Circular and Leaderless Bacteriocins: Biosynthesis, Mode of Action, Applications, and Prospects

Bacteriocins are a huge family of ribosomally synthesized peptides known to exhibit a range of bioactivities, most predominantly antibacterial activities. Bacteriocins from lactic acid bacteria are of particular interest due to the latter’s association to food fermentation and the general notion of them to be safe. Among the family of bacteriocins, the groups known as circular bacteriocins and leaderless bacteriocins are gaining more attention due to their enormous potential for industrial application. Circular bacteriocins and leaderless bacteriocins, arguably the least understood groups of bacteriocins, possess distinctively peculiar characteristics in their structures and biosynthetic mechanisms. Circular bacteriocins have N-to-C- terminal covalent linkage forming a structurally distinct circular peptide backbone. The circular nature of their structures provides them superior stability against various stresses compared to most linear bacteriocins. The molecular mechanism of their biosynthesis, albeit has remained poorly understood, is believed to possesses huge application prospect as it can serve as scaffold in bioengineering other biologically important peptides. On the other hand, while most bacteriocins are synthesized as inactive precursor peptides, which possess an N-terminal leader peptide attached to a C-terminal propeptide, leaderless bacteriocins are atypical as they do not have an N-terminal leader peptide, hence the name. Leaderless bacteriocins are active right after translation as they do not undergo any post-translational processing common to other groups of bacteriocins. This “simplicity” in the biosynthesis of leaderless bacteriocins offers a huge commercial potential as scale-up production systems are considerably easier to assemble. In this review, we summarize the current studies of both circular and leaderless bacteriocins, highlighting the progress in understanding their biosynthesis, mode of action, application and their prospects

Heterologous co-chaperone recognition

Streptococcus intermedius DnaK complements the temperature-sensitive phenotype of an Escherichia coli dnaK null mutant, only if co-chaperones DnaJ and GrpE are co-expressed. Therefore, we examined whether S. intermedius DnaK and E. coli DnaK could recognize heterologous co-chaperones in vitro. The addition of heterologous GrpE to DnaK and DnaJ partially stimulated ATPase activity, and almost completely stimulated the luciferase refolding activity. Addition of heterologous DnaJ to GrpE and DnaK also stimulated ATPase activity but significant luciferase refolding activity was not observed. Moreover, E. coli DnaJ had a negative effect on the luciferase refolding activity of the S. intermedius DnaK chaperone system. In E. coli chaperone mutants, with the exception of E. coli DnaJ higher expression of the heterologous co-chaperones partially or almost completely complemented the temperature-sensitive-phenotype. These results indicated that all heterologous co-chaperones could at least partially recognize DnaK of a distantly related species. A region of the ATPase domain that is present in the DnaK of gram-negative bacteria is absent in the DnaK of gram-positive bacteria. This region is believed to be important for the recognition of co-chaperones from gram-negative bacteria. However, insertion of this segment into the S. intermedius DnaK failed to increase its ability to recognize E. coli co-chaperones. This implied that this region might be unnecessary or insufficient for the recognition of E. coli co-chaperones. Therefore, our data suggested that a basic structural similarity is conserved among the components of the S. intermedius and E. coli DnaK chaperone systems, which allow weak associations between heterologous components

Kunkecin A, a New Nisin Variant Bacteriocin Produced by the Fructophilic Lactic Acid Bacterium, Apilactobacillus kunkeei FF30-6 Isolated From Honey Bees

Apilactobacillus kunkeeiFF30-6 isolated from healthy honey bees synthesizes the bacteriocin, which exhibits antimicrobial activity againstMelissococcus plutonius. The bacteriocin, kunkecin A, was purified through three-step chromatography, and mass spectrometry revealed that its relative molecular mass was 4218.3. Edman degradation of purified kunkecin A showed only the N-terminal residue, isoleucine. Hence, alkaline alkylation made the subsequent amino acid residues accessible to Edman degradation, and 30 cycles were sequenced with 11 unidentified residues. Whole genome sequencing ofA. kunkeeiFF30-6, followed by Sanger sequencing, revealed that the genes encoding the proteins involved in lantibiotic biosynthesis were within the plasmid, pKUNFF30-6. Most of the identified proteins exhibited significant sequence similarities to the biosynthetic proteins of nisin A and its variants, such as subtilin. However, the kunkecin A gene cluster lacked the genes corresponding tonisI,nisR, andnisKof the nisin A biosynthetic gene cluster. A comparison of the gene products ofkukAandnisA(kunkecin A and nisin A structural genes, respectively) suggested that they had similar post-translational modifications. Furthermore, the structure of kunkecin A was proposed based on a comparison of the observed and calculated relative molecular masses of kunkecin A. The structural analysis revealed that kunkecin A and nisin A had a similar mono-sulfide linkage pattern. Purified kunkecin A exhibited a narrow antibacterial spectrum, but high antibacterial activity againstM. plutonius. Kunkecin A is the first bacteriocin to be characterized in fructophilic lactic acid bacteria and is the first nisin-type lantibiotic found in the familyLactobacillaceae