CYANOBACTERIAL TOXINS - Cyanobacteria - page 502

Studies on the development of cyanobacterial blooms in eight lakes of northern Poland revealed the
existence of oscillatorian and nostocacean species reaching an average chlorophyll a concentration
of 10 µg L -1 corresponding to MC levels of 4-5 µg L -1 . Four variants of MCs, i.e. dm MC-RR,
MC-RR, MC-YR and MC-LR were detected. Briand et al . (2005) detected two main variants of MCs,
[D-Asp 3 ] MC-RR and [D-Asp 3 , Dhb 7 ] MC-RR, from natural bloom populations of P . rubescens with
high concentrations (6.7 µg L -1 ) during August-December every year. The cellular concentration of
MCs ranged from 0.1 to 0.3 pg cell -1 . Interestingly, laboratory cultures of the organism only produced
[D-Asp 3 ] MC-RR with a cellular quota of MC of 0.3 to 0.7 pg cell -1 . Species of Aphanizomenon ,
including a Scandinavian species, Aph . skujae (Skuja) Kom.-Legn. & Cromb, are predominantly noted
suggesting that the future toxin detection programmes should also focus attention on the evaluation
of impending potential danger from neurotoxins as well (Mankiewicz et al ., 2005).
i) Biosynthesis : Multimodular enzyme complexes known as peptide synthetases mediate synthesis
of small peptides through a thiotemplating mechanism rather than by mRNA. Cyclosporin (an
immunosuppressant), β-lactam group of antibiotics like penicillins, cephalosporins and certain others
such as gramicidin S, tyrocidin A and surfactins are synthesized through this route (Kleinkauf and
von Doehren, 1996; Neilan et al ., 1999). These enzyme complexes consist of polyketide synthases
(PKSs) and non-ribosomal peptide synthetases (NRPSs). Peptide synthetases possess a modular
structure containing specifi c functional domains and these domains exhibit a high degree of sequence
conservation (Stachelhaus et al ., 1995). The various functional domains for recognition, aminoacyl
adenylation and thioesterifi cation of its amino acid substrate as well as for the elongation of the
growing peptide are present (Kleinkauf and von Doehren, 1996). MCs are synthesized through the
pathway of mixed PKSs and NRPSs (Arment and Carmichael, 1996; Dittmann et al ., 1997). This is
also known as combinatorial biosynthesis (McDaniel et al ., 1993; Stachelhaus et al ., 1995). Welker
and von Doehren (2006) reviewed the synthesis of cyanobacterial peptides. A screening of the PKS
and NRPS genes, from axenic cultures of freshwater and marine cyanobacteria and 14 sequenced
genomes brought to light that their distribution is more common in the fi lamentous forms and the
degenerate primers used in the identifi cation of these modules would be helpful in the synthesis
of natural products from cyanobacteria (Ehrenreich et al ., 2005). The genetic basis of cyanobacterial
toxin production has been discussed (Kurmayer and Christiansen, 2009; Pearson et al ., 2010).
The origin of carbon atoms in the unusual amino acid, Adda and methylaspartic acid has been
traced by C 14 -incorporation. The methyl groups on C 6 and C 8 of Adda are methionine derived while
Me group of C 2 of Adda might be probably derived through methionine or acetate (or possibly from
propionate in the absence of acetate). Further it was shown that C 3 -C 8 segment of Adda is acetate
derived or likely from propionate in the absence of acetate. The remaining carbons in the Adda
are phenylalanine derived. Phenylacetyl-CoA has been suggested to be the most probable initiator
for Adda biosynthesis. The biosynthesis of Masp takes place as follows: A condensation reaction
involving Acetyl-CoA with pyruvic acid results in the formation of 2-hydroxy-2-methylsuccinic acid.
This gets converted to 2-hydroxy-3-methylsuccinic acid that upon oxidation gives rise to 2-oxo-3-
methylsuccinic acid. A transamination reaction fi nally yields Masp (Moore et al ., 1991).
With the help of conserved gene sequences in bacteria and fungi for NRPSs, it was possible to
identify the genes in Microcystis and Anabaena (Dittmann et al ., 1996). These genes were designated
as mcy genes. To confi rm this, insertional inactivation of these genes was done in the toxic M .
aeruginosa PCC 7806 that yielded non-toxic transformants. Thus it was proposed that mcyB gene
encodes MC synthetase (Dittmann et al ., 1997). The detection and characterization of MC and peptide
synthetase genes in strains of Anabaena , Aphanizomenon , Cylindrospermopsis , Microcystis , Nodularia ,