Biology Reference
In-Depth Information
microscopy. The corresponding genes appear to encode proteins needed
for Hgl deposition in the heterocyst envelope. Some such genes are found
in the Hgl island and have been named
hgd
genes for
Hg
l
d
eposition (
Fan
et al., 2005
), but some others are found in different chromosome locations.
These include the genes of the
devBCA
operon, which encodes an ABC-
type exporter (
Fiedler, Arnold, Hannus, & Maldener, 1998
), and
hgdD
,
which encodes an outer membrane TolC-like protein (
Moslavac et al.,
2007
). DevBCA and HgdD can form a protein complex that traverse the
cell wall and has been shown to constitute an ATP-driven exporter of Hgl
(
Staron, Forchhammer, & Maldener, 2011
). Which role the other identified
hgd
genes,
hgdA
,
hgdB
, and
hgdC
(
Fan et al., 2005
), may have in Hgl deposi-
tion remains to be established, as is also the case for HglK, a pentapeptide
repeat-containing protein that bears four transmembrane segments and is
necessary for localization of Hgl in the cell envelope (
Black, Buikema, &
Haselkorn, 1995
).
The first heterocyst differentiation gene that was identified,
hetA
(
Holland
& Wolk, 1990
), is involved in production of the heterocyst envelope poly-
saccharide layer (
Wolk et al., 1988
) and, therefore, has been renamed
hepA
(
Wolk, Elhai, Kuritz, & Holland, 1993
). This gene is part of the
hep
gene island
that includes from ORF
alr2822
to ORF
alr2841
of the genome of
Anabaena
sp. strain PCC 7120 (
Huang et al., 2005
). HepA is an ABC-type transporter
homologous to the
E. coli
exporter of the Lipid A of lipopolysaccharide (LPS).
Other genes encoding enzymes including glycosyl transferases and additional
genes encoding proteins homologous to LPS biosynthesis proteins are present
in the
hep
gene island.These observations made C. PeterWolk and co-workers
ask whether Hep may be a particular type of LPS (
Huang et al., 2005
). Hep
genes in other locations of the chromosome include genes encoding other
glycosyl transferases (
Maldener et al., 2003
;
Wang et al., 2007
) and
all4388
encoding a putative Wza-like periplasmic/outer membrane polysaccharide
export protein (
Lechno-Yossef et al., 2011
;
Maldener et al., 2003
).
Escherichia
coli
Wza is involved in the passage of the group 1 capsular polysaccharide
through the cell envelope and represents a new paradigm in outer membrane
proteins (
Collins & Derrick, 2007
). Thus, a number of genes encoding Hep
biosynthesis and export proteins have been identified, but a final description
of the process leading to production of Hep is pending.
2.2.3. Function
In cultures of different heterocyst-forming cyanobacteria incubated in
the absence of combined nitrogen under anoxic conditions, heterocyst
