Molecular genetics of carbon-phosphorus bond cleavage in bacteria

Abstract

Phosphonates (Pn) are a large class of organophosphorus molecules that have direct carbon-phosphorus (C-P) bonds in place of the carbon-oxygen-phosphorus ester bond. In bacteria two pathways exist for Pn breakdown for use as a P source: the phosphonatase and C-P lyase pathways. These pathways differ both in regard to their substrate specificity and their cleavage mechanism. The phosphonatase pathway acts on the natural Pn alpha-aminoethylphosphonate (AEPn). In a two-step process it leads to cleavage of the C-P bond by a hydrolysis reaction requiring an adjacent carbonyl group. In contrast the C-P lyase pathway has a broad substrate specificity. It leads to cleavage of substituted Pn (such as AEPn) as well as unsubstituted Pn by a mechanism involving redox or radical chemistry. Due to its broad substrate specificity, the C-P lyase pathway is generally thought to be responsible for the breakdown of Pn herbicides (such as glyphosate) by bacteria. As a way to gain a more in-depth understanding of these Pn degradative pathways, their respective genes have been isolated and characterized. In the absence of a biochemical assay for the C-P lyase pathway such molecular approaches have been especially valuable. The roles of individual genes have been inferred from DNA sequence analysis and mutational effects. Genes for the C-P lyase pathway exist in a fourteen-gene operon that appears to encode both a binding protein-dependent Pn transporter and a C-P lyase. Genes for the phosphonatase pathway also exist in a gene cluster containing Pn uptake and degradative genes. A combination of biochemistry, molecular biology, and molecular genetics approaches has provided more detailed understanding of the mechanisms of C-P bond cleavage. Such basic information may provide a new handle for improvement of Pn degradation capabilities in bacteria, or in other cells in which the respective genes may be introduced and expressed.