Biology Reference
In-Depth Information
of unicellular and fi lamentous cyanobacteria,
P
.
marinus
MED4,
Synechocystis
sp. strain PCC 6803,
T
.
erythraeum
IMS101, and
Anabaena
sp.strain PCC 7120 revealed 100, 186, 266 and 294 genes,
respectively (Yang
et al
., 2007). Thus the number of cell envelope related genes gradually increased
from unicellular to fi lamentous cyanobacteria (Table 6). Minimum number of genes has been found
for the biosynthesis of peptidoglycan, lipopolysaccharide, outermembrane proteins and genes of
unknown function in
P. marinus
MED4 while there is large number of genes found in each category
in
Anabaena
sp. strain PCC 7120.
Table 6:
Cell envelope-related genes in cyanobacterial genomes.
Sr. No. Cyanobacterium
Peptidoglycan
LPS
Exoplysaccharide
OM proteins
Unknown
Total
1.
P
.
marinus
MED4
29
40
14
16
2
100
2.
Synechocystis
sp. strain PCC 6803 37
73
28
40
8
186
3.
T
.
erythraeum
IMS101
47
90
48
63
18
266
4.
Anabaena
sp. strain PCC 7120
48
113
61
60
12
294
3) Energy metabolism
:
The number of genes allocated to this functional category seems to be lowest
in
T
.
elongatus
BP-1 (72) and highest in
G
.
violaceus
PCC 7421 (102) (Table 5). Detailed explanation
with reference to the processes governed is only available for
Cyanothece
sp. ATCC 51142. The genome
of this organism contains all the genes related to fermentation processes (yielding end products
like ethanol, lactate, acetate and hydrogen) that are known to occur in darkness coinciding with
nitrogen fi xation. A 20.2 kb cluster containing genes related to glucose and pyruvate metabolism
has been identifi ed on the linear chromosome while the genes related to carbohydrate and energy
metabolism are distributed at multiple loci on the circular chromosome. The presence of lactate
dehydrogenase (
ldh
) gene cluster on the linear chromosome that mediates the terminal step of
lactate fermentation is unique to this organism. Another unique feature is the presence of a gene
that encodes phosphoenolpyruvate carboxykinase that mediates the fi rst step in gluconeogenesis
required for dark metabolism.
4) Fatty acid, phospholipid and sterol metabolism
:
Genes related to fatty acid metabolism have
been found to be lowest in
T
.
elongatus
BP-1(29), followed by
Synechocystis
sp. strain PCC 6803 (39)
and
Anabaena
sp. strain PCC 7120 with maximum number of up to 53 found in
G
.
violaceus
PCC 7421 (Table 7). The standard nomenclature of fatty acids indicates the number of carbon atoms,
the number of double bonds and the position of double bonds. For example stearic acid is a 18
carbon compound with no double bonds and designated as 18:0. If a double bond is introduced into
stearic acid at carbon 9, it is converted to oleic acid, designated as 18:1, Δ
9c
. That is one double is
introduced between C9-C10. Saturated fats contain no double bonds-the more double bonds there
are, the greater the degree of unsaturation. These are then converted to acyl-CoA esters. Subsequently,
triacylglycerols are formed by step-wise acylation of glycerol-3-phosphate. Additional double bonds
may be added typically to Δ
12
, Δ
15
and Δ
6
. Fatty acid desaturases are the enzymes that introduce
the double bonds at specifi c sites. Fatty acid desaturation of membrane lipids is shown to increase
the fl uidity of membrane lipids and is suggested to help to overcome the stress caused by high
temperature and light intensity (Stubbs and Smith, 1984; Murata and Wada, 1995; Gombos
et al
., 1997).
In the thylakoid membranes of cyanobacteria monogalactosyldiacylglycerol (MGDG) comprises half
of the components and plays an important role in the development of membrane system. Awai
et
al
. (2006) compared the genomes of
Synechocystis
sp. strain PCC 6803 and
Anabaena
sp. strain PCC
7120 and identifi ed genes encoding monoglucosyldiacylglycerol (MGlcDG) synthase that mediates
galactolipid biosynthesis. The amino acid sequence of both the MGlcDG synthases showed signifi cant
