Biology Reference
In-Depth Information
except at its poles, which are surrounded by many more layers (
Fig. 8.1
B;
see Fig. 3A in
Moslavac et al., 2007
).
The homogeneous layer is largely composed of polysaccharide (
Dunn &
Wolk, 1970
), and the outermost fibrous layer that is sometimes observed in
electron micrographs of heterocysts has been suggested to represent a less
compacted material of the same polysaccharide (
Wolk, 1982
). The chemical
structure of the heterocyst polysaccharide (Hep) has been determined for a
few heterocyst-forming cyanobacteria and mainly consists, as described for
Anabaena cylindrica
, of a β-1,3-linked mannosyl-glucosyl-glucosyl-glucose
tetrasaccharide backbone to which side branches of mannose, glucose and
glucosyl glucose, galactose, and xylose are attached (
Cardemil &Wolk, 1979
).
2.2.2. Biosynthesis and deposition
To understand the processes of production and deposition of the hetero-
cyst envelope-specific Hgl and Hep layers, it is important to note that cya-
nobacteria are didermic bacteria in which a peptidoglycan layer and an
outer membrane are found outside of the cytoplasmic membrane (
Stanier
& Cohen-Bazire, 1977
;
Wolk, 1973
). As stated above, the heterocyst enve-
lope is external to the cell wall also found in the vegetative cells (
Lang &
Fay, 1971
;
Wildon & Mercer, 1963
; see also
Fig. 2
e in
Flores, Herrero, Wolk,
& Maldener, 2006
). Genetic analysis of diazotrophic growth has identified
many mutants that exhibit a Fox
−
phenotype (requiring
f
fixed nitrogen for
growth in the presence of
ox
ygen; see
Lechno-Yossef, Fan, Wojciuch, &
Wolk, 2011
). Many of these mutants bear inactivated
hgl
or
hep
genes, which
encode proteins needed for production of the Hgl and Hep layers, respec-
tively, and specifically lack the corresponding layer. Notably, many
hgl
genes
(
Fan et al., 2005
) on one hand and
hep
genes (
Huang et al., 2005
) on the
other hand are clustered together in 'gene islands' in the genome of
Anabaena
sp. strain PCC 7120, although related genes are found in other chromosomal
locations as well (
Maldener, Hannus, & Kammerer, 2003
;
Wang et al., 2007
).
In
Anabaena
sp. strain PCC 7120, a gene island including ORFs
all5343
-
alr5357
bears a number of
hgl
genes, whose mutation arrest production of
Hgl (
Fan et al., 2005
). These genes encode putative fatty acid synthases and
polyketide synthases that may account for the biosynthesis of the aglycone
of Hgl. This gene island may extend to
all5341
, which has been shown
to encode the putative glycosyl transferase (HglT) required to produce
the principal Hgl of
Anabaena
sp. strain PCC 7120 (
Awai & Wolk, 2007
).
A second set of
Anabaena
mutants synthesize Hgl detectable by thin layer
chromatography but do not produce the Hgl layer detectable by electron
