Mycobacterial Iron Acquisition Mechanisms - Iron Acquisition by the Genus Mycobacterium - page 45

irtAB , another channel may have been opened possibly through iron transferred
to cell-associated mycobactin from ferric-carboxymycobactin. Because many bac-
terial ferric reductases are flavin adenine dinucleotide (FAD) reductases and IrtA
binds a molecule of FAD, it could be a reductase of ferric-carboxymycobactin
imported by IrtAB similar to the cytosolic ViuB protein required for utilization
of the siderophore ferric-vibriobactin by Vibrio cholerae . In M. tuberculosis the
locus Rv2895c, a possible homolog of ViuB, was postulated to be involved in iron
acquisition but contrary to this prediction, analysis indicates that Rv2895c was not
required for iron uptake in M. tuberculosis [ 39 ].
Earlier research clarified, in part, the genes involved in uptake of ferric-exoche-
lin in M. smegmatis [ 30 ]. Uptake is accomplished by the products of four genes:
fxuA , fxuB, fxuC , and fxuD .
The mycobacterial cell wall may impede ready exchange of metabolites in
terrestrial microorganisms and in pathogenic mycobacteria during an infection.
The genetic locus esx - 3 encodes an IdeR regulated secretion process required
for acquisition of iron from the mycobactin siderophores [ 27 ]. Unlike patho-
genic mycobacteria which make only cell bound mycobactin and excreted car-
boxymycobactin, the exochelin-producing saprophyte M. smegmatis tolerated
deletion of esx - 3 . Using strains of M. smegmatis that lack esx - 3 with combina-
tions of deficiencies in the mycobactin/carboxymycobactin biosynthetic pathway
and in biosynthesis of exochelin, the potential interaction of Esx-3 and sidero-
phore production in iron uptake was assessed. An M. smegmatis mutant strain
unable to produce both the mycobactins and exochelin siderophores was rescued
by addition of exogenous mycobactin or carboxymycobactin; if Esx-3 was miss-
ing neither mycobactin nor carboxymycobactin rescued this strain. M. smeg-
matis lacking both Esx-3 and exochelin production, but still able to produce
mycobactin/carboxymycobactin, had a severe low iron growth defect that was
complemented by Esx-3. Therefore, Esx-3 was required for mycobactin utilization
and would be essential for growth in the host. In M. smegmatis , production of exo-
chelin appeared to by-pass the use of mycobactin; a role for Esx-3 was evident
only in strains lacking exochelin production in which Esx-3 was required for use
of mycobactin as a possible back-up process in iron uptake. The function of esx - 3
is uncertain, although it may be involved in correct positioning of components in
the cell wall or at the cell surface.
The saprophyte M. smegmatis has an ABC-type transporter ExiT that is respon-
sible for export of exochelin [ 32 ]. Given the external location of carboxymyco-
bactin, it is probable that an export system also exists in M. tuberculosis for this
siderophore. Recently, two iron (IdeR) regulated genes mmpS4 and mmpS5 that
encode two outer membrane proteins were identified [ 40 ]. Results obtained with
deletion mutants indicated that neither of the membrane proteins MmpS4 MmpS5
was involved in uptake of ferric-carboxymycobactin. Deletion of both genes cre-
ated a mutant strain with a growth defect in low iron culture medium. The double
mutant failed to proliferate in lungs and spleen of infected mice but the mutant
also made significantly less carboxymycobactin and mycobactin than the wild-
type parental culture. It was suggested that the lowered levels of the mycobactin