Biology Reference
In-Depth Information
(A) Nodularin (
nda
) synthetase
N. spumigena
, 48 kb
No. spumigena,
48kb
I H G ndaF E ndaD ndaC ndaA ndaB
(B) Microcystin (
mcy
) synthetase
M. aeruginosa,
55 kb
J I H mcyG F mcyE
mcyD mcyA mcyB mcyC
(C) Microcystin (
mcy
) synthetase
P. agardhii,
55.6 kb
mcyT mcyD mcyE mcyG H mcyA mcyB mcyC mcyJ
(D) Microcystin (
mcy
) synthetase
Anabaena
sp., 55.4 kb
H I F mcyE J mcyD mcyG mcyA mcyB mcyC
Figure 7:
Hepatotoxin gene clusters from various cyanobacteria. Structures of the nodularin gene cluster of
No. spumigena
(A)
and microcystin gene clusters of
M. aeruginosa
(B),
P. agardhii
(C)
and
Anabaena
sp. 90 (D). Diagram redrawn and not to scale.
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 [Pearson
et al
. (2010)
Mar
Drugs
8:
1650-1680; doi:10.3390/md8051650].
cluster could be possible. The important transcriptional events are as follows. (i) The gene cluster
is transcribed as two polycistronic transcripts in two opposite directions from a central promoter
located in between
mcyABC
and
mcyDEFGHIJ
. (ii) Two transcription start sites have been identifi ed
for both
mcyA
and
mcyD
that are dependent on high (68 µmol of photons m
-2
s
-1
) and low (16 µmol
of photons m
-2
s
-1
) intensities of light, respectively. (iii) The polyketide and tailoring genes
mcyE
to
mcyJ
appear to possess individual promoters. (iv) The inter-cistronic start sites for genes
mcyE
to
mcyJ
suggests that the gene regulation is very complex with multiple and alternate messages.
ii) Genetic diversity of MC producers
:
Mikalsen
et al
. (2003) identifi ed two groups of genetic variants
in closely related
Microcystis
strains (10 strains of
M
.
aeruginosa
, 2 strains of
M
.
viridis
, 2 strains
of
M
.
botrys
and 2
M
. spp.,
M
.
cf
.
fl os-aquae
,
M
.
cf
.
wesenbergii
,
M
.
cf
.
ichthyoblabe
) on the basis of
genetic characterization of a region of adenylation domain in module
mcyB1
. They suggested that
recombinational events in between
mcyB1
and the corresponding domains in
mcyC
are responsible
for the observed genetic variation in MC production. This is substantiated by the following: (i) the
strains containing
mcyABC
gene cluster produced detectable levels of MC; (ii) B-type variants of the
mcyB1
produced MC-LR isoforms and one strain (N-C 161/1) produced MC-LR as well as MC-YR.
Another strain N-C 31 produced only MC-LR. N-C 118/2 and PCC 7806 produced large amounts of
[Asp
3
] MC-LR and MC-LR and (iii) all strains with C-type variants of the
mcyB1
module produced
MC-RR and other members of sub-groups synthesized MC-LR in addition to MC-RR. Mikalsen
et al
.
(2003) further concluded that: (i) the
mcyABC
gene cluster is conserved in all strains of
Microcystis
,
(ii) the same gene arrangement in the synthetase-negative strains suggests that
mcyA
and
mcyB
genes have been lost in some strains, (iii) the MC synthesis is probably an ancestral feature of the
genus
Microcystis
and
(iv) LGT appears to be the mechanism for the evolution of structurally related
peptides in diverse microorganisms as indicated earlier by Kleinkauf and van Doehren (1996).
Kurmayer
et al
. (2004) compared two populations of
Planktothrix,
i.e.
P
.
agardhii
and
P
.
rubescens
to fi nd out the abundance of active and inactive MC genotypes.
P
.
agardhii
, a green-pigmented
