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at N-1 position due to the absence of
sxtX
gene in its cluster. The biosynthetic pathway for STX is
represented in Fig. 14. Moustafa
et al
. (2009) who traced the origin of STX genes in cyanobacteria
suggested that nine putative STX genes were horizontally transferred from non-cyanobacterial
sources and 13 belonged to cyanobacteria while four others are shared by toxic as well as non- toxic
strains. A comparison of gene clusters involved in the biosynthesis of STX in
R. brookii
D9 and CYN
in
C
.
raciborskii
CS-505 revealed a great deal of synteny. The STX gene cluster of the former spans
25.7 kb region in its genome and is organized into 24 ORFs. Of these, 20 ORFs bear resemblance
with those of
C
.
raciborskii
T3. Since 19 genes from among the STX gene clusters of
R
.
brookii
D9
and
C
.
raciborskii
T3 share 100% similarity, it has been suggested that this is the minimum number
of genes required for STX production (Stucken
et al
., 2010). Due to non-availability of molecular
markers, so far it has not been possible to undertake studies on genetic diversity of PST-producing
cyanobacteria. However, the sequencing of the
sxt
gene cluster in four cyanobacteria as described
above seems to have overcome this problem. Ballot
et al
. (2010) employed
sxtA
gene as a marker for
identifying PST-producing cyanobacteria from Lakes Melang and Scharmützel (northeast Germany)
Figure 18:
Structure of the paralytic shellfi sh toxin biosynthesis cluster identifi ed in (A).
Aphanizomenon. sp
. NH-5, (B)
Anabaena
circinalis
AWQC131C, C.
Cylindrospermopsis raciborskii
T3. The gene cluster schematic for
C. raciborskii
T3 has been adapted
from kellmann
et al
. 2008 [29]. Segments A-E denote cluster fragments homologous in the three strains. The scale indicates
length in thousand base pairs. ompR, transcriptional regulator of
ompR
family. 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.
(2009)
BMC Biochemistry
10:
8;doi:10.1186/1471-2091-10-8].
