Biology Reference
In-Depth Information
for specific case). The relevant information gathered from these studies are
that horizontal gene transfer events are probably responsible for the pres-
ence of the same clusters in the genome of cyanobacteria of different genus.
This is supported by the fact that transposase genes have been found next to
or within the clusters of gene responsible for the biosynthesis of secondary
metabolites. Other genetic events such as mutations, rearrangement of the
genes, recombinations, deletions and insertions have been proposed to be
responsible for the differences observed in the same cluster found in differ-
ent cyanobacteria.
3.4. The Metabolite Biosyntheses
We have not described, in this review, all the biosynthetic schemes that
have been derived from analysis of the clusters described in
Table 6.1
.
Instead, we have selected some representative examples that highlight the
salient issues concerning the biosynthesis of cyanobacterial secondary
metabolites. As noted above, the majority of the clusters so far described
code for PKSs, NRPSs and mostly PKS/NRPS hybrids, but the reac-
tions catalysed by these synthases show, in many cases, divergence from
the standard PKSs and NRPSs enzymology. Indeed, the starters of PKSs
are different from the standard acetyl or propionyl starter, and are very
diverse in structure (in the cylindrospermopsin, anatoxin-a, and saxitoxin
biosynthesis for instance). In the cyanobacterial PKSs, some domains
show new reactions as the Mannich-type cyclization in the biosynthe-
sis of anatoxin-a, or the numerous cyclization steps in the biosynthesis
of cylindrospermopsin. The termination steps in these PKSs-mediated
biosyntheses are frequently different from the standard case and involve
hydrolysis, decarboxylation, or reduction. These differences from the
standard PKS/NRPS reactions allow the formation of many various
metabolites by the cyanobacterial producers. Another, general trait is that
usually one cluster is responsible for the biosynthesis of several variants
of one metabolite. The variations observed are either a change in amino
acid residue, a methylation, a hydroxylation, a sulfatation, a halogena-
tion, or a change in configuration at some carbons. These variations are
either catalysed by tailoring enzymes or by some domains of the PKSs
or the NRPSs that show a relaxed specificity. For instance, the vari-
ants observed in the microcystin family are generated by promiscuous
adenylation domains or by mutations in these domains.
Not much is known concerning the regulation of the production of the
cyanobacterial secondary metabolites. In only a few cases, the transcriptional
