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Microcystis , Anabaena , or Planktothrix , and they contain an unusual 3-amino-
6-hydroxy-2-piperidone unit (Ahp), which, in fact, corresponds to a cyclic
glutamyl-γ-semialdehyde residue ( Welker & von Dohren, 2006 ). Several
variants of cyanopeptolins have been detected, and they usually are protease
inhibitors. Several clusters of genes responsible for cyanopeptolin biosyn-
thesis have been identified in diverse strains, the apd , mcn , and oci clusters
(see Table 6.1 and Rounge, Rohrlack, Tooming-Klunderud, Kristensen, &
Jakobsen, 2007 ). These clusters code for NRPSs and show interesting varia-
tions within strains.
4.2.4. Anabaenopeptins
Anabaenopeptins are cyclic hexapetides with an ureido linkage between
the variable residue at the N-terminal position and the D-lysine resi-
due usually found in the cyclic moiety ( Welker & von Dohren, 2006 ).
There are many anabaenopeptin variants described, and these peptides
that show protease inhibition activities are produced by diverse cyano-
bacteria. Five gene clusters responsible for the biosynthesis of anabae-
nopeptins from different strains have been identified - the apt , ana and
apn clusters (see Table 6.1 ). They code for NRPSs and two studies have
scrutinized the specificities of the isolated adenylation domains. In one
strain, Anabaena sp. 90, the apt cluster contains two genes, aptA1 and
aptA2 , that code for alternative starter modules, thus giving rise to dif-
ferent peptide with alternate residue (Tyr or Arg/Lys) at the N-terminal
position ( Rouhiainen, Jokela, Fewer, Urmann, & Sivonen, 2010 ). The
other study, in P. agardhii NIVA CYA-126/8, showed that, within the
apn cluster-encoded NRPSs, the first adenylation domain is promis-
cuous and accepts two residues as substrates (Arg or Tyr), giving thus
two anabaenopeptin variants ( Christiansen, Philmus, Hemscheidt, &
Kurmayer, 2011 ).
4.3. Secondary Metabolites from Earth Cyanobacteria
4.3.1. Cryptophycins
Cryptophycins are depsipeptides produced by cyanobacteria of the
genus Nostoc ( Magarvey et al., 2006 ). More than 25 variants of crypto-
phycins have been identified, with cryptophycin 1 as the major repre-
sentative ( Fig. 6.13 ). The modifications concern several positions like the
carbon-carbon double bonds, the epoxide, the methyl groups, and the
configuration at some positions.These metabolites and semisynthetic ana-
logues are promising anti-cancer drugs acting as tubulin-depolymerizing
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