Biology Reference
In-Depth Information
The origin of these structural variations is a very interesting aspect of the
research in this field and will be discussed below.
The majority of the metabolites described in
Table 6.1
are products of
PKS, NRPS or more frequently of PKS/NRPS hybrids, but several recent
studies suggest that RPs are much more represented than believed in the
cyanobacteria (
Li et al., 2010
;
Velasquez & van der Donk, 2011
;
Wang,
Fewer, et al., 2011
;
Wang, Xu, et al., 2011
). Thus, the data reported in
Table 6.1
are probably biased. Nevertheless, it is quite interesting to note
that PKSs and NRPSs are capable to produce very different molecules even
though their basic functions supported by the individual domains are con-
served, particularly for the PKSs. Not surprisingly, the biological activities
of the secondary metabolites are very diverse.
3.2. The Producing Strains
We have also indicated in
Table 6.1
the producers, in which the biosynthetic
genes have been sequenced. It can be seen that many genera are represented,
but as indicated earlier in this review, the producers are mostly filamentous
or colonial cyanobacteria. As expected, some cyanobacteria produce several
secondary metabolites. For instance, the strains
Planktothrix agardhii
NIVA-
CYA 126/8 and
Planktothrix rubescens
NIVA-CYA 98 are cited four and
six times, respectively in
Table 6.1
. These two strains have been well char-
acterized and their genome has been sequenced, and they produce several
secondary metabolites. This is a good illustration of the capacity of cyano-
bacteria to produce many secondary metabolites at the same time, and it is
expected that this is the case for many other cyanobacteria. It is also clear
from
Table 6.1
that metabolites from freshwater cyanobacteria are produced
by different genera (see, for instance, cylindrospermopsin producers). This
is very likely a result of horizontal gene transfer events as discussed below.
Thus, a particular metabolite can be found in many different freshwater
cyanobacteria. However, it seems that freshwater cyanobacteria and marine
cyanobacteria do not produce the same set of secondary metabolites, except
for the cyanobactins that have been found in either class of cyanobacteria
(
Donia & Schmidt, 2011
). This situation might of course be a simple bias
and it is possible that in the future, this apparent frontier show some leaks.
3.3. The Biosynthetic Clusters
The biosynthetic gene clusters identified are quite large, over 20 kb for
PKs, NRPs, and PK/NRP hybrids and of course smaller (about 10 kb)
