Fundamental Food Microbiology, Third Edition - Fuad Fathir

FOOD BIOPRESERVATIVES OF MICROBIAL ORIGIN 235
molecule (at +24 and +44 positions in pediocin AcH or PA-1); the presence of an
extra disulfide bond at the C-terminus makes the molecules more potent bactericidal
agents than those with one disulfide bond at the N-terminus region (e.g., leucocin
A). In general, the amino acid sequence of a bacteriocin determines the formation
of a-helix and b-sheet domains in the molecule, which are important for its bactericidal
efficiency.
C. Genetics and Gene Organization
The limited available information has revealed that the structural genes encoding
prebacteriocin molecules of lactic acid bacteria can be encoded in a plasmid (e.g.,
pediocin AcH or PA-1), in the genome (e.g., sakacin P), or in a genome-integrated
transposon (e.g., nisin A) of a producer strain. With plasmid-encoded bacteriocins,
several variations are observed: a single plasmid can encode only one bacteriocin
(e.g., pediocin AcH or PA-1) or more than one bacteriocin (e.g., lactococcin A, B,
and M); the same bacteriocin can be encoded in different-size plasmids in the same
species (e.g., lactococcin A in three separate strains of Lac. lactis) or different species
(e.g., pediocin AcH in Ped. acidilactici, Ped. pentosaceus, Ped. purvulus, and Lab.
plantarum); or a strain producing more than one bacteriocin that can be encoded in
different plasmids (e.g., carnobacteriocin A and canobacteriocin B1 and B2 are
encoded in two plasmids in a Car. piscicola strain).
In addition to the structural gene encoding for a bacteriocin, the DNA (plasmid
or genome) also encodes several other genes related to its production (Figure 16.1).
Some of them are quite simple (e.g., pediocin AcH) whereas others can be very
complex (e.g., nisin A). There are many other differences in the organization of
the genes associated with bacteriocin production in lactic acid bacteria. The genes
could present in a single operon (e.g., in pediocin AcH and nisin A) or in more
than one operon (sakacin P), and all genes are arranged in the same direction (e.g.,
sakacin P) or in opposite directions (e.g., leucocin A). In some, one or more genes
can overlap (e.g., nisin A, leucocin A). The gene organization in the simplest form,
as observed for pediocin AcH or PA-1 production, consists of an operon, with the
promoter located upstream, and four genes, encoding for four proteins: pap or ped
A for prepediocin, pap or ped B for immunity protein (Pap B or Ped B confer
immunity against pediocin), pap or ped C for helper protein (Pap C or Ped C
necessary for the efficient transport of pediocin), and pap or ped D (Pap D or Ped
D for the ABC-transporter protein). Almost similar gene expressions, but with two
promoters, are present in the production of leucocin A. In nisin A production, the
genetic expression is quite different. Thirteen genes, nis ABTCIPRKFEG, are
present in the operon, with the promoter located upstream and nis A encoding for
prenisin (Figure 16.1). The functions of the other proteins from the other genes
are as follows: Nis B and Nis C are involved in the dehydration of serine and
threonine to dehydroalanine and dehydrobutyrine and thioether ring formation;
Nis T is the ABC transporter involved in the transportation of nisin; Nis I is
responsible for producer cell immunity to nisin; Nis P removes the leader peptide
from prenisin following its transport by Nis T; Nis R and Nis K are regulatory
\