Biological Activities of Lasso Peptides and Structure–Activity Relationships - Lasso Peptides: Bacterial Strategies to Make and Maintain Bioactive Entangled Scaffolds - page 60

with a different spectrum of activity supports the idea that the spectrum of activity
of lasso peptides is mainly governed by the uptake process.
A very different mechanism of action has recently been reported for siamycin I
(Nakayama et al. 2007 ; Ma et al. 2011 ). Siamycin I (also named MS-271 and NP-
06) is a class I lasso peptide produced by Streptomyces . It exerts antibacterial activ-
ity against Gram-positive bacteria, including the hospital-acquired infection agent
Enterococcus faecalis (Tsunakawa et al. 1995 ; Yano et al. 1996 ; Nakayama et al.
2007 ; Table 3.2 ). Siamycin I has been shown to attenuate quorum-sensing-mediated
virulence in E. faecalis (Nakayama et al. 2007 ; Ma et al. 2011 ). Gelatinase is a ma-
jor virulence factor in E. faecalis, being involved in the formation of biofilms, and
thus adherence and pathogenicity (Su et al. 1991 ). Its expression is regulated by the
FsrABCD two-component regulation system. The kinase sensor FsrC sensor histi-
dine kinase, upon activation by the gelatinase biosynthesis-activating pheromone
(GBAP) peptide encoded by the fsrBD genes, phosphorylates the FsrA response
regulator (Qin et al. 2001 ; Hancock and Perego 2004 ; Del Papa and Perego 2011 ),
thus activating the transcription of different genes, including fsrBCD . In 2007, the
lasso peptide siamycin I was isolated during the screening of actinomycete cul-
ture supernatants for inhibition of quorum-sensing-mediated gelatinase activity
(Nakayama et al. 2007 ). In 2011, Ma et al. showed that siamycin I inhibits FsrC
sensor kinase activity (Ma et al. 2011 ). A study of the interaction between siamycin
I with FsrC by synchrotron radiation circular dichroism spectroscopy (SRCD) indi-
cated that the peptide binding occurs at a different, nonoverlapping site to the native
ligand, GBAP (Phillips-Jones et al. 2013 ). However, this inhibition was not specific
to FsrC, since siamycin I also inhibited several ATP-binding enzymes, including
nine membrane sensor kinases from E. faecalis (Ma et al. 2011 ). This observation
raises questions on the real origin of the antibacterial activity, and on the role of
lasso peptides in bacterial communication.

3.2 Structure-Activity Relationship

Extensive structure-activity relationship studies, involving chemical modifications,
enzymatic hydrolysis and saturation or site-directed mutagenesis, have permitted to
delineate the residues involved in the key stages of MccJ25 mechanism of action. In
addition, the comparison of the lasso peptide sequences, producing strains and spec-
trum of activity, has revealed general tendencies to account for the selectivity of the
antibacterial activity, at least for peptides active against Gram-negative bacteria.

3.2.1 MccJ25

The evaluation of the antibacterial activities and the ability of variants to inhib-
it RNAP of respiratory chain permitted to identify the key elements involved in
MccJ25 mechanism of action (Fig. 3.7 ). First of all, the branched-cyclic peptide