleotides with an average nucleotide diversity (π) of 0.012, while the IGRs ranged from 51 to 666 nucleotides with average π of 0.017. We present new evidence of recombination in multiple RRs including mga, leading to mga-2 switching, emm-switching and emm-like gene chimerization, and the first instance of an isolate that encodes both mga-1 and mga-2. Recombination was also evident in rofA/nra and msmR loci with 15 emm-types represented in multiple FCT (fibronectin-binding, collagen-binding, T-antigen)-types, including novel emm-type/FCT-type pairings. Strong associations were observed between concatenated RR allele types, and emm-type, MLST-type, core genome phylogroup, and country of sampling. No strong associations were observed between individual loci and disease outcome. We propose that 11 RRs may form part of future refinement of GAS typing systems that reflect core genome evolutionary associations. This subgenomic analysis revealed allelic traits that were informative to the biological function, GAS strain definition, and regional outbreak detection.

Structure-function studies of Rgg binding to pheromones and target promoters reveal a model of transcription factor interplay

Regulator gene of glucosyltransferase (Rgg) family proteins, such as Rgg2 and Rgg3, have emerged as primary quorum-sensing regulated transcription factors in Streptococcus species, controlling virulence, antimicrobial resistance, and biofilm formation. Rgg2 and Rgg3 function is regulated by their interaction with oligopeptide quorum-sensing signals called short hydrophobic peptides (SHPs). The molecular basis of Rgg-SHP and Rgg-target DNA promoter specificity was unknown. To close this gap, we determined the cryoelectron microscopy (cryo-EM) structure of Streptococcus thermophilus Rgg3 bound to its quorum-sensing signal, SHP3, and the X-ray crystal structure of Rgg3 alone. Comparison of these structures with that of an Rgg in complex with cyclosporin A (CsA), an inhibitor of SHP-induced Rgg activity, reveals the molecular basis of CsA function. Furthermore, to determine how Rgg proteins recognize DNA promoters, we determined X-ray crystal structures of both Streptococcus dysgalactiae Rgg2 and S. thermophilus Rgg3 in complex with their target DNA promoters. The physiological importance of observed Rgg-DNA interactions was dissected using in vivo genetic experiments and in vitro biochemical assays. Based on these structure-function studies, we present a revised unifying model of Rgg regulatory interplay. In contrast to existing models, where Rgg2 proteins are transcriptional activators and Rgg3 proteins are transcriptional repressors, we propose that both are capable of transcriptional activation. However, when Rgg proteins with different activation requirements compete for the same DNA promoters, those with more stringent activation requirements function as repressors by blocking promoter access of SHP-bound conformationally active Rgg proteins. While a similar gene expression regulatory scenario has not been previously described, in all likelihood it is not unique to streptococci.