CYANOBACTERIAL TOXINS - Cyanobacteria - page 506

organism inhabiting shallow, mesotrophic or hypertrophic water bodies, showed two genotypes:
those possessing mcyA gene and those lacking mcyA whereas all the 49 strains of P . rubescens , a red-
pigmented bacterium growing in deep stratifi ed oligo- or mesotrophic waters, showed the mcyA
gene. Although one strain of P . agardhii and eight strains of P . rubescens exhibited MC synthetase
genes but they did not produce MC. These results further demonstrate that the populations of
P . agardhii do not show proper distribution of mcy genes suggesting that the absence/inactivation
of mcy genes. This results in the development of non-toxic strains of P . agardhii that co-exist with
the toxin-producing strains. Kurmayer et al . (2005) identifi ed MC-producing ecotypes of P . agardhii
or P . rubescens that differed in production of specifi c MC forms and correlated the genetic variation
within adenylation domains ( Ad ). Thus the sequences of mcyA Ad1 , mcyB Ad1 and mcyC Ad 1 were
analysed in 17 Planktothrix strains. Two distinct Ad -genotypes were noted in the sequences of
mcyA Ad1 that differed in base pair composition and insertion of an N-methyltransferase (NMT)
domain. The genotype mcyA Ad1 possessing NMT, synthesized N-methyl-dehydroalanine while
mcyA Ad1 genotype without NMT produced dehydrobutyrine in position 7. The sequence of mcyB
Ad1 exhibited a lower variation where one Ad -genotype showed the formation homotyrosine and
another Ad -genotype synthesized arginine at position 2. Signifi cantly, the sequences of mcyC Ad1
were highly similar and all these formed arginine in position 4.
Mbedi et al . (2005) studied the variability of mcy gene cluster (8 genes) in 46 strains of Planktothrix .
The presence of PCR amplifi cation products for two mcy regions in non-toxic strains was noticed
whereas the MC-producing strains contained two variable (in sequence and length) mcy regions
and four regions that were highly conserved and specifi c. In order to fi nd out the development of
non-toxic strains (defi cient in MC production) in natural populations of Planktothrix , Christiansen et
al . (2006) analyzed 29 strains of P . rubescens and found that some of the strains were in fact represent
mutant strains of the organism due to either deletions or insertions in the mcy gene cluster. The
deletions are of the order of 400 bp long in mcyB region in one strain and 1869 bp long in mcyHA
region of three strains. Insertions of 1429 bp in mcyD region (in eight strains) and 1433 bp each in the
regions of mcyEG (in one strain) and mcyA (in one strain) have been identifi ed. The mosaic nature
of mcyABC gene cluster in naturally occurring strains of Microcystis has largely been understood by
the detection of point mutations, multiple recombination events, insertion element(s). Using probes
for dnaN , uma1 , mcyA (from M . aeruginosa PCC 7806), Tooming-Klunderud et al . (2008) found point
mutations in the NMT domain of mcyA and multiple recombination events gave rise to 'phylogenetic
mosaics' in mcyA (NMT domain) and the adenylation (A) domain sequences of mcyB and mcyC .
iii) Other toxins of Microcystis : Besides various forms of MCs, M . aeruginosa also produces certain
other toxins, i.e. micropeptins A and B that inhibit plasmin and trypsin (Okino et al ., 1993a),
microginin that inhibits angiotensin-converting enzyme (Okino et al ., 1993b) and microviridins
that inhibit elastase enzyme (Okino et al ., 1995). Microginins are linear tetra and pentapeptides in
which chlorination occurs at N-terminal aliphatic moiety (Ishida et al. , 1998) whereas in aeruginosins
and cyanopeptolins chlorination occurs at aromatic moieties. Agrawal et al . (2001) showed the
presence of a protease inhibitor in M . aeruginosa whose identity has not been determined. It means
that M . aeruginosa is a potential source of a number of MCs as well as enzyme inhibitors posing a
great challenge for the herbivores with complex heterogeneous mixture of inhibitors and toxins
(Fastner et al ., 2001; Welker et al ., 2003, 2004a,b). Cyanopeptolins possess an Arg or Lys residue at
the N-terminal of the modifi ed amino acid 3-amino-6-hydroxy-piperidone. These were identifi ed as
inhibitors of trypsin-like activity in Daphnia whereas peptides of microviridin class were moderately
active. Microginins and MCs did not inhibit trypsin-like activity (Czarnecki et al ., 2006). There is