A Genomic View of Secondary Metabolite Production in Cyanobacteria - Advances in Botanical Research - page 203

Figure 6.2 Comparisonofthreeselected mcy geneclustersfromdiferentgenera.Note
thatthe Planktothrix agardhii mcy clusterlacksthe mcyI and mcyF genes(implicatedin
thebiosynthesisof erythro -2-methyl- d -Asp)andhasasupplementarytype-IIthioester-
asegene, mcyT .Seethecolourplate.
However, the transcription of the mcy genes is repressed in the dark and
expressed by light, and thus, the production of microcystins occurs essen-
tially during the light period ( Straub, Quillardet, Vergalli, de Marsac, &
Humbert, 2011 ).
A biosynthetic scheme, for microcystins, was proposed based on the bio-
informatic analysis of the mcy cluster, involving two PKSs, one hybrid PKS/
NRPS and three NRPSs, one tailoring enzyme and one ABC transporter
( Tillett et al., 2000 ). The biosynthesis probably starts with the loading of
phenyllactate on McyG followed by extension and methylation ( Fig. 6.3 ).
In vitro experiments conducted on the isolated adenylation domain and
peptidyl carrier protein domain (A-PCP domains) of McyG showed that
the preferred starter for McyG is probably phenyllactate rather than phen-
ylacetate as first proposed ( Hicks, Moffitt, Beer, Moore, & Kelleher, 2006 ).
However, the carboxylate of the phenyllactate starter should be lost to give
the Adda moiety by an unspecified step. McyJ, a tailoring enzyme, is believed
to catalyse the O-methylation on the McyG product as evidenced by genetic
inactivation of mcyJ ( Christiansen et al., 2003 ). The chain is then loaded on
McyD for extension on two modules, and then on McyE for extension,
amino transfer and condensation of D-glutamate. The D-glutamate is teth-
ered to the PCP by its γ-carboxylate, and thus, the next peptide bond is a
γ-bond rather than an α-bond. A first genetic study ( Nishizawa, Asayama,
& Shirai, 2001 ) attributed the glutamate racemase activity to McyF but