Glycocin F

Glycocin F (GccF) is a unique diglycosylated bacteriocin peptide that possesses potent and reversible bacteriostatic activity against a range of Gram-positive bacteria. GccF is a rare example of a 'glycoactive' bacteriocin, with both the O-linked N-acetylglucosamine (GlcNAc) and the unusual S-linked GlcNAc moiety important for antibacterial activity. In this report, glycocin F was successfully prepared using a native chemical ligation strategy and folded into its native structure. The chemically synthesised glycocin appeared to be slightly more active than the recombinant material produced from Lactobacillus plantarum. A second-generation synthetic strategy was used to prepare 2 site selective 'glyco-mutants' containing either two S-linked or two O-linked GlcNAc moieties; these mutants were used to probe the contribution of each type of glycosidic linkage to bacteriostatic activity. Replacing the S-linked GlcNAc at residue 43 with an O-linked GlcNAc decreased the antibacterial activity, while replacing O-linked GlcNAc at position 18 with an S-linked GlcNAc increased the bioactivity suggesting that the S-glycosidic linkage may offer a biologically-inspired route towards more active bacteriocins.

The emergence of multidrug-resistant pathogens has motivated natural product research to inform the development of new antimicrobial agents. Glycocin F (GccF) is a diglycosylated 43-amino-acid bacteriocin secreted by Lactobacillus plantarum KW30. It displays a moderate phylogenetic target range that includes vancomycin-resistant strains of Enterococcus species and appears to have a novel bacteriostatic mechanism, rapidly inhibiting the growth of the most susceptible bacterial strains at picomolar concentrations. Experimental verification of the predicted role(s) of gcc cluster genes in GccF biosynthesis has been hampered by the inability to produce soluble recombinant Gcc proteins. Here, we report the development of pRV610gcc, an easily modifiable 11.2-kbp plasmid that enables the production of GccF in L. plantarum NC8. gcc gene expression relies on native promoters in the cloned cluster, and NC8(pRV610gcc) produces mature GccF at levels similar to KW30. Key findings are that the glycosyltransferase glycosylates both serine and cysteine at either position in the sequence but glycosylation of the loop serine is both sequence and spatially specific, that glycosylation of the peptide scaffold is not required for export and subsequent disulfide bond formation, that neither of the putative thioredoxin proteins is essential for peptide maturation, and that removal of the entire putative response regulator GccE decreases GccF production less than removal of the LytTR domain alone. Using this system, we have verified the functions of most of the gcc genes and have advanced our understanding of the roles of GccF structure in its maturation and antibacterial activity.IMPORTANCE The entire 7-gene cluster for the diglycosylated bacteriocin glycocin F (GccF), including the natural promoters responsible for gcc gene expression, has been ligated into the Escherichia coli-lactic acid bacteria (LAB) shuttle vector pRV610 to produce the easily modifiable 11.2-kbp plasmid pRV610gcc for the efficient production of glycocin F analogues. In contrast to the refactoring approach, chemical synthesis, or chemoenzymatic synthesis, all of which have been successfully used to probe glycocin structure and function, this plasmid can also be used to probe in vivo the evolutionary constraints on glycocin scaffolds and their processing by the maturation pathway machinery, thus increasing understanding of the enzymes involved, the order in which they act, and how they are regulated.

Here, we report on the biochemical characterization of a new glycosylated bacteriocin (glycocin), ASM1, produced by Lactobacillus plantarum A-1 and analysis of the A-1 bacteriocinogenic genes. ASM1 is 43 amino acids in length with Ser18-O- and Cys43-S-linked N-acetylglucosamine moieties that are essential for its inhibitory activity. Its only close homologue, glycocin F (GccF), has five amino acid substitutions all residing in the flexible C-terminal 'tail' and a lower IC50 (0.9 nm) compared to that of ASM1 (1.5 nm). Asm/gcc genes share the same organization (asmH← →asmABCDE→F), and the asm genes reside on an 11 905-bp plasmid dedicated to ASM1 production. The A-1 genome also harbors a gene encoding a 'rare' bactofencin-type bacteriocin. As more examples of prokaryote S-GlcNAcylation are discovered, the functions of this modification may be understood.