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

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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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