Biomedical Engineering Reference
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Fig. 2.1
Gene organization of lasso peptide clusters. Examples of known lasso peptide gene clus-
ters representing: (1) core cluster with ABC transporters:
mcj
(MccJ25 cluster from
Escherichia
coli
) and
lar
featuring split-B genes (lariatin from
Rhodococcus jostii
) and (2) core cluster with
isopeptidases:
axt
featuring accessory genes (astexin 1 from
Asticcacaulis excentricus
). Validated
genes are labelled with standard names. Genes coding for precursors (
red
), full length or split B
proteins (
green
), C proteins (
orange
), transporters (
dark blue
), isopeptidases (
purple
), GntR-like
transcription regulators (
light blue
), TonB-dependent receptors (
grey
), sigma factors (
yellow
) and
anti-sigma factors (
black
) are shown
their gene clusters validated. A global picture of lasso gene architectures emerged
from these functional and genomic data. The
A
,
B
and
C
genes are conserved in
all characterized and putative clusters comprising the core cluster essential for
lasso peptide biosynthesis. In most cases they are arranged in the same direction.
The
B
and
C
genes are generally transcriptionally coupled. Putative transcription
terminators are present in some instances in the intergenic region between
A
and
BC
genes (Severinov et al.
2007
). For MccJ25,
mcjA
is transcribed in an opposite direc-
tion to the
mcjBCD
operon and is under the control of its own promoter. The main
deviations in the core cluster include the presence of multiple
A
genes (up to three)
and split of
B
genes (
B1
and
B2
genes). Three types of lasso gene architectures can
be discerned: (1) core cluster only, (2) core cluster with ABC transporter genes and
(3) core cluster with an isopeptidase gene (Fig.
2.1
). The other accessory genes po-
tentially involved in the regulation or further post-translational modifications (e.g.
phosphorylation, sulfonation, acetylation, methylation) can be found clustered with
some 'core cluster/ABC transporter' and 'core cluster/isopeptidase' architectures
(Hegemann et al.
2013b
; Maksimov and Link
2014
); however, they have not been
experimentally validated yet. Currently, 6 out of 24 known lasso peptides with char-
acterized genes are from the 'core cluster/ABC transporter' type, while the rest are
products of the 'core cluster/isopeptidase' type. No lasso peptides from the 'core
cluster only' type have yet been isolated. A summary of characterized lasso gene
clusters is given in Table
2.2
.
2.2
Production and Purification
Early discovery of lasso peptides used a classical approach relying on the search
for antibiotic, antiviral or other biological activities of bacterial extracts, followed
by purification of the active fractions. The last lasso peptide discovered following
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