Antifungal azoxybacilin exhibits activity by inhibiting gene expression of sulfite reductase

Abstract

Azoxybacilin, produced by Bacillus cereus, has a broad spectrum of antifungal activity in methionine-free medium and has been suggested to inhibit sulfite fixation. We have further investigated the mode of action by which azoxybacilin kills fungi. The compound inhibited the incorporation of [35S] sulfate into acid-insoluble fractions of Saccharomyces cerevisiae under conditions in which virtually no inhibition was observed for DNA, RNA, or protein synthesis. It did not interfere with the activity of the enzymes for sulfate assimilation but clearly inhibited the induction of those enzymes when S. cerevisiae cells were transferred from rich medium to a synthetic methionine-free medium. Particularly strong inhibition was observed in the induction of sulfite reductase. Northern (RNA) analysis revealed that azoxybacilin decreased the level of mRNA of genes for sulfate assimilation, including MET10 for sulfite reductase and MET4, the transactivator of MET10 and other sulfate assimilation genes. When activities of azoxybacilin were compared for mRNA and enzyme syntheses from MET10, the concentration required for inhibition of transcription of the gene was about 10 times higher (50% inhibitory concentration = 30 micrograms/ml) than that required for inhibition of induction of enzyme synthesis (50% inhibitory concentration = 3 micrograms/ml). The data suggest that azoxybacilin acts on at least two steps in the expression of sulfite reductase; the transcriptional activation of MET4 and a posttranscriptional regulation in MET10 expression. We conclude that azoxybacilin exhibits antifungal activity by interfering with the regulation of expression of sulfite reductase activity.

New Targets and Inhibitors of Mycobacterial Sulfur Metabolism

“…Toward this end, mycobacterial sulfur metabolism represents a promising new area for anti-TB therapy [62, 65, 66]. Numerous studies have validated amino acid biosynthetic pathways and downstream metabolites as antimicrobial targets [67–70] and sulfur metabolic pathways are required for the expression of virulence in many pathogenic bacteria [71–74]. In particular, mutants in mycobacterial sulfur metabolism genes are severely impaired in their ability to persist and cause disease [49, 50, 73, 75–77].…”

Characterization of C-S Lyase fromC. diphtheriae: A Possible Target for New Antimicrobial Drugs

“…The biosynthesis of methionine is therefore of vital importance to microbial growth. This has been validated by the fact that several natural products, including 2-amino-5-hydroxy-4-oxopentanoic acid [3], azoxybacilin [4], and rhizocticin [5], target important enzymes for methionine biosynthesis and have antimicrobial properties. Moreover, most of the steps in the methionine biosynthesis pathway are unique to bacteria and plants.…”

Molecular characterization and analysis of the biosynthetic gene cluster for the azoxy antibiotic valanimycin

“…Despite the range of biological activities exhibited by this family of compounds, little is known about their mechanisms of action. Azoxybacilin has been shown to interfere with the expression of sulphite reductase (Aoki et al ., 1996), whereas valanimycin appears to inhibit DNA synthesis and causes the induction of DNA repair systems (Yamato et al ., 1987).…”