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

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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