Biology subjects

Nicholas, R. A.

Publications and source records attributed to Nicholas, R. A..

4 recordsLinked to original sources

bioRxiv2026

Energetic coupling of an active site residue in penicillin-binding protein 2 from Neisseria gonorrhoeae with a resistance-associated conformational switch in the β3-β4 loop

Mosaic penA alleles encoding highly mutated variants of penicillin-binding protein 2 (PBP2) are the principal determinants of ceftriaxone resistance in Neisseria gonorrhoeae. Resistance-associated mutations in PBP2 from the ceftriaxone-resistant strain H041 restrict formation of the inward conformation of the {beta}3-{beta}4 loop associated with efficient acylation, but how {beta}-lactam recognition is coupled to this conformational switch is unknown. Because the conserved active-site residue Tyr422 interacts with the R1 substituent of {beta}-lactams, we investigated its role in coupling ligand recognition and acylation activity. Mutation of Tyr422 to Ala lowered acylation rates by up to 120-fold for cefoperazone and piperacillin, whereas acylation rates of ceftriaxone increased 4-fold. Unexpectedly, the crystal structure of the Y422A mutant acylated by ceftriaxone revealed that the {beta}3-{beta}4 loop had adopted the inward, high-activity conformation, despite position 422 being spatially distant from the loop. Transformation experiments showed that cell viability requires a tyrosine at position 422, indicating the residue is essential for transpeptidase function. Together, these findings reveal an energetic coupling between an active-site residue in PBP2 and a conformational switch whose equilibrium is altered by resistance mutations. The previously observed higher activity of {beta}-lactams containing extended R1 groups is consistent with stronger interactions with Tyr422 that favor the conformational switch. Molecular modeling suggests that such groups enhance activity by mimicking the iso-Glu region of the pentapeptide substrate. Overall, we propose that access to the high-activity state of PBP2 where the {beta}3-{beta}4 loop is inward is regulated by interactions between Tyr422 and {beta}-lactam R1 groups, and that resistance mutations function by tilting the balance toward a lower activity state.

biochemistry↗

bioRxiv2025

Antibiotic-resistance mutations in penicillin-binding protein 2 from the ceftriaxone-resistant Neisseria gonorrhoeae strain H041 strike a delicate balance between increasing resistance and maintaining transpeptidase activity

The mosaic penA allele (penA41) from H041, the most ceftriaxone-resistant Neisseria gonorrhoeae strain identified to date, encodes a variant of the essential Penicillin-Binding Protein 2 (PBP2) with 60 amino acid mutations compared to PBP2 from the antimicrobial-susceptible strain, FA19. Based on previous work from our lab and others, we identified a minimal set of 10 mutations that, when introduced into the {beta}-lactam antibiotic-susceptible penA allele from FA19 (penA19), confers two-thirds of the ceftriaxone and cefixime resistance compared to the penA41 allele. Three mutations (A311V, I312M, and V316P) are in the 2 helix of PBP2 containing the catalytic serine (Ser310), two (F504L and N512Y) are in the 3-4 loop that is important in binding and acylation, and one (G545S) interacts with conserved amino acids in the active site. The seventh mutation, T483S, confers substantial resistance to ceftriaxone within the minimal mutant set but requires the presence of three epistatic mutations located on the other side of the protein that do not alter resistance on their own yet are necessary to retain essential transpeptidase activity. These epistatic mutations change the backbone dihedral angles at position-447, which may increase flexibility of the enzyme and help restore essential transpeptidation. Our results highlight the complex balance necessary for evolving cephalosporin resistance while also retaining sufficient transpeptidase function in PBP2. Author SummaryIn this study, we set out to understand how Neisseria gonorrhoeae, the bacterium that causes gonorrhea, is able to resist the last remaining recommended antibiotic, ceftriaxone. Gonorrhea is a common sexually transmitted infection worldwide, and rising resistance threatens to make it untreatable. We focused on penicillin-binding protein 2 (PBP2), which is essential for the bacteriums survival and is the lethal target of ceftriaxone. By incorporating a subset of the 60 PBP2 mutations found in a highly resistant strain into PBP2 from an antibiotic-susceptible strain, we discovered that resistance evolved from a combination of mutations that work together to directly reduce the capacity of ceftriaxone to inactivate the protein and others that act as "supporting" mutations to keep the protein functional despite the presence of the resistance mutations. Our study highlights how N. gonorrhoeae successfully negotiates the delicate balance between resistance and function to escape the lethal action of antibiotics.

microbiology↗

bioRxiv2025

The role of the L421P mutation in Penicillin-Binding Protein 1 (PBP1) in the evolution of chromosomally mediated penicillin resistance in Neisseria gonorrhoeae

ponAL421P encodes a mutated variant of penicillin-binding protein 1 (PBP1) and is a key resistance determinant that increases the penicillin MIC (MICPEN) above the clinical breakpoint in Neisseria gonorrhoeae. Despite the removal of penicillin from treatment guidelines for gonococcal infections in the 1980s, ponAL421P is present in nearly 50% of current N. gonorrhoeae isolates in the PubMLST database. Bioinformatic analysis indicates that ponAL421P is exclusive to N. gonorrhoeae isolates, whereas Leu-421 is 100% conserved in other Neisseria species. To understand the involvement of ponAL421P in antibiotic resistance, we introduced ponA variants encoding 16 different amino acids at position-421 into FA6140, a penicillin-resistant gonococcal isolate that naturally harbors ponAL421P. Proline-421 was the only mutation that increased the MICPEN to the same level as FA6140. We also assessed the fitness of strains with the 16 mutant ponA alleles over multiple serial passages, both with and without sub-MIC levels of penicillin. There was no fitness defect attributed to ponAL421P under these experimental conditions; instead, our analyses suggest that the widespread occurrence of ponAL421P is driven by its capacity to increase the MICpen above the clinical breakpoint. In FA6140 transformed with the mosaic penA allele from strain H041, a ceftriaxone-resistant isolate, ponAL421P increased the MIC of ceftriaxone, suggesting that ceftriaxone targets PBP1 in this strain. We conclude that the ponAL421P allele emerged in gonococcal isolates, increasing the MICPEN above the clinical breakpoint, and has remained in the population even after the removal of penicillin from treatment guidelines. ImportanceThe emergence of antibiotic-resistant Neisseria gonorrhoeae threatens effective treatment of gonorrhea, one of the most common sexually transmitted infections worldwide. Understanding the genetic changes that drive and maintain resistance is crucial for anticipating future resistance trends. Here, we investigated the impact of a key resistance mutation in PBP1 (encoded by ponAL421P). Although penicillin has not been used to treat gonorrhea for decades, this mutation remains widespread even in recent N. gonorrhoeae isolates. ponAL421P confers clinically relevant penicillin resistance without imposing an in vitro fitness cost. ponAL421P also increases resistance to ceftriaxone in strains with penA alleles that are associated with ceftriaxone resistance. This work highlights the role of the ponAL421P allele in shaping the current antibiotic resistance landscape and supports the need for ongoing surveillance and evolutionary studies of such mutations in the gonococcal population.

microbiology↗

bioRxiv2023

Mutational analysis of LtgC, a lytic transglycosylase required for cell separation in Neisseria gonorrhoeae

Lytic transglycosylases function to degrade peptidoglycan strands that comprise the bacterial cell wall. Degradation of peptidoglycan at the septum following cell division is necessary for cell separation, and a deletion of ltgC in Neisseria gonorrhoeae results in growth in clusters of around 6-20 cells rather than as normal diplococci or monococci. N. gonorrhoeae LtgC is a homolog of Escherichia coli MltA, and comparison of the two proteins shows that LtgC has an extra domain not found in MltA, referred to as domain 3. To better understand the function of LtgC, we characterized N. gonorrhoeae mutants with substitutions in amino acids predicted to be necessary for enzymatic activity or amino acids predicted to be on the surface of domain 3, and we characterized a mutant lacking domain 3. All the mutants showed defects in cell separation, and the bacteria failed to release peptidoglycan-derived disaccharides into the medium. Purified LtgC proteins with the amino acid substitutions had reduced peptidoglycan degradation activity. LtgC was found to bind AmiC in bacterial 2-hybrid assays, and domain 3 mutations reduced binding. In human blood, an ltgC mutant showed decreased survival, suggesting the cell wall defects in the mutant make the bacteria more sensitive to innate immune system components. ImportanceNeisseria gonorrhoeae uses a smaller set of proteins for peptidoglycan breakdown compared to Escherichia coli or other model systems. The peptidoglycan breakdown that occurs at the septum following cell division in N. gonorrhoeae requires three proteins, amidase AmiC, amidase activator NlpD, and lytic transglycosylase LtgC. LtgC has an unusual structure that includes a third domain not found in related proteins. Using mutants that lacked LtgC activity or had amino acid changes in the third domain, we found that the extra domain is involved in interaction of LtgC with AmiC and that it is required for LtgC function for cell separation. All of the ltgC mutants examined showed reduced survival in blood, indicating the importance of LtgC activity for infection.

microbiology↗