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PCC 6803 (
Marbouty, Saguez, Cassier-Chauvat, & Chauvat, 2009
). Interest-
ingly, the closest homologue of several bacterial tyrosine kinases, including
PCP proteins, is MinD.This protein is involved in bacterial cell division, allow-
ing the correct localization of the Z-ring during cell division (
Cuthbertson
et al., 2009
). Thus, it is reasonable to hypothesize that the proteins comprised
in cluster 9 may be involved in the cell division process. Similarly, the genes
encoding the proteins from the cluster 10 are all in the vicinity of
hepA
(with
the possible exception of N9414_00965, whose genome sequence is not
completely assembled yet). In addition, it is well established that, in
Nostoc
sp.
PCC 7120, the gene cluster comprising the ORFs alr2825-alr2841 is required
for the production of the polysaccharidic layer of the heterocysts envelope
(
Holland & Wolk, 1990
;
Huang et al., 2005
;
Xu, Elhai, & Wolk, 2008
). Due
to the synteny encountered for alr2833 and its orthologues in cluster 10, it is
likely that these PCP proteins also participate in this process. The involvement
of PCP proteins in the production of the heterocysts polysaccharidic layer is
not unexpected since previous studies have even suggested that the heterocyst
envelope may itself be an LPS (
Huang et al., 2005
). The high bootstrap value
of the large cluster I, comprising clusters 9 and 10, points towards a paralogous
duplication event in the last common ancestral organism of the heterocystous
cyanobacteria, followed by fine-tuning of each paralogue according to the
selective forces that acted upon them.
Altogether, the results obtained suggest the existence of a PCP-encoding
gene in the last common ancestor of cyanobacteria (hereafter designated
pcp
), which underwent a series of duplication events, specific losses and
HGT events, giving rise to the homologues comprised in clusters 1-17.
Based on the analysis of the PCP phylogenetic tree, its comparison with the
16S rRNA tree computed here, and the principle of parsimony, it is pos-
sible to reconstruct the major evolutionary events that lead to the pattern
observed nowadays (
Fig. 7.6
).
3.1.1.1. Unicellular cyanobacteria
Considering the presence of a single
pcp
in the last common cyanobacterial
ancestor, the ancestor of the cluster formed by
Synechococcus
and
Prochlorococ-
cus
spp. lost its
pcp
orthologue. Subsequently,
Prochlorococcus marinus
CCMP
1375 acquired a
pcp
by HGT as supported by the phylogenetic distance
from other cyanobacteria orthologues. On the other hand,
pcp
underwent
a paralogous duplication in
Thermosynechococcus
, and two duplications in
Gloeobacter violaceus
PCC 7421. In addition, in the last strain, three HGT are
likely to have occurred, giving rise to the sequences that compose cluster
