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Figure 6.9 Theproposedbiosynthesisforcylindrospermopsin.ThePKS intermediates
areshownhereaslinearmolecules.However,itispossiblethatsomecyclizationsteps
occuronthePKSs.Theirststepandthelaststephavebeencharacterizedin vitro.The
proteinCyrIshowsvariablestereospeciicitiesfromstraintostrain.Thedomainabbre-
viationsaredeinedintheabbreviationslist.
There are still uncertainties regarding the order of the PKSs steps because
CyrD and CyrE share exactly the same domains and will thus catalyse the
same reactions. Furthermore, the cyclization steps have been proposed to
occur sequentially on the PKSs or after the PKS-mediated steps. There is
no experimental data at this point to answer these interesting issues. CyrJ
has been annotated as a sulfotransferase and is supposed to branch a sulphate
group on the hydroxyl at position 12.Then, CyrI, a 2-oxoglutarate-dependent
iron oxygenase, hydroxylates 7-deoxycylindrospermopsin to give cylin-
drospermopsin and 7-epicylindrospermopsin. The stereoselectivity of the
isolated enzyme has been studied and varies from strain to strain ( Mazmouz,
Chapuis-Hugon, Pichon, Méjean, & Ploux, 2011 ).
4.2. Other Secondary Metabolites from Freshwater
Cyanobacteria
4.2.1. Aeruginosins and aeruginosides
The aeruginosin family comprises diverse linear PK/NRPs produced by
several Microcystis and Planktothrix strains. These peptides are inhibitors of ser-
ine and cysteine proteases with various potencies and selectivities ( Ersmark,
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