Biology Reference
In-Depth Information
Table 8:
Phycobilisome rod genes in different pigment types of
Synechococcus
strains (Six
et al.,
2007).
Pigment type
Strains
Total number of genes for PBP
Allocation for different components
Type 1
WH5701, RS9917
15
two
apcB
-
A
operons for C-PC α & β
subunits; two rod core linker genes
(
cpcGI
&
cpcGII
); two
cpcC
and
cpcD
rod linker genes; (an additional
cpcCIII
in WH5701); four genes encoding three
different lyases (CpcE/F, CpcS and
CpcT); one
pcy
gene for PCB: ferredoxin
oxidoreductase.
Type 2
WH7805
19
one set of PE α- and β-subunit encoding
genes three linker genes (
cpeC
,
cpeE
and
mpeD
-like gene); six putative lyase
genes; two genes for PEP biosynthesis
(
pebA
and
pebB)
and fi ve unkown genes.
Type 3a
WH7803
6
additional genes including
unk
7/8
and
unk 9
3d
12
additional genes (
mpeA
,
B
,
C
,
D
,
E
,
F
or
G
,
U
,
Y
, Z and
unk 7
,
8
and
9
) of
which those underlined are
novel PII genes. These include PEB
synthesis genes
PebA/B
and a number
of lyase genes.
Unk
=unknown
(of ~9 kDa) is generally found to be homologous to small PC rod-linker polypeptide CpcD. γ-subunits
serve as PE linker polypeptides and contain two subtypes of covalently attached linear tetrapyrrole
chromophores, phycoerythrobilin (PEB) and phycourobilin (PUB). Guan
et al.
(2007) downloaded
(http://img.jgi.doe.gov/cgi-bin/pub/main.cgi) 25 genomes of cyanobacteria and compared the
types and number of the linker polypeptides (Table 9). All
Prochlorococcus
strains revealed one FNR
component each. However, three strains [NATLA, CCMP1375 (SS120) and MIT9211 all three LL-
adapted strains) additionally possess a γ-subunit. In fi ve
Synechococcus
strains (CC9311, WH8102,
CC9605, CC9902 and WH5701) also a γ-subunit linker is present but in the rest of the cyanobacterial
genomes examined the γ-subunit linker is absent. Homologues for genes
cpcC
and
cpcD
that encode
PC-associated L
R
linker polypeptides are absent in the genome of
Synechococcus
sp. strain WH8102.
The absence of homologues for
nblA
and
nblB
, involved in the degradation of phycobilisomes during
nutrient stress in cyanobacteria, is another signifi cant feature of
Synechococcus
sp. strain WH8102
genome. Maximum number of linkers up to 13 is found in
C
.
watsonii
WH8501 (with 9 L
R
and one
each of the other four) and in
T
.
erythraeum
IMS101 (with 7 L
R
,
2
L
RC
and 2L
CM
and one each of L
C
and
FNR). The total number of linker genes in the three heterocystous nitrogen-fi xing species is found to
be 10 with a variation of L
R
linker genes up to fi ve and two L
RC
genes in
N
.
punctiforme
PCC 73102. In
all 25 cyanobacteria put together, the number of linker polypeptides number up to 192. Of these, 167
genes encode PBS-associated linkers and 25 genes relate to the synthesis of FNR. Another important
fi nding is that these genes, encoding PBS-associated linkers, PBSs as well as enzymes involved in
biosynthesis, exist in clusters. In total 36 such gene clusters (1.5 kb to 13.2 kb) were found in the
cyanobacterial genomes examined. FNR genes are widely distributed in the genomes but most of
the PBS-linkers and PBSs are clustered and transcribed in the same direction.
Homologues of all chlorophyll biosynthesis genes have been identified in
A
.
marina
MBIC11017
.
The presence of genes governing the synthesis of protoporphyrin IX monomethyl
ester oxidative cyclase (
AcsF
) and chlorophyll synthase (
ChlG
) that are responsible for the
