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Figure 6.6 Alignment of TrHb2s using Chlorella variabilis E1ZBP8 as the query. The top line
indicates secondary structure as found inMycobacterium tuberculosis trHbO (PDB ID 1NGK,
Milani et al., 2003
). Residues in bold are at positions B10, F8 and G8, as numbered by struc-
tural homology to the canonical 3/3 fold. The proteins and organisms are: E1ZBP8_CHLVA
C. variabilis (GA); Q946U7_ARATH Arabidopsis thaliana (Embryophyta); B8C835_THAPS
Thalassiosira pseudonana (Stramenopiles); B7G0D0_PHATC Phaeodactylum tricornutum
strain CCAP 1055/1 (Stramenopiles); E1ZF79_CHLVA C. variabilis (GA); Q5L1S0_GEOKA
Geobacillus kaustophilus strain HTA426 (Firmicutes); TRHBO_MYCTU Mycobacterium
tuberculosis (Actinobacteria).
recommendations, we refer to these as TrHb1-1 and TrHb1-2 when the
distinction between the two classes is necessary.
Figure 6.7
compares the
sequences of selected TrHb1-1s and TrHb1-2s. Analysis of the separation
of the two subgroups of TrHb1 has suggested that they are not the result
of evolutionary divergence but are the result of two separate instances of
horizontal gene transfer (
Vinogradov, Fernandez, et al., 2011
). These two
gene transfers are suggested to be the result of two separate endosymbiotic
events. This conclusion is in contrast with the idea that the rise of the
Archaeplastida group came from a single endosymbiotic event, making algae
and land plants monophyletic. Based upon the existing sequence informa-
tion, the endosymbiotic events that gave rise to the algae could have
occurred not only with cyanobacteria (as is the current hypothesis) but
may also have happened following the incorporation of an ancestor of the
gamma-proteobacteria. In the same study, it was noted that TrHb2 proteins
(not to be confused with TrHb1-2) do not fit into either the monophyletic or
the (cyanobacterial/gamma-proteobacteria) polyphyletic model (
Vinogradov,
Fernandez, et al., 2011
). Rather it appears that TrHb2 genes are the result of
an additional independent instance of horizontal gene transfer that occurred
via a bacterial ancestor of Chloroflexi, Firmicutes or epsilon-proteobacteria
(
Vinogradov,Hoogewijs,etal.,2011
). Consequently, the origin of the pri-
mary endosymbiotic event at the beginning of Archaeplastida has been put
