Biology Reference
In-Depth Information
eukaryotes and bacterial species, including Mycobacteria, have been shown
to contain trHbs (
Vuletich & Lecomte, 2006
), both through the analysis of
sequenced genomes and experimental data; these include
Campylobacter
jejuni
(
Wainwright, Elvers, Park, & Poole, 2005
),
Pseudoalteromonas
haloplanktis
(
Giordano et al., 2007
),
Thermobifida fusca
(
Droghetti et al.,
2010
) and other Mycobacterial species such as
M. leprae
(
Visca et al.,
2002
). Phylogenetic analysis of Mycobacterial Hbs revealed that the trHb
family branches into three main groups, designated as type I, II and III
(named as HbN, HbO and HbP, respectively); the trHbO gene (
glbO
)
has been predicted as the ancestral gene, with group I and group III genes
appearing as a result of duplication and transfer events (
Vuletich & Lecomte,
2006
). All three groups of trHbs are represented in Mycobacterial species,
and in some cases, two or three groups of trHbs coexist in the same organ-
ism, indicating that they have evolved to perform distinct cellular functions
(
Table 5.1
). Detailed analysis of the co-occurrence of these three classes of
trHbs in Mycobacteria indicates that all Mycobacterial genomes sequenced
so far carry genes for the group II (trHbO), and the majority of them have
group I (trHbN) trHbs, the exceptions being
M. leprae
,
Mycobacterium
abscessus
,
Mycobacterium massiliense
and
Mycobacterium thermoresistibile
that lack
the gene encoding for trHbN (
Table 5.1
).
In addition to these single-domain trHbs, the occurrence of more than
one flavohaemoglobin (flavoHb)-encoding gene has been identified in
many pathogenic and non-pathogenic Mycobacteria (
Gupta et al., 2012,
2011
). An interesting pattern in the occurrence of flavoHb-encoding genes
in pathogenic and non-pathogenic Mycobacteria has been observed. The
majority of opportunistic pathogens and fast-growing Mycobacteria display
the presence of two flavoHb-encoding genes: one for conventional
flavoHbs (Type I) similar to other microbial flavoHbs, and the other one
for a new class of flavoHb (Type II), carrying unusual structural features
in their haem and reductase domains (
Gupta et al., 2012
). Notably, the
new class of flavoHb is present in the majority of Mycobacterial species
and co-exists along with a conventional flavoHb in several Mycobacterial
species. The occurrence of trHbs and flavoHbs in Mycobacterial genomes
varies in different species and, to some extent, depends on their natural
niche. Based on the available genome data, the distribution and
co-existence of single-domain trHb and two-domain flavoHbs in various
Mycobacterial species are presented in
Table 5.1
.
