Mallery, C. | Search bioRxiv

bioRxiv2025

How medically important antimicrobials bind to the 30S ribosomal subunit in a bacterial pathogen

Ribosomes translate the genetic code in mRNA to synthesize proteins in all living organisms. Decoding of mRNA occurs in the small subunit of the ribosome and is mediated by tRNA anticodons. Regions near the decoding center are a target for antibiotics, such as aminoglycosides and tetracyclines, where their presence results in errors in protein synthesis. More than two decades of high-resolution structural studies have shown how such medically important antimicrobials (MIAs) bind to the small subunit of the bacterial ribosome. Here, we comprehensively analyze these previously reported structures to help understand the variability with which MIAs bind to small subunits of bacterial ribosomes. We previously solved the hibernating 70S ribosome structure of the bacterial pathogen Borrelia burgdorferi (Bbu), the causative agent of Lyme disease, but there is no structure of any MIA bound to this ribosome reported. Our structural analysis makes it possible to use inexpensive computational methods to predict binding of these MIAs to the Bbu ribosomal 30S small subunit. For this, we used structural analogy, restrained energy minimization, and single-point binding free energy computations. We find the single-point binding free energy of the MIAs to be very sensitive to small conformational changes in the MIA and its environment. Incorporating this knowledge in structure-guided design could aid in development of narrow-spectrum MIAs targeting specific bacterial pathogens.

biophysics↗

bioRxiv2024

Ligand binding determines proteolytic stability of Vibrio LuxR/HapR quorum sensing transcription factors

In Vibrio species, quorum sensing signaling culminates in the production of the master transcription factor SmcR that regulates group behavior genes in a density-dependent manner. Previously, we identified a small molecule thiophenesulfonamide inhibitor called PTSP that targets the SmcR family of proteins and blocks activity in vivo. Here, we used structure-function analyses to identify eight PTSP-interacting residues in the ligand binding pocket that are required for PTSP inhibition of Vibrio vulnificus SmcR. Binding of PTSP to SmcR drives allosteric unfolding of the N-terminal DNA-binding domain and, in this state, SmcR is degraded by the ClpAP protease. SmcR degradation controls the timing of the phenotypic switch between high and low cell density, and strains expressing degradation-resistant smcR alleles are impervious to changes in cell density state. These studies implicate ligand binding as a mediator of SmcR protein stability and function, which dictates the timing of quorum sensing gene expression in three Vibrio pathogens. Significance StatementSmcR family proteins were discovered in the 1990s as central regulators of quorum sensing gene expression and later discovered to be conserved in all studied Vibrio species. SmcR homologs regulate a wide range of genes involved in pathogenesis, including but not limited to genes involved in biofilm production and toxin secretion. As archetypal members of the broad class of TetR-type transcription factors, each SmcR type protein has a predicted ligand binding pocket. However, no ligand has been identified for these proteins that control their function as regulators. Here, we used SmcR-specific chemical inhibitors to determine that ligand binding drives proteolytic degradation in vivo, the first demonstration of SmcR function connected to ligand binding for this historical protein family.

microbiology↗