Biology Reference
In-Depth Information
Figure 6.4
Thestructureofanatoxin-aandhomoanatoxin-a.
Deaths of animals due to ingestion of these toxins are regularly reported
in the world (
Cadel-Six et al., 2007
). Although anatoxin-a was isolated in
1972, its biosynthesis has only been deciphered recently. In 2009, Méjean
et al. used degenerate primers to amplify, by PCR, sequences coding for
ketosynthase (KS) domain of PKSs, in the genome of
Oscillatoria
sp. PCC
6506, a homoanatoxin-a producer (
Cadel-Six et al., 2009
). They identi-
fied a 1.7-kb sequence, called
ks2
, that was specific for
Oscillatoria
strains
producing anatoxin-a or homoanatoxin-a. This sequence was then found
within a cluster of nine genes, in the genome sequence of
Oscillatoria
sp.
PCC 6506 (
Mejean et al., 2010
). The cluster, designated
ana
, was proposed
to be responsible for the biosynthesis of anatoxin-a and homoanatoxin-a,
and a complete biosynthetic scheme for these alkaloids was thus proposed
based on a combination of bioinformatic analysis, feeding experiments
and in vitro biochemical experiments on isolated enzymes (
Méjean et al.,
2009
). The
ana
cluster was afterwards identified in the genome of
Anabaena
flos-aquae
sp. 37, an anatoxin-a producer (
Rantala-Ylinen et al., 2011
). The
clusters are quite similar although two genes,
orf1
and
anaA
, are differently
located in the clusters (
Fig. 6.5
). The genes
anaB
,
C
,
D
,
E
,
F
and
G
and
the encoded proteins share strong sequence identities suggesting a com-
mon origin. Unfortunately, neither genetic inactivation of these
ana
genes
nor transcriptional studies have yet been reported. No transporter has been
identified for anatoxin-a, and it is thus assumed that the toxin is released
when the cells lyse during senescence.
The biosynthesis of anatoxin-a and homoanatoxin-a starts from pro-
line (
Fig. 6.6
), which is loaded on an ACP, AnaD and then oxidized to
pyrroline-5-carboxyl-ACP, by AnaB. The biosynthesis then takes place on
three consecutive PKSs, AnaE, F, and G. The first PKS, AnaE, should add one
acetate unit that is completely reduced. The second PKS, AnaF, should add
one acetate unit and should catalyse a Mannich cyclization that forms the
bicyclic structure of anatoxin-a and homoanatoxin-a. It has been proposed,
on the basis of bioinformatic analysis, that the protein coded by
orf1
partici-
pates in this cyclization reaction. The last PKS, AnaG should add the final
