ORCID

Joseph Harrison: 0000-0002-2118-6524

Document Type

Article

Publication Title

Journal of Bacteriology

Department

Chemistry

ISSN

0021-9193

Volume

189

Issue

18

DOI

10.1128/JB.00487-07

First Page

6714

Last Page

6722

Publication Date

1-1-2007

Abstract

Mycobacterium tuberculosis places an enormous burden on the welfare of humanity. Its ability to grow and its pathogenicity are linked to sulfur metabolism, which is considered a fertile area for the development of antibiotics, particularly because many of the sulfur acquisition steps in the bacterium are not found in the host. Sulfite reduction is one such mycobacterium-specific step and is the central focus of this paper. Sulfite reduction in Mycobacterium smegmatis was investigated using a combination of deletion mutagenesis, metabolite screening, complementation, and enzymology. The initial rate parameters for the purified sulfite reductase from M. tuberculosis were determined under strict anaerobic conditions [kcat = 1.0 (±0.1) electron consumed per second, and Km(SO3−2) = 27 (±1) μM], and the enzyme exhibits no detectible turnover of nitrite, which need not be the case in the sulfite/nitrite reductase family. Deletion of sulfite reductase (sirA, originally misannotated nirA) reveals that it is essential for growth on sulfate or sulfite as the sole sulfur source and, further, that the nitrite-reducing activities of the cell are incapable of reducing sulfite at a rate sufficient to allow growth. Like their nitrite reductase counterparts, sulfite reductases require a siroheme cofactor for catalysis. Rv2393 (renamed che1) resides in the sulfur reduction operon and is shown for the first time to encode a ferrochelatase, a catalyst that inserts Fe2+ into siroheme. Deletion of che1 causes cells to grow slowly on metabolites that require sulfite reductase activity. This slow-growth phenotype was ameliorated by optimizing growth conditions for nitrite assimilation, suggesting that nitrogen and sulfur assimilation overlap at the point of ferrochelatase synthesis and delivery.

Comments

This work was supported by National Institutes of Health grants GM54469 (T.S.L.) and RO1 AI26170 (W.R.J.).

We thank Martin Warren and Evelyne Raux, Department of Biosciences, University of Kent, for kindly providing the E. coli cysG mutant strains and Ann-Francis Miller, Department of Chemistry and Biochemistry, University of Kentucky, for her generous guidance and support in executing the anaerobic, sulfite reductase assays.

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