Biology Reference
In-Depth Information
signals have been identified and the transcription regulation studied. It
seems that light plays a central role in the transcription level in
mcy
or
cyr
operons for instance.
4. DESCRIPTION OF THE BIOSYNTHESIS AT THE
GENOMIC AND BIOCHEMICAL LEVEL FOR
SELECTED CYANOBACTERIAL SECONDARY
METABOLITES
4.1. Cyanotoxins from Freshwater Cyanobacteria
4.1.1. The hepatotoxins - microcystins and nodularins
Microcystins are nonribosomal cyclic heptapeptides containing an unusual
β-amino acid (Adda: (2
S
, 3
S
, 8
S
, 9
S
)-3-amino-9-methoxy-2,6,8-trimethyl-
10-phenyldeca-4,6-dienoic acid) and several nonproteogenic amino acids
including
N
-methyldehydroalanine (
Fig. 6.1
). These toxins are produced by
freshwater cyanobacteria belonging to different genera such as
Microcystis
,
Planktothrix
, or
Anabaena
(
Pearson, Mihali, Moffitt, Kellmann, & Neilan,
2010
). More than 90 variants of microcystins have been identified, with
Figure 6.1
Structureofmicrocystin-LR.Themostfrequentvariationsencounteredare
shown schematically according to their position. Abbreviations: Hty, Homotyrosine;
Hph,homophenylalanine;Hil,homoisoleucine;Har,homoarginine;
d
-mAsp,
d
-
erythro
-
3-methylaspartate;DesMeAdda,theAddamoietylackingamethylgroupatposition9;
MeDha,
N
-methyldehydroalanine;Dha,dehydroalanine.
