Biology Reference
In-Depth Information
Loew, Mejean, & Ploux, 2011
;
Méjean et al., 2010
). AnaC is an adenylation
protein very specific for proline and AnaB is a flavoprotein oxidase homolo-
gous to isovaleryl-CoA dehydrogenase in terms of reaction mechanism and
three-dimensional structure.
Saxitoxins and paralytic shellfish toxins are structurally closely related
alkaloids that provoke the paralytic shellfish poisoning. There are more than
30 analogues of saxitoxin that have been identified (
Kellmann et al., 2008
;
Wiese, D'Agostino, Mihali, Moffitt, & Neilan, 2010
). These toxins act as
blockers of the voltage-gated sodium and calcium channels. The biosynthe-
sis of saxitoxin has been studied by feeding experiments and the putative
biosynthetic genes have been identified in several cyanobacterial producers
belonging to different genera:
Anabaena
,
Aphanizomenon
,
Cylindrospermop-
sis
,
Raphidiopsis
, and
Lyngbya
(
Kellmann et al., 2008
;
Mihali, Kellmann, &
Neilan, 2009
;
Murray, Mihali, & Neilan, 2011
;
Stucken et al., 2010
). The
strategy relied on using degenerate PCR amplifications targeting carbam-
oyltransferase genes because these enzymes were putatively involved in
the biosynthesis (
Kellmann et al., 2008
). The cluster of genes spans 35 kb
and there are 31 coding sequences. A biosynthetic pathway has been pro-
posed on the basis of bioinformatic analysis, although none of the steps
has been studied in vitro. Several studies have shown that the
sxt
clusters,
from different species, have been remodelled after loss, rearrangements and
recombination events (
Murray et al., 2011
).
4.1.3. The cytotoxin cylindrospermopsin and its analogues
Cylindrospermopsin (
Fig. 6.7
) is an alkaloid that has been isolated from
a fresh water cyanobacterium,
Cylindrospermopsis raciborskii
, that was iso-
lated in a lake after a serious intoxication episode in Australia (
Griffiths
& Saker, 2003
). Its chemical structure was revised when the two epi-
mers at position 7 were synthesized (
Heintzelman, Fang, Keen, Wallace,
& Weinreb, 2001
,
2002
). In fact, three variants have been detected in
cyanobacteria: 7-deoxycylindrospermopsin, cylindrospermopsin and
7-epicylindrospermopsin, depending on the substituents on carbon 7
(
Fig. 6.7
). Cylindrospermopsin and its natural analogues are cytotoxins, and
provoke hepatotoxicity in humans (
Froscio, Fanok, & Humpage, 2009
).
Several genera of cyanobacteria produce cylindrospermopsin or its ana-
logues:
Anabaena
,
Aphanizomenon
,
Cylindrospermopsis
,
Oscillatoria
,
Raphidiop-
sis
, and
Umezakia
(
Pearson et al., 2010
).
The biosynthesis of cylindrospermopsin was first studied by feeding
experiments and it was concluded that PKSs were involved and that the
