GlcNAc-1,6-anhydro-MurNAc Moiety Affords Unusual Glycosyl Acceptor that Terminates Peptidoglycan Elongation.

Peptidoglycan (PG), an essential exoskeletal polymer in bacteria, is a well-known antibiotic target. PG polymerization requires the action of bacterial transglycosylases (TGases), which couple the incoming glycosyl acceptor to the donor. Interfering with the TGase activity can interrupt the PG assembly. Existing TGase inhibitors like moenomycin and Lipid II analogues always occupy the TGase active sites; other strategies to interfere with proper PG elongation have not been widely exploited. Inspired by the natural 1,6-anhydro-MurNAc termini that mark the ends of PG strands in bacteria, we hypothesized that the incorporation of an anhydromuramyl-containing glycosyl acceptor by TGase into the growing PG may effectively inhibit PG elongation. To explore this possibility, we synthesized 4-O-(N-acetyl-ß-d-glucosaminyl)-1,6-anhydro-N-acetyl-ß-d-muramyl-l-Ala-γ-d-Glu-l-Lys-d-Ala-d-Ala, 1, within 15 steps, and demonstrated that this anhydromuropeptide and its analogue lacking the peptide, 1-deAA, were both utilized by bacterial TGase as noncanonical anhydro glycosyl acceptors in vitro. The incorporation of an anhydromuramyl moiety into PG strands by TGases afforded efficient termination of glycan chain extension. Moreover, the preliminary in vitro studies of 1-deAA against Staphylococcus aureus showed that 1-deAA served as a reasonable antimicrobial adjunct of vancomycin. These insights imply the potential application of such anhydromuropeptides as novel classes of PG-terminating inhibitors, pointing toward novel strategies in antibacterial agent development.

Functional analysis of germline RAD51C missense variants highlight the role of RAD51C in replication fork protection.

Monoallelic or biallelic RAD51C germline mutations results in chromosome instability disorders such as Fanconi anemia and cancers. The bona fide function of RAD51C is to assist RAD51 nucleoprotein filament onto single-strand DNA to complete homologous recombination (HR) repair. In addition to HR repair, the role of RAD51C in DNA replication is emerging when replication forks are transiently or irreversibly stalled. We identified novel RAD51C variants of uncertain significance (VUS) from breast, ovarian, pancreatic and gastric cancer patients and functionally characterized the effect of these variants in replication fork protection and double-strand breaks (DSB's) repair. In RAD51C-deficient Chinese hamster CL-V4B cells, expression of RAD51C F164S, A87E, L134S and E49K variants heightened sensitivity to mitomycin C (MMC), etoposide and PARP inhibition. Differently, expression of subset of RAD51C variants R24L, R24W and R212H displayed mild sensitivity to MMC, etoposide and PARP inhibition. Further functional characterization of a subset of variants revealed that Rad51C F164S, A87E, L134S and E49K variants displayed reduced RAD51 foci formation and increased overall nuclear single strand DNA levels in the presence of replication stress. Additionally, DNA fiber assay revealed that RAD51C F164S, A87E, L134S and E49K variants displayed defective replication fork protection upon prolonged fork stalling. Investigations using patient-derived lymphoblastoid cell line carrying heterozygous RAD51C L134S variant showed an impairment in RAD51 chromatin association and replication fork protection, suggestive of deleteriousness of this VUS variant. Overall, our findings provide more insights into molecular roles of RAD51C in replication fork integrity maintenance and in DSB repair.