Circular bacteriocins

Borrero, J., Brede, D.A., Skaugen, M., et al. (2011) Characterization of garvicin ML, a novel circular bacteriocin produced by Lactococcus garvieae DCC43, isolated from mallard ducks (Anas platyrhynchos). Appl Environ Microbioin, 369-373. 

Class IIc bacteriocins comprise the circular bacteriocins, characterized by a head-to-tail covalent bond forming a circular stmcture (Figure 5.1) (van Belkum et al. 2011 Gabrielsen et al. 2014). These bacteriocins are known for a relatively broad inhibitory spectrum and resistance to heat, extreme pH, and even many proteases. 

It is well known that transporters play a role in bacteriocin producer self-immunity for different classes of bacteriocins, including lantibiotics (Draper et al. 2008) and circular bacteriocins (Martinez-Bueno et al. 1998 Kempaman et al. 2003). Different transporter complexes have also been shown... 

Gabrielsen, C., Brede, D.A., Herndndez, P.F., et al. (2012). The maltose ABC transporter in Lactococctts facft sfaciU-tates high-level sensitivity to the circular bacteriocin garvicin ML. Antimicrob Agents Chemother S6, 2908-2915. 

Maqueda, M., Sanchez-Hidalgo, M., Fernandez, M., et al. (2008). Genetic features of circular bacteriocins produced by Gram-positive bacteria. FEMS Microbiol Rev 32, 2-22. 

Martin-Visscher, L.A., Gong, X., Duszyk, M., and Vederas, J.C. (2009). The three-dimensional structure of camocychn A reveals that many circular bacteriocins share a common structural motif. J Biol Chem 284, 28674-28681. 

Cyclic bacteriocins. At least seven bacteriocins discovered over recent years have been shown to have the unusual feature of a head-to-tail circular backbone.56 This post-translational modification is believed to stabilise the 3D structures of the peptides in a conformation needed for their membrane-disrupting function. 

Database searches aimed at identifying related proteins found that the closest structural relative to bacteriocin AS-48 is the effector polypeptide from porcine lymphocytes, NK-lysin.63 Similar to AS-48, this 76-residue protein adopts a globular fold with five helices surrounding a hydrophobic core but lacks a circular backbone.64 Instead, NK-lysin comprises three cross-bracing disulphide bonds that stabilise the structure. Despite the linear backbone of NK-lysin creating differences in the orientation of helix 5 in relation to the one in AS-48, and despite no significant sequence homology between them, the backbone... 

Plasmids are autonomous, circular DNA molecules (from around 1 kilobase pairs (kb) to several hundreds of kb in size) that are capable of self-replication within a bacterial cell (Figure 1). Plasmids contain some genes involved in their own replication and transfer between bacteria, but can also harbor additional genes that can impart such biochemical capabilities as antibiotic resistance, utilization of additional nutrients, production of pathogenic factors, nitrogen fixation, or the production of bacteriocins. 

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