Biology Reference
In-Depth Information
OPX homologue. Exceptions were observed for only three unicellular cya-
nobacteria, namely,
P. marinus
str. MIT 9301,
S. elongatus
PCC 6301 and
Cyanobacterium
UCYN-A. Most of the putative OPX identified also pos-
sess a soluble ligand-binding beta-grasp domain (SLBB; IPR019554). This
domain is typical of proteins that contain a beta-grasp, and it has been
hypothesized that it may play a role in soluble or small-molecule ligand
recognition. SLBB is found not only among bacterial polysaccharide export
proteins, but also in other proteins, such as the competence DNA recep-
tor ComEA and the Nqo1 subunit of the respiratory electron transport
chain (
Burroughs, Balaji, Iyer, & Aravind, 2007
). Regarding the cyanobac-
terial OPX proteins, 13 monophyletic clusters (1-13) were observed (
Fig.
7.5
B). Similar to that observed for PCP, the OPX proteins belonging to
each strain were also grouped in different clusters, emphasizing the role of
duplication events in the evolution of the genes encoding OPX proteins
(hereafter referred to as
opx
). In general, the clusters are consistent with
the morphologies displayed by the strains, with clusters 5 and 13 including
OPX sequences from unicellular strains, clusters 2, 3, 4, 6, and 11 compris-
ing the OPX representatives from filamentous cyanobacteria, and clusters 1
and 12 including the sequences of the heterocystous strains (
Fig. 7.5
B). In
addition, three larger clusters could also be distinguished (I-III), with clus-
ters I and III comprising at least one bootstrap-supported cluster associated
with each morphological group (
Fig. 7.5
B). These results strongly suggest
that the last common cyanobacterial ancestor possessed two
opx
paralogues,
here designated as
opx
A and
opx
B, which underwent a series of duplication
events, specific losses and HGT, giving rise to the homologues comprised in
clusters I and III, respectively (
Fig. 7.7
).
3.1.2.1. Unicellular cyanobacteria
According to the hypothesis of the presence of two paralogues,
opxA
and
opxB
, in the last common ancestor of cyanobacteria, it is likely that these
two genes were lost in the lineage that gave rise to the monophyletic clus-
ter composed by
Synechococcus
and
Prochlorococcus
spp. (
Fig. 7.7
). Later on,
P. marinus
CCMP 1375 acquired an
opx
gene by HGT, being the encoded
protein phylogenetically separated from other cyanobacterial OPX. On the
other hand, both
Thermosynechococcus
and
Synechocystis
lost the
opxB
in two
separate events.The
Gloeobacter
OPX sequences comprised in cluster 8 show
a high degree of sequence similarity, being likely the result of two paralo-
gous duplications from one of the ancestor
opx
. The other putative OPX
probably originated by an HGT followed by duplication. The fact that none
