G. Schumann et U. Mollmann, Screening system for xenosiderophores as potential drug delivery agents inmycobacteria, ANTIM AG CH, 45(5), 2001, pp. 1317-1322
In order to establish a screening system for xenosiderophores which can be
utilized by mycobacteria, we generated a set of mutants of Mycobacterium sm
egmatis that are blocked in different steps of the well-known iron acquisit
ion system. One mutant with a block in mycobactin biosynthesis was generate
d from strain mc(2)155 by chemical mutagenesis, The exochelin biosynthesis
gene fxbA and the ferric exochelin uptake gene fxuA, previously identified
by Fiss et al, (E, H, Fiss, S, Yu, and W, R, Jacobs, Jr,, Mel. Microbiol, 1
4:557-559, 1994), were knocked out by gene replacement. Adjacent chromosoma
l fragments were used for homologous recombination in order to replace wild
-type genes by the kanamycin resistance gene from transposon Tn903, Gene re
placement was confirmed by PCR, The isolated mutants show the expected phen
otype: fxbA mutants are defective in exochelin biosynthesis, whereas fxuA m
utants excrete a significantly larger amount of exochelin compared to the a
mount excreted by the parent strain, This is due to their defectiveness in
ferriexochelin uptake, as demonstrated in growth promotion assays. This new
set of mutants allows differentiation of siderophores that supply mycobact
eria with iron by ligand exchange with exochelin or mycobactin, by the use
of separate siderophore uptake routes, or by the use of the exochelin perme
ase, All these types of iron uptake routes were identified with 25 exogenou
s siderophores as test substances. Siderophores that act without ligand exc
hange are potential candidates as drug vectors that can be used to overcome
permeability-mediated resistance.