Biology Reference
In-Depth Information
Figure 9:
Model depicting the molecular machinery involved in the Z-scheme of oxygenic photosynthesis. Abbreviations:
Lhc
a
/
b
proteins: Light (
hv
) is captured and channelled to the P680 reaction centre of PSII, or the P700 reaction centre of PSI.
PSII: In caps=
psb
gene products; P680= primary donor; Phe= pheophytin; Q
A
,Q
B
= secondary plastoquinone acceptors; PQ=
plastoquinone. Cyt b
6
f: caps=
pet
gene products; Cyt b
H
= high potential; Cyt b
L
= low potential; Note: total of 4 haem centres
per monomer (Fe), including c
i
= link for cyclic e
-
transport; Q= quinone, Car= carotenoid, Chl
a
= Chlorophyll
a
molecule.
PSI: In caps=
psa
gene products; P700= primary donor; PC= plastocyanin; A
0
(Chl
a
), A
1
(phylloquinone)= primary and
intermediate e
-
acceptors; F
A
, F
B
and F
X
= bound FeS centres; Fd= ferredoxin. A, A' accessory chls are not shown. Approximate
dimensions as indicated, in nm. Courtesy of Jon Nield, Queen Mary, University of London, UK; freely available from http://
www.queenmaryphotosynthesis.org/nield/psIIimages/oxygenicphotosynthmodel.html.
Color image of this figure appears in the color plate section at the end of the topic.
and 15 putative genes (9 encoding 2Fe-2S
petF
type and 6 encoding 4Fe-4S type), respectively
(Meeks
et al
., 2001). Rocap
et al
. (2003) observed that the genomes of
P
.
marinus
strains, MED4 and
MIT9313 lacked phycobilisome structural proteins and enzymes involved in their biosynthesis.
P
.
marinus
MED4 has a single gene (
psbA
) where as
P
.
marinus
MIT9313 has two such genes. Both
possess a single copy of the gene
psbD
. The
psbV
gene is absent in
P
.
marinus
MED4. As
P
.
marinus
MED4 is a HL-adapted strain, it has 22 genes that synthesize high-light inducible proteins (HLIPs).
One copy of the gene encoding chlorophyll
a
/
b
-binding antenna protein is present in
P
.
marinus
MED4 whereas
P
.
marinus
MIT 9313 has two copies of this gene. Both of the strains lacked the
typical cyanobacterial RUBP carboxylase/oxygenase (RuBisCO form IB) but instead possess genes
for the synthesis of RuBisCO of form IA. These genes are located adjacent to carboxysome shell
protein encoding genes. The similarity of these genes to those of γ-proteobacterium
Acidothiobacillus
ferroxidans
suggests that they might have been acquired through LGT. Both these strains have genes
for oligopeptide transporters and some sugar transporters which indicates that they can be partial
heterotrophs if the need arises. Genes encoding some enzymes of TCA cycle such as 2-oxoglutarate
dehydrogenase, succinyl-CoA synthase and succinyl-CoA-acetoacetate-CoA transferase are absent
in both these strains. In a very exhaustive study, Dufresne
et al
. (2008) compared the genomes of all
