An Aldolase-dependent Phloroglucinol Degradation Pathway in Sp. Zg1085

Phloroglucinol (1,3,5-trihydroxybenzene) is a key intermediate in the degradation of polyphenols such as flavonoids and hydrolysable tannins and can be used by certain bacteria as a carbon and energy source for growth. The identification of enzymes that participate in the fermentation of phloroglucinol to acetate and butyrate in was recently reported. In this study, we present the discovery and characterization of a novel metabolic pathway for phloroglucinol degradation in the bacterium sp. zg1085, from marmot respiratory tract. In both the and pathways, phloroglucinol is first reduced to dihydrophoroglucinol by the NADPH-dependent phloroglucinol reductase (PGR), followed by ring opening to form ()-3-hydroxy-5-oxohexanoate by a Mn-dependent dihydrophloroglucinol cyclohydrolase (DPGC). In the pathway, ()-3-hydroxy-5-oxohexanoate is then cleaved to form malonate semialdehyde and acetone by a newly identified aldolase (HOHA). Finally, a NADP-dependent malonate-semialdehyde dehydrogenase converts malonate semialdehyde to CO and acetyl-CoA, an intermediate in carbon and energy metabolism. Recombinant expression of the PGR, DPGC, and HOHA in enabled the conversion of phloroglucinol into acetone, providing support for the proposed pathway. Experiments with , another bacterium containing the gene cluster of interest, show that the PGR, DPGC, HOHA, and MSDH are induced by phloroglucinol. Our findings add to the variety of metabolic pathways for the degradation of phloroglucinol, a widely distributed phenolic compound, in the anaerobic microbiome.IMPORTANCEPhloroglucinol is an important intermediate in the bacterial degradation of polyphenols, a highly abundant class of plant natural products. Recent research has identified key enzymes of the phloroglucinol degradation pathway in butyrate-producing anaerobic bacteria, which involves cleavage of a linear triketide intermediate by a beta ketoacid cleavage enzyme, requiring acetyl-CoA as a co-substrate. This paper reports a variant of the pathway in the lactic acid bacterium sp. zg1085, which involves cleavage of the triketide intermediate by a homolog of deoxyribose-5-phosphate aldolase, highlighting the variety of mechanisms for phloroglucinol degradation by different anaerobic bacterial taxa.