The opportunistic pathogen Burkholderia cenocepacia produces the yellow-green fluorescent
siderophore,
pyochelin. To isolate mutants which do not produce this
siderophore, we mutagenized B. cenocepacia with the transposon mini-Tn5Tp. Two nonfluorescent mutants were identified which were unable to produce
pyochelin. In both mutants, the transposon had integrated into a gene encoding an orthologue of CysW, a component of the
sulfate/
thiosulfate transporter. The cysW gene was located within a putative operon encoding other components of the transporter and a
polypeptide exhibiting high homology to the LysR-type regulators CysB and Cbl.
Sulfate uptake assays confirmed that both mutants were defective in
sulfate transport. Growth in the presence of
cysteine, but not
methionine, restored the ability of the mutants to produce
pyochelin, suggesting that the failure to produce the
siderophore was the result of a depleted intracellular pool of
cysteine, a biosynthetic precursor of
pyochelin. Consistent with this, the wild-type strain did not produce
pyochelin when grown in the presence of lower concentrations of
sulfate that still supported efficient growth. We also showed that whereas
methionine and certain organosulfonates can serve as sole
sulfur sources for this bacterium, they do not facilitate
pyochelin biosynthesis. These observations suggest that, under conditions of
sulfur depletion,
cysteine cannot be spared for production of
pyochelin even under
iron starvation conditions.