Rapid evolutionary divergence of Photosystem I core subunits PsaA ...

Photosynthesis Research 65: 131–139, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
131
Regular paper
Rapid evolutionary divergence of Photosystem I core subunits PsaA and
PsaB in the marine prokaryote Prochlorococcus
Georg W.M. van der Staay 1,2 , Seung Yeo Moon-van der Staay 1,3 , Laurence Garczarek 1 &
Frédéric Partensky 1,∗
Observatoire Océanologique de Roscoff, CNRS-UPR 9042 et Université Pierre et Marie Curie, BP 74, F-29682
Roscoff Cedex, France; 2 Present address: Botanisches Institut, Lehrstuhl III, Universität zu Köln, Gyrhofstraße 15,
D-50931 Köln, Germany; 3 Present address: Botanisches Institut, Lehrstuhl I, Universität zu Köln, Gyrhofstraße
15, D-50931 Köln, Germany; ∗ Author for correspondence (e-mail: partensky@sb-roscoff.fr; fax: +33-98292324)
Received 2 December 1999; accepted in revised form 9 August 2000
Key words: cyanobacteria, Photosystem I, Synechococcus, prochlorophyte, Prochlorococcus
Abstract
The nucleotide sequences of the genes coding for the subunits of the Photosystem I (PS I) core, PsaA and PsaB
were determined for the marine prokaryotic oxyphototrophs Prochlorococcus sp. MED4 (CCMP1378), P. marinus
SS120 (CCMP1375) and Synechococcus sp. WH7803. Divergence of these sequences from those of both freshwater
cyanobacteria and higher plants was remarkably high, given the conserved nature of PsaA and PsaB proteins. In
particular, the PsaA of marine prokaryotes showed several specific insertions and deletions with regard to known
PsaA sequences. Even in between the two Prochlorococcus strains, which correspond to two genetically different
ecotypes with shifted growth irradiance optima, the sequence identity was only 80.2% for PsaA and 88.9% for
PsaB. Possible causes and implications of the fast evolution rates of these two PS I core subunits are discussed.
Abbreviations: Chl – chlorophyll; PS – photosystem
Introduction
PS I is a pigment–protein complex containing 11 different
polypeptides in cyanobacteria and 17 in higher
plants and binding about 90 molecules of Chl a (reviewed
in Golbeck 1994; Chitnis et al. 1995; Chitnis
1996; Scheller et al. 1997). Most of the pigments
and components of the electron-transport chain in PS
I are bound to the large subunits PsaA and PsaB
(Schubert et al. 1997). These are hydrophobic proteins
with molecular masses of 83–84 kDa, that are
chloroplast-encoded in eukaryotes. The structure of
PS I structure has recently been determined at 4 Å
resolution (Schubert et al. 1997). Both PsaA and PsaB
have 11 transmembrane helices, that are organized in
a pseudo two-fold symmetry. Based on a combination
of X-ray diffraction studies and biochemical analyses
on purified PS I reaction centers, a topological model
for PsaA and PsaB has been proposed (Sun et al.
1997). Comparison of their three-dimensional structures
showed remarkable similarities in the topologies
of PS I and PS II (Schubert et al. 1998; Klukas et al.
1999). The six N-terminal helices of PsaA and PsaB,
the antenna binding domain, are organized like the six
helices of the inner PS II antenna proteins CP43 and
CP47. The 5 C-terminal helices of PsaA and PsaB, that
bind the components of the electron transport chain,
are organized comparably to the PS II core proteins
D1 and D2.
PS I and PS II are remarkably conserved between
prokaryotes and eukaryotes, whose chloroplasts are
thought to have derived from an endosymbiotic cyanobacterial
ancestor (Douglas 1998). With amino
acid identities between cyanobacteria and chloroplasts
of about 80% and many conservative amino acid replacements,
PsaA and PsaB were considered to be

132
very conserved proteins (Cantrell and Bryant 1987).
This point of view has recently been challenged by the
determination of the psaA and psaB sequences from
the dinoflagellate Heterocapsa triquetra (Zhang et al.
1999), that showed a much lower degree of sequence
conservation. This surprising result suggests that the
available sequences are not sufficiently representative
of the diversity of PsaA and PsaB proteins.
Prochlorococcus (Chisholm et al. 1992) is a genus
of planktonic photosynthetic prokaryotes very abundant
in the ocean [see Partensky et al. (1999) for a
review]. In contrast to other cyanobacteria, that use
phycobiliproteins to collect light, the major light harvesting
complexes in Prochlorococcus are membrane
intrinsic proteins that bind mainly divinyl chlorophylls
a and b. This genus is closely related to the marine
AandBSynechococcus species, as shown by analysis
of 16S rRNA, rpoC1, psbA, psbB, petB and petD
gene sequences (Hess et al. 1995; Palenik and Swift
1996; Urbach et al. 1998). PS I in Prochlorococcus
shows several peculiar features. Recently, we have
shown that the PS I subunits PsaF and PsaL in Prochlorococcus
are longer and less conserved than in
other organisms (van der Staay et al. 1998; van der
Staay and Partensky 1999). Additionally, PS I in Prochlorococcus
contains divinyl Chl b, which seems to
be linked to the PS I core itself (Garczarek et al. 1998).
To investigate the sequence conservation of the
two large PS I subunits PsaA and PsaB, we have
isolated and cloned these genes from two strains of
Prochlorococcus. P. marinus SS120 (CCMP1375) is
a low light adapted strain which has a divinyl chlorophyll
a to b ratio lower than 1 (Moore et al. 1995).
This strain also contains some ‘normal’ (monovinyl)
chlorophyll b (Partensky et al. 1993) and low amounts
of phycoerythrin (Hess et al. 1996). Prochlorococcus
sp. MED4 (CCMP1378) is adapted to grow at higher
photon fluxes and has a divinyl chlorophyll a to b ratio
of about 10 (Moore et al. 1995). Phylogenetic analyses
using 16S rRNA consistently show MED4 belongs
to the most evolved Prochlorococcus clade which includes
all high light-adapted strains, whereas SS120
and other low-light adapted strains belong to separate
clusters genetically closer to the marine Synechococcus
(Moore et al. 1998; Urbach et al. 1998; Moore and
Chisholm 1999). As, prior to our study, there were
no psaA and psaB gene sequences known from typical
marine cyanobacteria, we have also determined their
sequences from Synechococcus WH 7803. This strain
is closely related to the other marine cyanobacteria
and Prochlorococcus, as determined e.g. by analysis
of DNA dependent RNA polymerase gene sequences
(Palenik and Swift 1996). Our data confirm the close
relatedness between Prochlorococcus and the marine
Synechococcus. We show that in these three marine
oxyphototrophs, the psaA and psaB genes evolved
very rapidly.
Materials and methods
Isolation of clones
The isolation of Lambda vectors containing the psaIpsaL
gene clusters from Prochlorococcus sp. MED4
and P. marinus SS120 has been previously described
(van der Staay et al. 1998). The same vectors also
contained the psaA-psaB genes (see ‘Results’).
A genomic library of Synechococcus WH7803
in Charon 35 was kindly provided by Dr D.
Scanlan. A 499 bp psaB gene fragment from P.
marinus SS120 was amplified by PCR with the
primers CCGCTACATATTCGCCCAA and AGTGC-
CGAAAACAGCATC. An initial denaturation at 94
◦ C for 4 min was followed by 35 cycles of 94 ◦ C, 50
◦ C and 72 ◦ C for 1 min each, with a final elongation at
72 ◦ C for 7 min, in the presence of 1.5 mM Mg 2+ .The
PCR product was cloned into the pCR 2.1 vector (In-
Vitrogen). For probe labeling, the insert was excised
with Eco RI. About 25 pg total DNA, corresponding to
about 3 pg insert, was amplified by PCR in a volume
of 50 µl as described above, except that the nucleotide
mix was replaced by 5 µl of a fluorescein nucleotide
mix (DuPont NEN). The labeled PCR product was
used as a probe. Plaque hybridization proceeded as
described in van der Staay et al. (1998). Hybridization
occurred at 45 ◦ C, blots were washed with 5 ×
SSC/0.1%SDS and 1 × SSC/0.1% SDS at 50 ◦ C. All
other methods are described in van der Staay et al.
(1998).
Sequences used in this study
The sequences determined in this study were deposited
in the EMBL databank under the accession numbers
AJ133190 for Synechococcus WH7803 psaA/B,
AJ133191 for Prochlorococcus sp. MED4 psaA/B,
AJ133192 for P. marinus SS120 psaA/B.
The other sequences are available at GenBank under
the following accession numbers: Anabaena variabilis:
L26326, Mastigocladus laminosus (Fischerella
PCC7605): AF038558, Synechococcus elongatus:

133
X63768, Synechococcus PCC7002: M18165; Synechocystis
PCC6803: D90906; Cyanophora paradoxa:
U30821, Porphyra purpurea: U38804, Guillardia
theta AF041468, Odontella sinensis Z67753, Heterocapsa
triquetra: psaA AF130031, psaB AF130032;
Euglena gracilis: X70810; Nephroselmis olivacea:AF
137379; Chlamydomonas reinhardtii: psaA X05845,
psaB X05848; Chlorella vulgaris: AB001684;
Marchantia polymorpha: X04465; Pinus thunbergii
(pine): D17510; Zea mays (maize): M11203; Oryza
sativa (rice): X15901, Spinacia oleracea (spinach):
X04131; Nicotiana tabacum (tobacco): Z00044.
Protein alignments
PsaA and PsaB sequences were aligned with the program
ClustalX 1.64 (Thompson et al. 1994) using
the PAM 250 matrix (Dayhoff et al. 1978). Alignments
made with this matrix were very similar to the
ones obtained using other matrices. Alignments were
manually refined, keeping the number of gaps caused
by insertions and deletions to a minimum. This program
was also used to calculate the percentages of
sequence identity.
Phylogenetic analysis
Phylogenetic analysis used the parsimony and the
neighbor-joining distance algorithms. PHYLIP Version
3.5c (Felsenstein 1992) was used for all analyses.
Distance matrices were constructed with the program
PROTDIST using the PAM 250 matrix (Dayhoff et al.
1978). Neighbor-joining distance trees were determined
with the neighbor-joining (NEIGHBOR) method
(Saitou and Nei 1987). Bootstrap analyses (SEQ-
BOOT) with 100 replicates were applied to test the
stability of the tree topology.
Results
Further upstream sequencing of a genomic clone containing
the previously described genes coding for the
PS I subunits PsaL and PsaI of Prochlorococcus sp.
MED4 (van der Staay et al. 1998) revealed that this
genomic fragment also contained the genes for the
PS I core subunits PsaA and PsaB. The latter were
separated from psaL and psaI by a gene encoding a
protein that showed the closest similarity to a dolicholphosphate
mannosyltransferase from Aquifex aeolicus
(GenBank accession number AE000762), as determined
by partial sequencing. Analogously, psaA and
psaB from P. marinus SS120 were located on a genomic
clone containing psaL and psaI. The psaA and
psaB genes of the marine Synechococcus sp. WH
7803 were isolated from a genomic library in lambda
Charon 35, using a psaB probe from P. marinus
SS120. As found in a study on the psbB and petB/D
genes of several strains of Prochlorococcus and marine
Synechococcus (Urbach et al. 1998), Prochlorococcus
shows a third codon bias towards A and T
(not shown), with a more pronounced bias in Prochlorococcus
sp. MED4 than SS120. In contrast, Synechococcus
WH7803 has a bias towards G and C at
the third codon position, like reported for several other
marine Synechococcus strains (Urbach et al. 1998).
An alignment of the deduced amino acid sequences
is shown in Figure 1. The corresponding proteins
of the cyanobacterium Synechocystis PCC 6803, for
which a topological model of the PS I core complex
has been presented (Sun et al. 1997), are included in
the alignment. The numbering of the putative membrane
spanning helices and the extramembrane loops
is done according to the model proposed for Synechocystis.
The deduced PsaA proteins from P. marinus
SS120, Prochlorococcus sp. MED4 and Synechococcus
WH7803 are, respectively, 773, 767 and 767
amino acids long. This is longer than PsaA sequences
reported for other organisms, which vary between 732
and 761 amino acids, with most species having a PsaA
750–755 amino acids long. None of the insertions or
deletions are situated in the 11 potential membrane
spanning regions, nor in regions known to be involved
in the binding of components of the electron transport
chain (Chitnis 1996). The PsaA proteins of all three
marine prokaryotes contain a 13 amino acid insertion
between the transmembrane helices III and IV (extramembrane
loop D), the least conserved area of this
protein (Figure 1A). A conserved insertion of three
amino acids (F-P-A) is located between the helices IV
and V (loop E). This loop also shows a low degree of
sequence identity. PsaA proteins from both Prochlorococcus
strains have an additional insertion between
helices VII and VIII and a deletion of 10 amino acids
between the helices XI and X (loop J) not found in any
other species.
PsaB from P. marinus SS120 is with 747 amino
acids the longest known PsaB sequence with the exception
of the one of Heterocapsa triquetra, which
contains 776 amino acids (Zhang et al. 1999). The
PsaB proteins from Prochlorococcus sp. MED4 and
Synechococcus sp. WH7803 fall, with 742 and 738
amino acids, respectively, within the usual range of

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Figure 1. Alignment of the PsaA (A) and PsaB (B) from the marine prokaryotes Prochlorococcus sp. MED4, Prochlorococcus marinus SS120
and Synechococcus WH7803. For comparison, the corresponding sequences of the freshwater cyanobacterium Synechocystis PCC6803 is
included. The putative membrane spanning helices I–XI and the extramembrane loops A–L, determined and numbered based on the analogy
to the model presented by Sun et al. (1997) are indicated. The cysteines that ligate the F x electron acceptor are indicated with a block. For
positions where amino acids were identical, only the amino acid for Prochlorococcus sp. MED4 is given. Gaps are indicated by -.
731–743 amino acids reported for most other organisms.
Most diverse is loop E between the helices IV
and V and loop H between helices VII and VIII, the
areas that are the least conserved in all PsaB pro-

135
Figure 1. Continued.
teins. The C-terminal domains of PsaA and PsaB, from
helices VIII to XI, that are thought to bind the components
of the electron transport chain including the
F x -binding domain, are the most conserved regions.
In all phylogenetic analyses using the concatenated
PsaA and PsaB sequences, the Prochlorococcus
strains grouped together with the marine Synechococcus
strain (Figure 2), separately from the other cyanobacteria.
Analyses of either PsaA or PsaB lead
to comparable results. Inclusion of the sequences
from the dinoflagellate Heterocapsa triquetra resulted
in a destabilization of the trees, due to the high
divergence of these sequences. Depending on small
differences in the alignments and the inclusion or
omission of gaps, Heterocapsa grouped together with
Guillardia and Odontella or with the Prochlorococcus/Synechococcus
cluster (not shown).

136
Figure 2. Phylogenetic analysis of concatenated PsaA and PsaB proteins using the neighbor-joining distance method. For this analysis, all
gaps created by the alignment were excluded. The first number at nodes represent the bootstrap percentage from 100 replicates for the
neighbor-joining distance method, the second number indicates the bootstrap support for this node found by the maximum parsimony method.
Values below 50% are indicated by – or not shown. Synechocystis PCC6803 was arbitrarily chosen as an outgroup. The scale bar indicates
substitutions per amino acid position.
The neighbor-joining PsaA/B tree exhibits unexpectedly
long branches within the Synechocococcus/Prochlorococcus
cluster (Figure 2), despite the
fact that these species are phylogenetically close. This
suggests considerable differences in the evolutionary
rates of the psaA/B operon both in between marine
cyanobacteria and between these and the other phototrophs.
The high divergence in PS I protein sequences
between these three strains is confirmed by the comparison
of sequence identities (Table 1). The PsaA sequences
are more divergent than the PsaB sequences.
Prochlorococcus sp. SS120 shows the highest deviation
of both the PsaA and PsaB sequences. For
both PsaA and PsaB, Heterocapsa showed by far the
highest deviation (not shown). Since the larger number
of insertions and deletions in these proteins from Heterocapsa
complicated the alignment, leading to more
gaps, they were excluded from the Table. Compared
to the species indicated in Table 1, PsaA from H. triquetra
showed sequence identities between 51.4% to
P. marinus SS120 and 56.6% to Cyanophora paradoxa.
For Heterocapsa PsaB, the sequence identity
ranged between 51.3% to Prochlorococcus sp. MED4
and 55.9% to Cyanophora paradoxa.
Discussion
The amino acid sequence analysis of the PS I subunits
PsaA and PsaB from one Synechococcus strain
(WH 7803), representative of the Marine-Cluster A
(Waterbury and Rippka 1989), and two Prochlorococcus
strains, representative of the low-light (SS120)
and high-light adapted (MED4) ecotypes (Moore et al.
1995, 1998; Urbach et al. 1998), are in agreement with
previous molecular studies showing that these marine
oxyphototrophs are closely related and probably
evolved from a common ancestor (Hess et al. 1995;
Urbach et al. 1998; Honda et al. 1999; Turner et al.
1999).
The high evolutionary rate of the PsaA and PsaB
sequences from all the three marine oxyphototrophs

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Table 1. Sequence identity of PsaA and PsaB of cyanobacteria and chloroplasts. Sequences were aligned with the program
ClustalX and manually refined. The percentage sequence identity was calculated from a distance matrix constructed with the
program ClustalX. Positions with gaps were excluded from the alignment. Upper triangle: PsaA, lower triangle: PsaB
PsaA
1 2 3 4 5 6 7 8 9 10 11 12 13 14
1 Prochlorococcus sp. MED4 – 80.2 75.7 69.9 70.3 70.9 69.1 69.7 70.9 69.4 67.2 70.2 69.9 68.8
2 Prochlorococcus marinus SS120 88.9 – 70.8 68.2 67.1 68.9 65.8 66.4 67.9 66.8 65.4 67.5 67.4 65.8
3 Synechococcus WH7803 81.5 80.6 – 77.6 77.7 78.4 75.8 77.7 77.3 75.1 71.3 76.1 77.7 74.7
4 Anabaena variabilis 76.1 74.5 81.9 – 94.4 87.6 82.9 85.0 82.6 80.9 77.2 82.6 83.2 80.6
5 Mastigocladus 74.4 74.5 80.3 90.6 – 87.4 81.4 84.4 82.6 80.7 76.7 81.9 82.6 80.9
6 Synechococcus elongatus 76.8 76.0 81.6 89.6 87.7 – 84.3 87.9 84.3 85.0 79.3 85.1 86.1 85.6
7 Synechococcus PCC7002 79.0 78.5 87.6 87.0 85.5 87.1 – 88.4 81.5 82.1 75.9 81.8 81.5 79.0
8 Synechocystis PCC6803 76.8 76.8 84.1 86.4 85.6 87.1 92.6 – 83.1 84.2 75.7 83.7 83.6 80.9
9 Cyanophora paradoxa 74.6 74.8 82.0 82.3 82.4 84.8 86.4 84.8 – 85.6 79.9 85.4 85.0 82.4
10 Porphyra purpurea 74.4 74.1 80.4 80.5 81.1 81.7 84.4 83.5 83.5 – 78.9 86.2 85.1 82.3
11 Euglena gracilis 70.9 71.3 76.4 77.4 76.7 79.2 80.5 78.3 80.4 79.0 – 82.5 82.2 79.3
12 Chlamydomonas reinhardtii 73.0 73.0 79.7 79.0 79.0 81.6 82.7 80.9 82.2 81.5 80.4 – 89.8 86.8
13 Marchantia polymorpha 73.8 73.6 81.2 79.3 79.3 81.8 84.1 82.3 83.8 84.5 83.2 85.0 – 93.2
14 Spinacia oleracea 72.7 72.2 79.8 77.9 77.5 80.5 81.8 79.8 81.7 83.2 82.3 83.8 92.4 –
PsaB
is unexpected. Most striking is the low percent identity
(80.2%) between the PsaA proteins of the two
Prochlorococcus strains. This percent identity is as
low as the identity between freshwater cyanobacteria
and chloroplasts, that are thought to have separated
about one billion years ago. By comparison, the respective
16S rRNA sequences of these ecotypes are
98.4% identical (Urbach et al. 1998). One may wonder
which evolutionary constraint led to this rapid divergence
of PsaA (and to a lesser extent PsaB). It has
been observed for other proteins that their evolutionary
rate differs among taxa. (Lockhart et al. 1996;
Lopez et al. 1999). The underlying mechanism is not
well understood yet. For Prochlorococcus, themost
dramatic difference (with regard to photosynthesis)
between the respective niches of these ecotypes in the
field is certainly the amount of available light (Campbell
and Vaulot 1993; Moore et al. 1998; Moore and
Chisholm 1999). Besides having shifted growth irradiance
optima and divinyl Chl a to divinyl Chl b ratios
(Partensky et al. 1993, Moore et al. 1995), the Prochlorococcus
ecotypes MED4 and SS120 also have
dissimilar thylakoid protein profiles (Partensky et al.
1997; Garczarek et al. 1998). The antenna system
is probably the component of the photosynthetic apparatus
the most differentiated between these strains
(LaRoche et al. 1996). We recently discovered that
SS120 possesses multiple pcb genes encoding seven
different antenna proteins whereas MED4 possesses
a single pcb gene (Garczarek et al. 2000). Another
major difference between SS120 and MED4 is the
presence of a large gene cluster implicated in the
biosynthesis of phycoerythrin and its associated phycobilins
in the latter but not the former strain (Hess et
al. 1999). Thus, it seems that light has been a major
driving force in the evolution of several key photosynthetic
proteins in Prochlorococcus and this might be
the case for the PS I core as well. In the open ocean,
the wavelengths of photons found at depths below
100 m (i.e. in the environment where the low light adapted
Prochlorococcus ecotype thrives), are narrowly
centered around 470 nm (Morel 1978). These photons
are most efficiently captured by the divinyl Chl b and
much less by divinyl Chl a. So to cope with these low
blue light levels, the low-light adapted ecotypes must
have evolved to optimize the capture of photons by PS
I, e.g. either by binding Chl b molecules (Garczarek et
al. 1998) or by recruiting as an antenna one or several
of the multiple divinyl Chl a/b-binding Pcb proteins
present in the cell. Comparative analysis of PsaA/B
and other proteins associated with the photosystems
from other representatives of the low-light and high
light ecotypes should help us to get more insight in the
underlying adaptation processes.
The analysis of PS I-enriched fractions from
MED4 and SS120 strains previously showed that they

138
have similar PS I protein profiles, but both possess
two proteins with apparent molecular mass of 21 and
25 kDa, which have no equivalent in cyanobacteria,
including the marine Synechococcus WH 8103 (Garczarek
et al. 1998). These proteins were identified as
the PS I subunits PsaF and PsaL, respectively, and
their anomalous length was found to be due to specific
gene insertions (van der Staay et al. 1998; van
der Staay and Partensky 1999). Here we demonstrate
that the two large core proteins PsaA and PsaB in
Prochlorococcus show some specific insertions, too,
but also deletions. Not surprisingly, the C-terminal
part of both these proteins is the most conserved. This
part is thought to bind the components of the electron
transport chain. Therefore, a differentiation of this region
is severely restricted. In the remaining part of the
regions, variations are more tolerated, allowing insertions
and deletions. The main insertions in PsaA of the
three marine prokaryotes occur in loop D, located in
the lumen [compared to topographic model of the PS
I core proteins proposed by Sun et al. (1997)] and the
cytoplasmic loop E. Both these loops contain less conserved
regions in all species. Whereas no function has
been assigned to loop D, loop E might interact with the
subunit PsaE. An insertion in PsaA of Prochlorococcus,
but not of other species, is located in the luminal
loop H. This loop is thought to interact with PsaF. It is
tempting to speculate that both this insertion in PsaA
and the ones in PsaF from Prochlorococcus might be
involved in the interaction between these proteins. A
significant deletion of 10 amino acids is present in
the loop J of PsaA from Prochlorococcus. Thecorresponding
loop in PsaB was shown to be involved in
the interaction with soluble electron transporters (Sun
et al. 1999). Assuming a pseudo twofold symmetry of
PsaA and PsaB in the PS I complex, loop J might have
a similar function in PsaA. In PsaB, major insertions
occur in the cytoplasmic loop E and the luminal loop
H. Insertions in both of these loops, that are the least
conserved in all species, are found in PsaB from other
species, too.
Our results on marine cyanobacteria, in addition to
the ones obtained with the dinoflagellate Heterocapsa
triquetra (Zhang et al. 1999), show that PS I core proteins
can be more variable than previously assumed.
We demonstrated that PsaA of marine cyanobacteria
has characteristic features that distinguish them from
the corresponding proteins of all other groups. In addition,
based on the characteristic insertion and deletion
in the PsaA sequence, and the much lower GC content
in the psaA and psaB genes, representatives of the
genus Prochlorococcus can probably be distinguished
from marine Synechococcus. Therefore, these genes
might constitute useful genetic markers for studies on
the biodiversity of natural picoplanktonic communities.
To get a more complete picture, sequences from
other organisms should be obtained. Of special interest
would be the cyanobacterium Gloeobacter violaceus.
It is considered as a representative of the most primitive
group of cyanobacteria (Honda et al. 1999; Turner
et al. 1999) and, like Prochlorococcus (Garczarek et
al. 1998), its PS I lacks the characteristic fluorescence
at 77 K (Koenig and Schmidt 1995). PS I from Prochlorococcus
appears to be unique, because it binds a
divinyl form of Chl a, and probably divinyl Chl b as
well (Garczarek et al. 1998). Since Chl b has also been
reported in the PS I of Prochloron and Prochlorothrix
(Hiller and Larkum 1985; van der Staay et al. 1992),
obtaining sequences from the PsaA and PsaB of these
two organisms might cast some light on the potential
effect at the protein sequence level of the kind of
bound pigment. Also of particular interest in this context
is Acaryochloris marina, an oxygenic prokaryote,
the PS I of which contains almost exclusively Chl d
(Hu et al. 1998).
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