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Nostoc , Oscillatoria , Plectonema and Pseudoanabaena was described by Neilan et al . (1999) who not only
demonstrated their existence in cultures of these cyanobacteria but also from naturally occurring
bloom populations.
The precursors required for synthesis are phenylacetate, malonyl coenzyme A, S-adenyl-L-
methionine, glutamate, serine, alanine, leucine, D-methyl-isoaspartate and arginine. The gene
cluster responsible for the synthesis of MC has now been sequenced (Nishizawa et al ., 2000; Tillett
et al ., 2000). The role of PKS has been identifi ed in the production of fatty acid side chain of Adda.
The biosynthetic pathway of MC-LR starting with the biosynthesis of Adda is presented. Tillett et
al . (2000) recognized two operons (from a large gene cluster) for mcyABC (peptide synthetase) and
mcyDE (hybrid polyketide-peptide synthetase), as confi rmed by the cloning and sequencing of
MC synthetase gene cluster in two different strains of M . aeruginosa . The two operons recognised
are mcyABC and mcyDEFGHIJ consisting of 10 genes. The genes mcyA , mcyB and mcyC have
been identifi ed as responsible for activation and incorporation of fi ve amino acid constituents of
the heptapeptide (Nishizawa et al ., 1999). Thus mcyA1 is shown to be responsible for N-methyl-
dehydroalanine; mcyA2 for D-alanine; mcyB1 for Xaa (at position 2); mcyB2 for D-erythro-β-methyl-
iso-aspartic acid; mcyC1 for Zaa (at position 4). The second operon mcyD - J is responsible for Adda
and D-glutamate synthesis (Nishizawa et al ., 2000; Tillett et al ., 2000). The two operons mcyABC
and mcyDEFGHIJ are transcribed in opposite directions. Apart from the six genes ( mcyA to mcyE
and mcyG ) responsible for the incorporation of precursors, the rest of the four genes were shown to
encode monofunctional proteins that possess tailoring functions. Thus these four genes, i.e. mcyJ ,
mcyF , mcyI and mcyH help in O-methylation, epimerization, dehydration and cellular localization,
respectively (Kaebernick et al ., 2002).
There are three D-amino acids in MCs, i.e. D-glutamate, D-alanine and D-Me-Asp/D-aspartate
but there are only two racemase activities present in the gene cluster. An integral epimerase domain
present in the NRPS module is responsible for the incorporation of D-alanine into heptapeptide
backbone. Since an integrated epimerase domain for incorporating D-glutamate and D-MeAsp/D-
aspartate is lacking, it is suggested that these two amino acids might be converted by racemases
either before or after their incorporation. Nishizawa et al . (2001) showed that mcyF mediates synthesis
of D-glutamate since gene disruption studies abolished MC production in M . aeruginosa K-139 but
did not affect cell growth. Further, mcyF supported D-Glu-independent growth of an auxotroph
of E . coli for D-Glu. Thus these workers reported for the fi rst time the existence of the racemase in
prokaryotic NRPS genes. On the contrary, Sielaff et al . (2003) presented evidences for the synthesis
of aspartate racemases rather than glutamate racemases by mcyF gene and also detected a gene for
L-glutamate specifi c racemase that is located outside the mcy gene cluster.
It has now been possible to identify the gene clusters for heptapeptide biosynthesis in Planktothrix
and Anabaena strain 90. There are certain differences in the organization of the mcy gene cluster
among the three MC producers (Table 4; Fig. 7). The signifi cant features of MC biosynthesis in
Planktothrix CYA 126/8 are that (i) sequence analysis revealed mcy region to consist of 55-kb cluster
having 9 genes, (ii) of these, 8 genes ( mcyA , B , C , D , E , G , H and J ) are similar to M . aeruginosa genes
governing peptide synthetases, (iii) mcyT , an additional gene encoding thioesterases, is present which
is lacking in M . aeruginosa , (iv) the racemase genes mcyF and mcyI (the products of which are similar
to 3-phosphoglycerate dehydrogenase genes) present in M . aeruginosa are reported to be absent in
P . agardhii , (v) while mcyA - C and mcyD - J constitute two operons that are transcribed bidirectionally
in M . aeruginosa , the whole of mcy gene cluster except mcyT represents a single operon and (vi) the
gene mcyJ synthesizes O-methyltransferase. The probability of lateral gene transfer (LGT) of complete
mcy gene clusters between two different genera has been ruled out (Christiansen et al ., 2003). In
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