Biodegradation of 4-nitrobenzoate, 4-aminobenzoate and their mixtures: new strains, unusual metabolites and insights into pathway regulation

Abstract

Strains PB4 ( Burkholderia cepacia) and SB4 ( Ralstonia paucula) were isolated on 4-aminobenzoate and found to also grow on 4-nitrobenzoate. Nevertheless, and although reduction of a nitro group into an amino group is a thermodynamically favorable reaction, the main 4-nitrobenzoate degradation pathway used by these bacteria did not involve 4-aminobenzoate as an intermediate. Rather strains PB4 and SB4 used the previously described partial reduction into 4-hydroxylaminobenzoate, subsequently converted into protocatechuate. Remarkably, both microorganisms also harbored a mutase, through which two dead-end metabolites, 3-hydroxy-4-aminobenzoate and 3-hydroxy-4-acetamidobenzoate, were produced. Regulation of the pathways appeared to differ in both strains. In strain PB4, both 4-nitro- and 4-aminobenzoate were able to induce their own degradation as well as the degradation of the corresponding aminated or nitrated derivative. On the other hand, when strain SB4 was incubated with mixtures of 4-nitro- and 4-aminobenzoate, 4-aminobenzoate strongly interfered with the degradation of 4-nitrobenzoate, even when the nitro compound was the inducer. Since aminoaromatic compounds are common co-contaminants of nitroaromatic-polluted sites, bacteria such as strains PB4 and SB4, which can mineralize both a nitroaromatic compound and its corresponding amino derivative, are relevant subjects of investigation in the interest of a complete site remediation.