CYANOBACTERIAL TOXINS - Cyanobacteria - page 538

Table 6: Differences between sxt gene clusters of four PST-producing cyanobacteria.

A. circinalis AWQC131C (3)

sxt gene
cluster

C . raciborskii
T3 (1)

Aphanizomenon sp.
NH-5 (2)

A. circinalis AWQC131C L . wollei

(4) (3)

Length >35 kb 27.5 kb 29 kb 36 kb

Flanking
regions

5'- orf1
3'- orf34

5'- psbH
3'- ublA

5'-β-lactamase
3'- smf gene

5'- orf6
3'unknown

Number 26 genes 22 genes 21 genes 31 genes

Genes absent - sxtF, sxtO, sxtY, sxtZ,

ompR

sxtF,sxtY,sxtZ, ompR,sxtX,sxtW sxtL

Additional
known genes

- sxtPER sxtPER sxtdiox, sxtN2, sxtSUL

sxtM1, sxtM2, sxtACT

Additional
unkown
genes

- - orf3,orf5, orf7 truncated

intersected by transposases

orf1, orf2, orf3, orf4,
orf5, orf6, orf24

PSTs STX, neoSTX,

dcSTX,
GTX5

STX, neoSTX STX, dcSTX, GTX2, GTX3,

dcGTX3, dcGTX2, O- and
N-sulfurylated C-1 and C-2
variants

STXdcSTX, dcGTX2,
dcGTX3, 6 novel
variants known as L . wollei
toxins

References Kellmann et al .

(2008)

Mihali et al . (2009) Mihali et al . (2009) Mihali et al . (2011)

Note: Flanking regions: The 5'- orf1 and 3'- orf34 in (1) are of unknown function. The 5'- psbH and 3- ublA genes in (2) encode a
photosystem II reaction center protein and a prenyltransferase involved in the biosynthesis of ubiquinone, respectively. The
3'- smf gene in (3) is presumably involved in DNA uptake. The 5'- orf6 and 3'-fl anking gene in (4) are of unknown function.
Genes absent: The sxtY , sxtZ and ompR genes are involved in signal transduction and transcriptional regulation of PST
production in (1). sxtF is involved in PST transport; sxtO encodes an adenylsulfate kinase; the gene product of sxtW resembles
a ferredoxin; sxtX is responsible for the production of neoSTX so that is why (3) does not produce neoSTX. sxtL encodes GDSL
lipase-like enzyme. Additional known genes: sxtPER belongs to drug and metabolite transport family of proteins probably
fulfi lls the role of sxtY , sxtZ and ompR in both (2) and (3). sxtdiox in (4) encodes a dioxygenase; gene products of sxtN2 and
sxtSUL represent sulfotransferases; sxtM1 and sxtM2 encode exporters; sxtACT represents novel acyltransferase.

dependent sodium channels. A single molecule of STX is reported to interact with a sigle sodium
channel (Catterall et al ., 1979). Since the guanidinium group of STX is positively charged it interacts
with negatively charged carboxyl groups at the opening of the pore of the sodium channel (Cestele
and Catterall, 2000). The fl ow of potassium or the resting potential of the membrane or membrane
resistance are not affected (Adelman et al ., 1982; Gorham and Carmichael, 1988). Due to inhibition of
sodium channels, acetylcholine cannot be released. Symptoms of tingling and numbness around lips
are seen at low doses of STXs but at very high doses paralysis and death results due to respiratory
failure (Carmichael and Falconer, 1993; Duy et al ., 2000).
In 1994, after an outbreak of PSP in Alaska, patients admitted to hospital with acute toxicity
could recover in less than 24 h and the toxin was mostly cleared via urine (Gessner et al., 1997). In
North America, eating of contaminated mussels, clams, cockles, scallops, oysters, crabs and lobsters
commonly leads to PSP from east coast (New Foundland to Massachusetts) to the west coast (Alaska
to California). Though currently no antidote for PSP is available, anti-STX monoclonal antibodies
show some protection against STX binding and reduction in peripheral nerve action potential in
rat tibial nerve (Huot et al ., 1989; Chanh et al ., 1991). Potassium channel blocker, 4-amonipurine
signifi cantly reversed the effects of STX in rats suggesting that it can be used as an antidote for PSP
(Chen et al ., 1996).