Biology Reference
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is mycosporine-glycine (
Gao & Garcia-Pichel, 2011a
,
2011b
). A com-
parative genomic study of this pathway in diverse cyanobacteria showed
that the genes coding for the first three enzymes are conserved (
Gao &
Garcia-Pichel, 2011a
,
2011b
). However, a recent publication showed that
there are probably, in
A. variabilis
ATCC 29413, two redundant biosynthetic
pathways leading to 4-deoxygadusol, the precursor of shinorine (
Spence,
Dunlap, Shick, & Long, 2012
).
4.7. Alkanes, Alkenes and Terpenes
4.7.1. Alkanes and alkenes
The biosynthesis of alkanes in cyanobacteria was elegantly deciphered by
Schirmer, who used a genomic approach (
Schirmer et al., 2010
). By com-
paring the genome of cyanobacteria producing or not producing alkanes,
potential biosynthetic genes were identified and finally, two candidate genes
were found in
Synechococcus elongatus
PCC 7942. These two genes were
found in other cyanobacteria, and they are clustered in the genome. They
code for an acyl-ACP reductase and an aldehyde decarbonylase. The alkanes
are thus produced from fatty acid by reduction to the aldehyde followed by
decarbonylation, giving alkanes containing one less carbon than the par-
ent fatty acid (the
n
− 1 rule). Heterologous expression of these genes
in
E. coli
confirmed their functions. The enzymology of the reduction/
decarbonylation has also been studied in vitro, supporting the proposed
functions (
Krebs, Bollinger, & Booker, 2011
). Interestingly, the biosynthesis
of alkenes was recently identified in
Synechococcus
PCC 7002, a cyanobacte-
rium that does not produce alkanes. In this cyanobacterium, a single gene of
8 kb codes for a PKS, which transforms C18 fatty acids into C19 α-olefins
(
Mendez-Perez, Begemann, & Pfleger, 2011
). The function of this giant
PKS was probed using genetic inactivation experiments.
4.7.2. Terpenes
Geosmin and 2-methylisoborneol (MIB) biosyntheses have been well
studied because these compounds are odorant and their presence in water
or foodstuff is sometimes problematic (
Cane & Ikeda, 2012
). Geosmin is
formed from farnesyl-diphosphate by a single enzyme whose gene has
been identified in
N. punctiforme
PCC 73102. The enzyme was cloned and
expressed in
E. coli
and it produces geosmin together with other com-
pounds, from farnesyl-diphosphate. MIB is produced in two steps from
geranyl-diphosphate by a methyltransferase and a monoterpene cyclase.
This biosynthesis has been extensively characterized in
Streptomyces
species.
