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mutation in the
M. smegmatis
ortholog of
fad33
disrupted mycobactin synthesis
resulting in a molecule with an altered side chain that may reveal an intersection
between iron acquisition and lipid metabolism [
34
]. The product of
fad33
may be
an acyl-coenzyme A synthase and mutations in this gene could produce altera-
tions in mycobactin side chains with resultant changes in iron uptake through the
mycobactin system.
As described above, many of the genes and systems required for high affin-
ity mycobacterial iron acquisition have been identified, including siderophore
production, siderophore export, uptake of ferric-siderophores, assembly of iron
storage proteins, uptake of heme, and regulation of these processes, as well as
possible low affinity acquisition through multiple function porins and iron trans-
port subsequent to its reduction. In pathogenic
M. tuberculosis
, most of the high
affinity systems appear necessary for maintenance of infection, indicating a role
in iron acquisition in the macrophage. Better definition of the functions of the
identified genes, as well as genes yet to be discovered (including a possible outer
receptor for ferric-carboxymycobactin), will refine our understanding of myco-
bacterial iron acquisition. Schematic drawings illustrating the complex interplay
of the iron uptake processes were not attempted here. Reference is made to Fig.
tems and which outlines the platform on which many of the present genetic stud-
ies were based.
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