Acetylation of aminobenzoates controls the degradation of benzoate in Rhodopseudomonas

Dissertation Defense Seminar

Chelsey M. VanDrisse

ABSTRACT

The use of lignin as an alternative, renewable source of energy has attracted much attention because of the high-energy content of the phenyl derivatives that comprise this important natural polymer. The degradation of lignin-derived aromatics such as benzoate has been extensively studied in Rhodopseudomonas palustris and other bacteria. In R. palustris, the chemistry underpinning the conversion of benzoate to acetyl-CoA is well understood, however, gaps of knowledge regarding the regulation of the bad (benzoic acid degradation) genes encoding the enzymes of the pathway remain. Here we report the function of the BadL protein, the missing link in our understanding of R. palustris bad gene expression. In this bacterium, BadL function was required for benzoate degradation under photosynthetic conditions (i.e., light plus anoxia). On the basis of results of phenotypic and bioinformatics analyses, qRT-PCR, mass spectrometry, electrophoretic mobility assays, and UV-vis spectroscopy data we show that BadL is a bona fide member of the Gcn5-related N-acetyltransferase family of proteins (GNATs, PF00583) that acetylates aminobenzoates to yield acetamidobenzoates. The latter relieve repression of the badDEFGAB operon by binding to BadM, triggering the synthesis of enzymes that activate and dearomatize the benzene ring. We uncovered an unexpected connection between acetamidobenzoates and the expression of genes encoding the photosynthetic reaction center light harvesting complexes and provide evidence that acetamidobenzoates positively affect the expression of genes encoding reaction center proteins in this bacterium. The effect of acetamidobenzoates on photosynthesis is new and different than their effect on the catabolism of benzoate. Acetamidobenzoates link benzoate degradation to proton motive force generation.