Maize smut caused by the fungus Ustilago maydis is a widespread disease characterized by the development of large plant tumours. U. maydis is a biotrophic pathogen that requires living plant tissue for its development and establishes an intimate interaction zone between fungal hyphae and the plant plasma membrane. U. maydis actively suppresses plant defence responses by secreted protein effectors. Its effector repertoire comprises at least 386 genes mostly encoding proteins of unknown function and expressed exclusively during the biotrophic stage. The U. maydis secretome also contains about 150 proteins with probable roles in fungal nutrition, fungal cell wall modification and host penetration as well as proteins unlikely to act in the fungal-host interface like a chorismate mutase. Chorismate mutases are key enzymes of the shikimate pathway and catalyse the conversion of chorismate to prephenate, the precursor for tyrosine and phenylalanine synthesis. Root-knot nematodes inject a secreted chorismate mutase into plant cells likely to affect development. Here we show that the chorismate mutase Cmu1 secreted by U. maydis is a virulence factor. The enzyme is taken up by plant cells, can spread to neighbouring cells and changes the metabolic status of these cells through metabolic priming. Secreted chorismate mutases are found in many plant-associated microbes and might serve as general tools for host manipulation.

The cytoskeleton has a key function in the temporal and spatial organization of both prokaryotic and eukaryotic cells. Here, we report the identification of a new class of polymer-forming proteins, termed bactofilins, that are widely conserved among bacteria. In Caulobacter crescentus, two bactofilin paralogues cooperate to form a sheet-like structure lining the cytoplasmic membrane in proximity of the stalked cell pole. These assemblies mediate polar localization of a peptidoglycan synthase involved in stalk morphogenesis, thus complementing the function of the actin-like cytoskeleton and the cell division machinery in the regulation of cell wall biogenesis. In other bacteria, bactofilins can establish rod-shaped filaments or associate with the cell division apparatus, indicating considerable structural and functional flexibility. Bactofilins polymerize spontaneously in the absence of additional cofactors in vitro, forming stable ribbon-or rod-like filament bundles. Our results suggest that these structures have evolved as an alternative to intermediate filaments, serving as versatile molecular scaffolds in a variety of cellular pathways.

Anaerobic oxidation of methane (AOM) in marine sediments is an important microbial process in the global carbon cycle and in control of greenhouse gas emission. The responsible organisms supposedly reverse the reactions of methanogenesis, but cultures providing biochemical proof of this have not been isolated. Here we searched for AOM-associated cell components in microbial mats from anoxic methane seeps in the Black Sea. These mats catalyse AOM rather than carry out methanogenesis. We extracted a prominent nickel compound displaying the same absorption spectrum as the nickel cofactor F430 of methyl-coenzyme M reductase, the terminal enzyme of methanogenesis; however, the nickel compound exhibited a higher molecular mass than F430. The apparent variant of F(430) was part of an abundant protein that was purified from the mat and that consists of three different subunits. Determined amino-terminal amino acid sequences matched a gene locus cloned from the mat. Sequence analyses revealed similarities to methyl-coenzyme M reductase from methanogenic archaea. The abundance of the nickel protein (7% of extracted proteins) in the mat suggests an important role in AOM.

Recently, a microbial consortium was shown to couple the anaerobic oxidation of methane to denitrification, predominantly in the form of nitrite reduction to dinitrogen gas. This consortium was dominated by bacteria of an as yet uncharacterized division and archaea of the order Methanosarcinales. The present manuscript reports on the upscaling of the enrichment culture, and addresses the role of the archaea in methane oxidation. The key gene of methanotrophic and methanogenic archaea, mcrA, was sequenced. The associated cofactor F(430) was shown to have a mass of 905 Da, the same as for methanogens and different from the heavier form (951 Da) found in methanotrophic archaea. After prolonged enrichment (> 1 year), no inhibition of anaerobic methane oxidation was observed in the presence of 20 mM bromoethane sulfonate, a specific inhibitor of MCR. Optimization of the cultivation conditions led to higher rates of methane oxidation and to the decline of the archaeal population, as shown by fluorescence in situ hybridization and quantitative MALDI-TOF analysis of F(430). Mass balancing showed that methane oxidation was still coupled to nitrite reduction in the total absence of oxygen. Together, our results show that bacteria can couple the anaerobic oxidation of methane to denitrification without the involvement of Archaea.

c Flavin-based electron bifurcation is a recently discovered mechanism of coupling endergonic to exergonic redox reactions in the cytoplasm of anaerobic bacteria and archaea. Among the five electron-bifurcating enzyme complexes characterized to date, one is a heteromeric ferredoxin-and NAD-dependent [FeFe]-hydrogenase. We report here a novel electron-bifurcating [FeFe]-hydrogenase that is NADP rather than NAD specific and forms a complex with a formate dehydrogenase. The complex was found in high concentrations (6% of the cytoplasmic proteins) in the acetogenic Clostridium autoethanogenum autotrophically grown on CO, which was fermented to acetate, ethanol, and 2,3-butanediol. The purified complex was composed of seven different subunits. As predicted from the sequence of the encoding clustered genes (fdhA/hytA-E) and from chemical analyses, the 78.8-kDa subunit (FdhA) is a selenocysteine-and tungsten-containing formate dehydrogenase, the 65.5-kDa subunit (HytB) is an ironsulfur flavin mononucleotide protein harboring the NADP binding site, the 51.4-kDa subunit (HytA) is the [FeFe]-hydrogenase proper, and the 18.1-kDa (HytC), 28.6-kDa (HytD), 19.9-kDa (HytE1), and 20.1-kDa (HytE2) subunits are iron-sulfur proteins. The complex catalyzed both the reversible coupled reduction of ferredoxin and NADP ؉ with H 2 or formate and the reversible formation of H 2 and CO 2 from formate. We propose the complex to have two functions in vivo, namely, to normally catalyze CO 2 reduction to formate with NADPH and reduced ferredoxin in the Wood-Ljungdahl pathway and to catalyze H 2 formation from NADPH and reduced ferredoxin when these redox mediators get too reduced during unbalanced growth of C. autoethanogenum on CO (E 0= ‫؍‬ ؊520 mV). Five years ago it was discovered that in butyric acid-forming clostridia, the exergonic reduction of crotonyl coenzyme A (crotonyl-CoA; E 0 = ϭ Ϫ10 mV) with NADH (E 0 = ϭ Ϫ320 mV) is coupled with the endergonic reduction of ferredoxin (Fd) (E 0 = ϭ Ϫ400 mV) with NADH (reaction 1) catalyzed by the cytoplasmic butyryl-CoA dehydrogenase/electron transfer flavoprotein complex Bcd/EtfAB (1, 2).The available evidence indicates that electron bifurcation is flavin based: a protein-bound flavin is reduced by NADH to the hydroquinone, which is subsequently reoxidized by crotonylCoA to the semiquinone radical that has a redox potential sufficiently negative to reduce ferredoxin (3). The proposed mechanism is analogous to the mechanisms of ubiquinonebased electron bifurcation in the bc1 complex of the aerobic respiratory chain and plastoquinone-based electron bifurcation in the b6f-complex in oxygenic photosynthesis (4-6). The main differences between the electron bifurcation mechanisms are that flavin-based electron bifurcation is associated with cytoplasmic proteins and operates at more negative redox potentials (Ϫ300 mV Ϯ 200 mV), whereas ubiquinone/plastoquinone-based electron bifurcation is associated with membranes and operates at more positive redox potentials (ϩ100 mV Ϯ 200 mV) (3,(7)(8)(9).Since t...