Mycobacterial Iron Acquisition Mechanisms - Iron Acquisition by the Genus Mycobacterium - page 44

All mycobacteria with very few exceptions produce two salicylate contain-
ing siderophore subgroups with the same core structure [ 23 ]. Mycobactin is a
cell-associated molecule, whereas the second salicylate siderophore, called ini-
tially carboxymycobactin (see Chap. 2 ), contains a side chain that confers water
solubility and is excreted from the mycobacterial cells. A cluster of genes (des-
ignated mbt ) encodes both mycobactin and carboxymycobactin; mbt mutants fail
to synthesize the mycobactin core and produce neither mycobactin nor carboxy-
mycobactin [ 29 ]. A third siderophore called exochelin is excreted by only the sap-
rophytic mycobacteria (see Chap. 2 for siderophore structures). Biosynthesis of
exochelin is encoded by at least three genes: fxbA which adds the formyl group
to exochelin and fxbB , fxbC which share homology with motifs common to non-
ribosomal peptide synthetases [ 30 - 32 ]. Another gene product ExiT is a member
of the superfamily of ABC transporters and may be part of the exochelin export
system that delivers exochelin to the exterior of the saprophyte Mycobacterium
smegmatis [ 32 ].
Similar to all siderophores, production of mycobactin, carboxymycobactin, and
exochelin is increased by iron restrictive cultivation. The iron dependent regula-
tory protein IdeR controls production of the mycobactin class of siderophores in
M. tuberculosis [ 33 ]. Consensus “IdeR boxes” were identified in the saprophyte
M. smegmatis [ 34 ], suggesting that M. smegmatis also employs IdeR control. In
low iron conditions, IdeR relieves repression of siderophore genes, ultimately pro-
moting iron uptake. When sufficient iron is present in the mycobacterial cell, IdeR
activates transcription of genes for ferritin and bacteroferritin to sequester iron and
protect the mycobacterial cell from iron induced damage [ 20 , 24 , 33 ]. Multiple
other genes also are either repressed or derepressed by IdeR regulation [ 33 ].
It was suggested that mycobactin could be a temporary iron storage agent [ 18 ]
and iron exchange from ferric-carboxymycobactin to mycobactin has been noted
[ 35 ]. As both of the mycobactin class of siderophores have the same binding affin-
ities, exchange should favor iron movement into mycobactin from a greater con-
centration of ferric-carboxymycobactin obtained by external chelation. Such iron
exchange between the two siderophores may reveal only what is plausible, not the
major route of metal uptake because mbt mutants that make neither of the myco-
bactin siderophores can use exogenously provided ferric-carboxymycobactin,
suggesting that cell-associated mycobactin is non-essential for iron uptake from
ferric-carboxymycobactin [ 20 ]. Inactivation of two IdeR-regulated genes irtA and
irtB that encode a putative ABC transporter showed these genes to be involved in
uptake of ferric-carboxymycobactin in M. tuberculosis in macrophages [ 36 , 37 ].
The gene products IrtAB are not required for carboxymycobactin excretion but
are components of the ferric-carboxymycobactin uptake mechanism. Using recon-
stituted proteoliposomes, it was argued by others that IrtA exported carboxymy-
cobactin and IrtB imported ferric-carboxymycobactin [ 38 ]; however, other work
indicated that both IrtAB were required for uptake of ferric-carboxymycobactin
[ 37 ]. Despite the blockade in M. tuberculosis high affinity uptake of ferric-carbox-
ymycobactin caused by mutations in the irtA and irtB genes, iron acquisition was
not completely eliminated and some iron was accumulated. Upon inactivation of