amino acids [49]. However, based on its striking homology with PlnI, it could
also be considered as an immunity protein for PlnN [49]. Upstream from plnN,
a 66 amino acid protein, PlnM with one putative transmembrane helix was
found. Besides PlnP, PlnM could hence be considered as a second valid candidate for PlnN immunity, although located in the 5¢ end of the operon. Two
proteins, PlnG (ABC transporter) and plnH (accessory protein), shown to
constitute an ATP-dependent transport apparatus, were located downstream of
the plnABCD operon [49]. The homologous counterparts in the sakacin A
operon were named SapT and SapE, respectively [133]. The region encoding
plantaricin A activity has proven to be a multiple gene locus consisting of not
less than 22 different open reading frames in the same or opposite orientation
to the previously described plnABCD operon [49, 134]. Besides the above
described proteins, PlnROSTUV and orf1 display no homology with other
protein sequences, and their function in plantaricin A activity has not been
elucidated [49].
The sakacin A region was transcribed as two operons: the first one encompassed the structural sakacin A gene sakA, and its immunity factor SaiA, and
the second covered the sapK, sapR, sapT and sapE genes involved in transcription regulation and sakacin A export [133]. Northern blot analysis revealed that
the putative SapR/SapK system probably acted as a transcription activator
[133]. A 35-bp region, upstream from the putative sapA promoter, and a similar
sequence upstream from sapK were necessary for proper expression and could
be possible targets for transcriptional activation [133]. Five promoters (upstream from plnA, plnE, plnJ, plnM and plnG) and six rho-independent transcription terminators (downstream from the operons plnABCD, plnJKLR,
plnMNOP, plnEFI and the ORFs plnF and plnN) have been mapped in the
plantaricin A cluster, resulting in a complex expression pattern [49, 134]. The
–10 consensus sequences were located 6 to 7 bp upstream from the transcription start site, but the –35 consensus was more difficult to identify [49]. Just
upstream from the putative – 35 region, all promoters were seen to harbor two
direct repeats spaced by an A+T-rich strand of 12 bp [49].
3.5.5
Class IIB Bacteriocins
Two bacteriocins and an immunity protein, organized in one operon, comparable with the lactococcin M operon, were detected in the case of lactacin F,
lactococcin G, and in two operons (plnEFI and plnJKLR) of the plantaricin A
gene cluster. All four depend on complementation of two bacteriocin peptides
for highest activity and therefore belong to the class IIB bacteriocins [46, 49,
124, 128].
Purification of lactococcin G identified two peptides, a and b, that individually exhibited marginal levels of activity. Upon complementation of the two
peptides in a 7a:lb ratio, a seven-fold increase in activity was noted [47].
Allison et al. [48] proved that lactacin F activity and host range were expanded
upon the complementation of two heterologously expressed peptides of the lactacin F operon, although initially only one bioactive peptide (LafA) was purified
Antimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
35
also be considered as an immunity protein for PlnN [49]. Upstream from plnN,
a 66 amino acid protein, PlnM with one putative transmembrane helix was
found. Besides PlnP, PlnM could hence be considered as a second valid candidate for PlnN immunity, although located in the 5¢ end of the operon. Two
proteins, PlnG (ABC transporter) and plnH (accessory protein), shown to
constitute an ATP-dependent transport apparatus, were located downstream of
the plnABCD operon [49]. The homologous counterparts in the sakacin A
operon were named SapT and SapE, respectively [133]. The region encoding
plantaricin A activity has proven to be a multiple gene locus consisting of not
less than 22 different open reading frames in the same or opposite orientation
to the previously described plnABCD operon [49, 134]. Besides the above
described proteins, PlnROSTUV and orf1 display no homology with other
protein sequences, and their function in plantaricin A activity has not been
elucidated [49].
The sakacin A region was transcribed as two operons: the first one encompassed the structural sakacin A gene sakA, and its immunity factor SaiA, and
the second covered the sapK, sapR, sapT and sapE genes involved in transcription regulation and sakacin A export [133]. Northern blot analysis revealed that
the putative SapR/SapK system probably acted as a transcription activator
[133]. A 35-bp region, upstream from the putative sapA promoter, and a similar
sequence upstream from sapK were necessary for proper expression and could
be possible targets for transcriptional activation [133]. Five promoters (upstream from plnA, plnE, plnJ, plnM and plnG) and six rho-independent transcription terminators (downstream from the operons plnABCD, plnJKLR,
plnMNOP, plnEFI and the ORFs plnF and plnN) have been mapped in the
plantaricin A cluster, resulting in a complex expression pattern [49, 134]. The
–10 consensus sequences were located 6 to 7 bp upstream from the transcription start site, but the –35 consensus was more difficult to identify [49]. Just
upstream from the putative – 35 region, all promoters were seen to harbor two
direct repeats spaced by an A+T-rich strand of 12 bp [49].
3.5.5
Class IIB Bacteriocins
Two bacteriocins and an immunity protein, organized in one operon, comparable with the lactococcin M operon, were detected in the case of lactacin F,
lactococcin G, and in two operons (plnEFI and plnJKLR) of the plantaricin A
gene cluster. All four depend on complementation of two bacteriocin peptides
for highest activity and therefore belong to the class IIB bacteriocins [46, 49,
124, 128].
Purification of lactococcin G identified two peptides, a and b, that individually exhibited marginal levels of activity. Upon complementation of the two
peptides in a 7a:lb ratio, a seven-fold increase in activity was noted [47].
Allison et al. [48] proved that lactacin F activity and host range were expanded
upon the complementation of two heterologously expressed peptides of the lactacin F operon, although initially only one bioactive peptide (LafA) was purified
Antimicrobial Peptides of Lactic Acid Bacteria: Mode of Action, Genetics and Biosynthesis
35