18-residue N-terminal leader peptide at a very conserved double-glycineresidue processing site. Site-directed mutagenesis of the lactacin F (LafA)
precursor was employed to modify the glycine residues at positions –1 and –2,
and the valine (–3 position) and arginine (+1 position) residues [77].
Replacement of glycine at position –1 with valine, or replacement of the glycine
residue at position –2 with arginine, or even the polarityneutral amino acid
serine eliminated lactacin F expression. Replacement of valine at position –3
with a charged residue (Asp) or arginine at position +1 with another positively
charged residue did not disrupt bacteriocin activity [77]. Based on these experiments and sequence alignments, a more conserved pattern for the bacteriocin leader peptide was proposed [77] including (i) two conserved glycines at
positions –1, and –2, (ii) hydrophobic residues at positions 4, –7, –12, and –15,
(iii) a core of charged amino acids at positions –8 and –10, and (iv) a serine at
position –11. Heterologous expression of lactacin F peptides in Carnobacterium
piscicola LV17, and Leuconostoc gelidum resulted in the production of mature,
bioactive lactacin F [204, 205].
These results confirm the conservative nature of the processing and secretion apparatus involved in class II non-lantibiotic maturation, as it was capable
of recognizing and properly processing heterologously expressed lactacin F
[204, 205]. Heterologous expression of the lactococcin A operon in Pediococcus
and expression of the pediocin PA-1 operon in Lactococcus lactis also resulted
in fully matured bioactive peptides [206]. Recently, the new bacteriocin
divergicin A was shown to be secreted by the sec-dependent pathway [51]. The
N-terminal extension of divergicin A had a -A-S-A-(positions –3 to –1) cleavage
site and acts as a signal peptide that accessed the general export system of the
cell [51]. Production of divergicin A was demonstrated in heterologous hosts
containing the two genes associated with the bacteriocin and immunity [51].
These data indicate that a fully functional bacteriocin molecule can be produced in the absence of the typical leader peptide consensus and the corresponding ABC transporter gene.
Comparison of the leader peptides and the ABC transporter systems of the
class IA II lantibiotics and the class II non-lantibiotic bacteriocins suggest that
both are processed and secreted in the same manner [21, 40, 146]. A striking
feature of the class IA II lantibiotic leader peptides consists of a very conserved
glutamate residue at position –13, missing in the class II leaders and resulting
in the ELS consensus [40]. The class IA II lantibiotics, in contradiction to the
class II non-lantibiotic bacteriocins undergo post-translational modification
and their leader peptide would hence not only be involved in proper processing
of the precursor, but might fulfill a comparable role in conformational stabilization during post-translational modification, as the class IA I leader peptides
[21, 40, 82].
7
Conceptual Model for Bacteriocin Maturation
A combination of the above described findings could hence result in the following hypothetical model for bacteriocin biosynthesis (Figs. 8 and 9). Firstly,
Antimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
47
precursor was employed to modify the glycine residues at positions –1 and –2,
and the valine (–3 position) and arginine (+1 position) residues [77].
Replacement of glycine at position –1 with valine, or replacement of the glycine
residue at position –2 with arginine, or even the polarityneutral amino acid
serine eliminated lactacin F expression. Replacement of valine at position –3
with a charged residue (Asp) or arginine at position +1 with another positively
charged residue did not disrupt bacteriocin activity [77]. Based on these experiments and sequence alignments, a more conserved pattern for the bacteriocin leader peptide was proposed [77] including (i) two conserved glycines at
positions –1, and –2, (ii) hydrophobic residues at positions 4, –7, –12, and –15,
(iii) a core of charged amino acids at positions –8 and –10, and (iv) a serine at
position –11. Heterologous expression of lactacin F peptides in Carnobacterium
piscicola LV17, and Leuconostoc gelidum resulted in the production of mature,
bioactive lactacin F [204, 205].
These results confirm the conservative nature of the processing and secretion apparatus involved in class II non-lantibiotic maturation, as it was capable
of recognizing and properly processing heterologously expressed lactacin F
[204, 205]. Heterologous expression of the lactococcin A operon in Pediococcus
and expression of the pediocin PA-1 operon in Lactococcus lactis also resulted
in fully matured bioactive peptides [206]. Recently, the new bacteriocin
divergicin A was shown to be secreted by the sec-dependent pathway [51]. The
N-terminal extension of divergicin A had a -A-S-A-(positions –3 to –1) cleavage
site and acts as a signal peptide that accessed the general export system of the
cell [51]. Production of divergicin A was demonstrated in heterologous hosts
containing the two genes associated with the bacteriocin and immunity [51].
These data indicate that a fully functional bacteriocin molecule can be produced in the absence of the typical leader peptide consensus and the corresponding ABC transporter gene.
Comparison of the leader peptides and the ABC transporter systems of the
class IA II lantibiotics and the class II non-lantibiotic bacteriocins suggest that
both are processed and secreted in the same manner [21, 40, 146]. A striking
feature of the class IA II lantibiotic leader peptides consists of a very conserved
glutamate residue at position –13, missing in the class II leaders and resulting
in the ELS consensus [40]. The class IA II lantibiotics, in contradiction to the
class II non-lantibiotic bacteriocins undergo post-translational modification
and their leader peptide would hence not only be involved in proper processing
of the precursor, but might fulfill a comparable role in conformational stabilization during post-translational modification, as the class IA I leader peptides
[21, 40, 82].
7
Conceptual Model for Bacteriocin Maturation
A combination of the above described findings could hence result in the following hypothetical model for bacteriocin biosynthesis (Figs. 8 and 9). Firstly,
Antimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
47