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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.
11 of Chap. 2 of this volume which depicts the mycobactin and exochelin sys-
tems and which outlines the platform on which many of the present genetic stud-
ies were based.
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