CYANOBACTERIAL TOXINS - Cyanobacteria - page 507

a lot of structural diversity in the aeuginosins, microginins, cyanopeptolins and anabaenopeptins
because of variability in the amino acid residues and modifi cations like glycosylation, sulfatation,
methylation or halogenation (Ishida et al ., 1999, 2007; Rouhiainen et al ., 2000; von Elert et al ., 2005;
Cadel-Six et al ., 2008). The gene clusters for the biosynthesis of cyanopeptolins, aeruginosins and
microcyclamide have now been identifi ed and the organization of genes in these is depicted in Fig.
8 A. Moreover, 22 structural variants of cyanopeptolins have been identifi ed with 13 such structural
variants produced by a single strain of M . aeruginosa . Trichamide, a cyclic peptide from Trichodesmium
erythraeum predicted on the basis of its genome sequence, bears close resemblance to patellamide
produced by cyanobacterial symbionts of ascidians. The genes of trichamide biosynthetic pathway
include a precursor peptide gene, a putative heterocyclization gene, an oxidase, two proteases and
hypothetical genes (Sudek et al ., 2006).
An operon for cyanopeptolin-984 ( mcn ) has been identied in Microcystis c.f. wesenbergii NIVA-
CYA 172/5 which consists of four genes encoding peptide synthetases and one halogenase gene.
Two additional ORFs are present in the 5'-fl anking region that transcribe in the opposite direction,
of which one encodes an ABC-transporter. The mcn operon bears resemblance to anabaenopeptilide
synthetase operon ( apd ) from Anabaena strain 90 (Tooming-Klunderud et al ., 2007). The existence
of a halogenase gene in 17 out of 28 axenic strains of Microcystis sp. that mediates chlorination of
cyanopeptolins and aeruginosins has been demonstrated. It is located in between two genes coding
for NRPS modules and is present only in the toxic strains of the genus and absent in the non-toxic
strains (Cadel-Six et al ., 2008). A cyanopeptolin gene cluster has been identifi ed in Planktothrix
strain NIVA CYA 116, Microcystis sp. and Anabaena strain. While the latter two members possessed
a halogenase gene, it is absent in the former member. Sequencing of this gene cluster showed that
both Microcystis sp. and Planktothrix are related to each other rather than Anabaena . Because there is
no clear evidence of both recombination and LGT between cyanopeptolin and MC gene sequences,
it is suggested that the cyanopeptolin gene clusters in the three genera have evolved independently
(Rounge et al ., 2007). In the aeruginosin gene cluster ( aer ) of M . aeruginosa (strains PCC 7806,
NIES-98 and NIES-843) the genes encoding the fi rst three NRPS modules exhibit a high degree of
similarity with the exception of the genes encoding tailoring enzymes such as the halogenases and
sulfotransferases. This is largely responsible for the diversity of aeruginosin congeners produced
by the three strains (Ishida et al ., 2009).
M . aeruginosa NIES-298 produces a cytotoxic cyclic hexapeptide known as microcyclamide that
is synthesized by ribosomal pathway. The biosynthetic pathway of microcyclamide is akin to the
synthesis of patellamide of Prochloron didemni . From structural analysis of microcyclamide to genes
and vice-versa revealed the mediation of two subtisilin-type proteases, a heterocyclization enzyme
and six other gene products of which two bear no resemblance to patellamide synthetic intermediates
(Ziemert et al ., 2008). A cluster of nine such genes for cyclamide synthesis in M . aeruginosa PCC 7806
revealed the presence of patA to patG , these seven genes are present in two groups patA to patE with
intervening mic3999 and mic3998 and then patF and patG (Fig. 8 B; Frangeul et al ., 2008).
iv) Factors affecting MC biosynthesis : The factors affecting biosynthesis of MCs in all the organisms
known to produce this toxin are discussed in this section collectively. The effects of environmental
factors such as light intensity, temperature and nutrients (nitrogen, phosphorus and trace elements)
have been investigated in a number of MC-producing cyanobacteria. The results from these studies
are not comparable with each other as the parameters taken for expressing MC content of cultures
varied greatly.