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Phytophthora capsici Leonian is an important oomycete pathogen of crop vegetables, causing significant economic losses each year. Its cell wall, rich in cellulose, is vital for cellular integrity and for interactions with the host organisms. Predicted cellulose synthase (CesA) proteins are expected to catalyze the polymerization of cellulose, but this has not been biochemically demonstrated in an oomycete. Here, we present the properties of the four newly identified CesA proteins from P. capsici and compare their domain organization with that of CesAs from other lineages. Using a newly constructed glucosyltransferase-deficient variant of Saccharomyces cerevisiae with low residual background activity, we have achieved successful heterologous expression and biochemical characterization of a CesA protein from P. capsici (PcCesA1). Our results demonstrate that the individual PcCesA1 enzyme produces cellobiose as the major reaction product. Co-immunoprecipitation studies and activity assays revealed that several PcCesA proteins interact together to form a complex whose multiproteic nature is most likely required for cellulose microfibril formation. In addition to providing important insights into cellulose synthesis in the oomycetes, our data may assist the longer term identification of cell wall biosynthesis inhibitors to control infection by pathogenic oomycetes.

The oomycetes are filamentous eukaryotic microorganisms, distinct from true fungi, many of which act as crop or fish pathogens that cause devastating losses in agriculture and aquaculture. Chitin is present in all true fungi, but it occurs in only small amounts in some Saprolegniomycetes and it is absent in Peronosporomycetes. However, the growth of several oomycetes is severely impacted by competitive chitin synthase (CHS) inhibitors. Here, we shed light on the diversity, evolution and function of oomycete CHS proteins. We show by phylogenetic analysis of 93 putative CHSs from 48 highly diverse oomycetes, including the early diverging Eurychasma dicksonii, that all available oomycete genomes contain at least one putative CHS gene. All gene products contain conserved CHS motifs essential for enzymatic activity and form two Peronosporomycete-specific and six Saprolegniale-specific clades. Proteins of all clades, except one, contain an N-terminal microtubule interacting and trafficking (MIT) domain as predicted by protein domain databases or manual analysis, which is supported by homology modelling and comparison of conserved structural features from sequence logos. We identified at least three groups of CHSs conserved among all oomycete lineages and used phylogenetic reconciliation analysis to infer the dynamic evolution of CHSs in oomycetes. The evolutionary aspects of CHS diversity in modern-day oomycetes are discussed. In addition, we observed hyphal tip rupture in Phytophthora infestans upon treatment with the CHS inhibitor nikkomycin Z. Combining data on phylogeny, gene expression, and response to CHS inhibitors, we propose the association of different CHS clades with certain developmental stages.

A well-balanced human diet includes a significant intake of non-starch polysaccharides, collectively termed 'dietary fibre', from the cell walls of diverse fruits and vegetables. Owing to the paucity of alimentary enzymes encoded by the human genome, our ability to derive energy from dietary fibre depends on the saccharification and fermentation of complex carbohydrates by the massive microbial community residing in our distal gut. The xyloglucans (XyGs) are a ubiquitous family of highly branched plant cell wall polysaccharides whose mechanism(s) of degradation in the human gut and consequent importance in nutrition have been unclear. Here we demonstrate that a single, complex gene locus in Bacteroides ovatus confers XyG catabolism in this common colonic symbiont. Through targeted gene disruption, biochemical analysis of all predicted glycoside hydrolases and carbohydrate-binding proteins, and three-dimensional structural determination of the vanguard endo-xyloglucanase, we reveal the molecular mechanisms through which XyGs are hydrolysed to component monosaccharides for further metabolism. We also observe that orthologous XyG utilization loci (XyGULs) serve as genetic markers of XyG catabolism in Bacteroidetes, that XyGULs are restricted to a limited number of phylogenetically diverse strains, and that XyGULs are ubiquitous in surveyed human metagenomes. Our findings reveal that the metabolism of even highly abundant components of dietary fibre may be mediated by niche species, which has immediate fundamental and practical implications for gut symbiont population ecology in the context of human diet, nutrition and health.

Hitherto the production of the biopolymer cyanophycin (CGP) using recombinant Escherichia coli strains and cheap mineral salts medium yielded only trace amounts of CGP (0.5%, w/w) of the cell dry matter (CDM). This was probably due to the instability of the plasmids encoding the cyanophycin synthetase.In this study, we developed an anabolism-based media-dependent plasmid addiction system (PAS) to enhance plasmid stability, and we established a process based on a modified mineral salts medium yielding a CGP content of 42% (w/w) at the maximum without the addition of amino acids to the medium for the first time. This PAS is based on different lysine biosynthesis pathways and consists of two components: (1) a knockout of the chromosomal dapE disrupts the native succinylase pathway in E. coli and (2) the complementation by the plasmid-encoded artificial aminotransferase pathway mediated by the dapL gene from Synechocystis sp. PCC 6308, which allows the synthesis of the essential lysine precursor L,L-2,6-diaminopimelate. In addition, this plasmid also harbors cphAC595S, an engineered cyanophycin synthetase gene responsible for CGP production.Cultivation experiments in Erlenmeyer flask and also in bioreactors in mineral salts medium without antibiotics revealed an at least 4.5-fold enhanced production of CGP in comparison to control cultivations without PAS.Fermentation experiments with culture volume of up to 400 l yielded a maximum of 18% CGP (w/w) and a final cell density of 15.2 g CDM/l. Lactose was used constantly as an effective inducer and carbon source. Thus, we present a convenient option to produce CGP with E. coli at a technical scale without the need to add antibiotics or amino acids using the mineral salts medium designed in this study.