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MdnD to give the final metabolite. The last gene, mdnE , codes for an ABC
transporter. Interestingly, several putative microviridin precursors have been
identified by genome mining in diverse cyanobacteria ( Weiz et al., 2011 ).
4.6. Sunscreens
Cyanobacteria, like many other microorganisms, produce secondary metab-
olites to protect themselves against UV radiations. In cyanobacteria, these
metabolites, the so-called sunscreens, are of two types: scytonemin and
mycosporines ( Gao & Garcia-Pichel, 2011a , 2011b ).
4.6.1. Scytonemin
The cluster of genes responsible for the production of scytonemin was iden-
tified in Nostoc punctiforme by random genetic inactivations ( Soule, Stout,
Swingley, Meeks, & Garcia-Pichel, 2007 ). The cluster comprises six genes,
scyA-F , directly involved in the biosynthesis of scytonemin and several
other genes likely involved in the formation of precursors. The biosynthesis
of this metabolite was then studied in vitro using isolated enzymes, ScyA,
B and C, and it was shown that scytonemin is produced from tryptophan
and prephenate ( Balskus & Walsh, 2008 , 2009 ). The initial steps of the bio-
synthesis take place in the cytoplasm, whereas the final steps would occur
in the periplasm. The final yellow pigment is excreted and deposited on
the extracellular polysaccharide polymer. The scy cluster of genes has been
detected in numerous cyanobacterial genera such as Lyngbya , Anabaena , and
Nodularia ( Soule et al., 2009 ).
4.6.2. Shinorine
Shinorine belongs to the mycosporine family of sunscreens and its biosyn-
thesis has been studied in Anabaena variabilis ATCC 29413 and N. punctiforme
ATCC 29133. A biosynthesis for shinorine, in A. variabilis ATCC 29413,
has been proposed based on the bioinformatic analysis of the putative bio-
synthetic gene cluster, which comprises four genes. The entire gene cluster
of 6.5 kb was expressed in E. coli and the transformant produced shinorine,
thus validating the function of this cluster. The pathway was also supported
by in vitro experiments, using isolated enzymes ( Balskus & Walsh, 2010 ).
The biosynthesis starts from sedoheptulose-7-phosphate, which is trans-
formed into 4-deoxygadusol. This intermediate is in turn condensed with
glycine to give mycosporine-glycine. Finally, an NRPS-like protein con-
denses mycosporine-glycine and serine to give shinorine. In N. punctiforme
ATCC 29133, the last enzyme is lacking, and thus, the final metabolite
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