Oxidative desulfurization pathway for complete catabolism of sulfoquinovose by bacteria

Abstract

Catabolism of sulfoquinovose (SQ,, the ubiquitous sulfosugar produced by photosynthetic organisms, is an important component of the biogeochemical carbon and sulfur cycles.Here, we describe a new pathway for SQ degradation that involves oxidative desulfurization to release sulfite and enable utilization of the entire carbon skeleton of the sugar to support the growth of the plant pathogen Agrobacterium tumefaciens. SQ or its glycoside sulfoquinovosyl glycerol (SQGro) are imported into the cell by an ABC transporter system with an associated SQ binding protein. A sulfoquinovosidase hydrolyses the SQ glycoside and the liberated SQ is acted on by a flavin mononucleotide-dependent sulfoquinovose monooxygenase, in concert with an NADHdependent flavin reductase, to release sulfite and 6-oxo-glucose. An NADPH-dependent oxidoreductase reduces the 6-oxo-glucose to glucose, enabling entry into primary metabolic pathways. Structural and biochemical studies provide detailed insights into the recognition of key metabolites by proteins in this pathway. Bioinformatic analyses reveal that the sulfoquinovose monooxygenase (smo) pathway is distributed across Alpha-and Betaproteobacteria and is especially prevalent within the Rhizobiales order. This strategy for SQ catabolism is distinct from previously described pathways as it enables the complete utilization of all carbons within SQ by a single organism with concomitant production of inorganic sulfite. Significance StatementSulfoquinovose, a sulfosugar derivative of glucose, is produced by most photosynthetic organisms and contains up to half of all sulfur in the biosphere. Several pathways for its breakdown are known, though they provide access to only half of the carbon in sulfoquinovose and none of its sulfur. Here, we describe a fundamentally different pathway within the plant pathogen Agrobacterium tumefaciens that features oxidative desulfurization of sulfoquinovose to access all carbon and sulfur within the molecule. Biochemical and structural analyses of the pathway's key proteins provided insights how the sulfosugar is recognized and degraded. Genes encoding this sulfoquinovose monooxygenase pathway are present in many plant pathogens and symbionts, alluding to a possible role for sulfoquinovose in plant host-bacteria interactions.

The sulfoquinovosyl glycerol binding protein SmoF binds and accommodates plant sulfolipids

“… Liu et al., 2021 ). In contrast to all other known pathways, the sulfoglycolytic SQ monooxygenase (sulfo-SMO) pathway involves the cleavage of the sulfur-carbon bond of SQ with excretion of inorganic sulfur (predominantly as sulfite) and enables the utilization of all six carbons of SQ ( Sharma et al., 2022 , J. Liu et al., 2021 ).…”

“…The sulfo-SMO pathway of Agrobacterium tumefaciens utilizes a two-component system, comprised of an FMNH 2 -dependent SQ monooxygenase and a flavin reductase, to cleave the carbon-sulfur bond of SQ to form 6-oxo-glucose (6-OG) and sulfite ( Sharma et al., 2022 ), ( Fig. 1 ).…”

“…The smo gene cluster encodes an ATP-binding cassette (ABC) transport system consisting of a pair of identical ATPase domains (SmoE) and two distinct transmembrane domains (SmoG, SmoH). The ABC transporter engages with the periplasmic solute binding protein SmoF, which binds SQGro with sub-micromolar affinity and recruits it for import into the cell ( Sharma et al., 2022 ). The sulfo-ED pathway gene cluster in Rhizobium leguminosarum also contains an ABC transporter and putative SQGro binding protein, suggesting that ABC transporters may be utilized in other sulfoglycolytic pathways in different organisms ( Li et al., 2020 ).…”

“…The adoption of the closed conformation is essential for productive interaction of MalE with the cytoplasmic-membrane components of the ABC transporter complex and importation of maltooligosaccharides across the membrane ( Duan et al., 2001 ). The SQ binding protein SmoF, like other solute binding proteins, has a structural fold comprised of two globular lobes (interconnected by polypeptide chains), which undergo conformational changes upon ligand binding ( Berntsson et al., 2010 ; Sharma et al., 2022 ). Studies of solute binding proteins show that the ligand-free form undergoes equilibration between open and semi-closed states ( Tang et al., 2007 ).…”

“…Studies of solute binding proteins show that the ligand-free form undergoes equilibration between open and semi-closed states ( Tang et al., 2007 ). In the case of SmoF, once the open ligand-free form binds SQGro ( Sharma et al., 2022 ), it undergoes a domain rotation to a closed conformation that encapsulates the ligand.…”