Biology Reference
In-Depth Information
Figure 17:
Structure of the saxitoxin analogs identifi ed in
Lyngbya wollei
. With the kind permission of B. A. Neilan, Cyanobacteria
and Astrobiology Research Laboratory, School of Biotechnology and Biomolecular Sciences, The University of New South
Wales, Sydney 2052, NSW, Australia. [Mihali
et al
. (2011)
PLoS ONE
6(2):
e14657. doi:10.1371/journal.pone.0014657]
doi:10.1371/journal.pone.0014657.g001.
sequencing of
sxt
gene cluster in this organism as well (Fig. 19; Mihali
et al
., 2011). The
sxt
gene
cluster of
C
.
raciborskii
T3 reveals 26 candidate STX genes (
sxtA
to
sxtZ
). The biosynthesis of STX
is initiated by the gene product of
sxtA
which consists of four catalytic domains known as SxtA1
(methyltransferase), SxtA2 (GCN5-related N-acetyltransferase, GNAT domain), SxtA3 (an acyl carrier
protein, ACP domain) and SxtA4 (aminotransferase domain). The precursors for STX biosynthesis
arginine, S-adenosylmethionine and acetate are acted upon by SxtA. The fi rst reaction in biosynthesis
of STX is initiated by the loading of ACP (SxtA3) with acetate by SxtA1 forming propionyl ACP which
then undergoes a Claisen condensation reaction (SxtA4) with arginine resulting in the formation
of 4-amino-3-oxo-guanidinoheptane. This is followed by three heterocyclizations resulting in STX
and the activity of tailoring enzymes facilitates the formation of numerous isoforms of the toxin
(Kellmann
et al
., 2008). The general pattern of biosynthesis in the four PST-producing cyanobacteria
is almost the same with differences in the species of STXs produced. However, there are a number
of differences in the structural organization of the
sxt
gene cluster. These are depicted in Fig. 20
and summarized (Table 6). It is pertinent to point out here that
A
.
circinalis
AWQC131C does not
produce neoSTX because it is unable to convert STX into neoSTX by introducing a hydroxyl group
