Fundamental Food Microbiology, Third Edition - Fuad Fathir

236 FUNDAMENTAL FOOD MICROBIOLOGY
proteins and modulate transcription of nis-operon; Nis E, Nis F, and Nis G provide
accessory protection to the producer cells against nisin to host. Similar genes for
regulatory proteins are found in other bacteriocin operons such as sakacin P.
Biosynthesis of a bacteriocin involving transcription, translation, structural
modification, and secretion occur in a concerted way. In pediocin AcH or PA-1
and similar bacteriocins, probably a single mRNA is produced for all four genes,
which is then translated into four separate proteins (Figure 16.1). As soon as the
62-amino-acid prepedicin is translated in the cytoplasm, the leader peptide directs
transport of the molecule outside the cell by the help of Pap or Ped D (ABC
transporter) and C (helper). During transportation, the catalytic site of protein D
recognizes –G–G– at the –1 and –2 positions of prepediocin and cleaves the leader
peptide (18 amino acids); the transporter then secretes the 44-amino-acid pediocin
in the environment. In an oxidized environment, the four cysteine molecules form
two disulfide bonds, one at positions +9 and +14 and another at +24 and +44. The
location and mechanism of the immunity protein Pap or Ped B in the producing
cells are not known. In nisin A production, initially proteins Nis R and Nis K
induce transcription of the cluster, probably to produce a single mRNA, which is
then translated into 13 separate proteins. Nis B dehydrates serine and threonine
to their respective dehydroamino acids, and Nis C with Nis B then enable dehydroamino
acids to form thioether rings with cysteine residues in the molecule. The
modified molecule is then translocated through the membrane by Nis T, and Nis
P removes the 14-amino-acid leader peptide, releasing the 34-amino-acid nisin
outside the cells. Nis E, Nis F, and Nis G provide extra protection to producer
cells against nisin.
Nisin A
Pediocin AcH
Leucocin A
Sakacin A
Sakacin P
A B T C I P
A B C D
R K F E G
B A E C D
Transposon
Plasmid
Plasmid
ORF 1 I A ORF 2 ORF 3 ORF 4 K R T E
ORF 1 K R A I T E
Figure 16.1 Organization of several well-characterized bacteriocin gene clusters. Structural
genes are shown as open bars, promoters as black arrow heads, and rho-independent
terminators as lollipop symbols. Genes of unknown functions are designated
ORFs.