Our goal was to develop a field soil biodegradation assay using 13 C-labeled compounds and identify the active microorganisms by analyzing 16S rRNA genes in soil-derived 13 C-labeled DNA. Our biodegradation approach sought to minimize microbiological artifacts caused by physical and/or nutritional disturbance of soil associated with sampling and laboratory incubation. The new field-based assay involved the release of 13 Clabeled compounds (glucose, phenol, caffeine, and naphthalene) to soil plots, installation of open-bottom glass chambers that covered the soil, and analysis of samples of headspace gases for 13 CO 2 respiration by gas chromatography/mass spectrometry (GC/MS). We verified that the GC/MS procedure was capable of assessing respiration of the four substrates added (50 ppm) to 5 g of soil in sealed laboratory incubations. Next, we determined background levels of 13 CO 2 emitted from naturally occurring soil organic matter to chambers inserted into our field soil test plots. We found that the conservative tracer, SF 6 , that was injected into the headspace rapidly diffused out of the soil chamber and thus would be of little value for computing the efficiency of retaining respired 13 CO 2 . Field respiration assays using all four compounds were completed. Background respiration from soil organic matter interfered with the documentation of in situ respiration of the slowly metabolized (caffeine) and sparingly soluble (naphthalene) compounds. Nonetheless, transient peaks of 13 CO 2 released in excess of background were found in glucose-and phenol-treated soil within 8 h. Cesium-chloride separation of 13 C-labeled soil DNA was followed by PCR amplification and sequencing of 16S rRNA genes from microbial populations involved with 13 C-substrate metabolism. A total of 29 full sequences revealed that active populations included relatives of Arthrobacter, Pseudomonas, Acinetobacter, Massilia, Flavobacterium, and Pedobacter spp. for glucose; Pseudomonas, Pantoea, Acinetobacter, Enterobacter, Stenotrophomonas, and Alcaligenes spp. for phenol; Pseudomonas, Acinetobacter, and Variovorax spp. for naphthalene; and Acinetobacter, Enterobacter, Stenotrophomonas, and Pantoea spp. for caffeine.Achieving a mechanistic understanding of microorganisms where they dwell, in terrestrial and aquatic field habitats, is one of the major goals of microbial ecology; such understanding is facilitated by an ability to directly measure microbial metabolic processes and to identify microorganisms responsible for particular field biogeochemical reactions (4,24,35,41,48). But a variety of methodological obstacles have traditionally prevented investigators from simultaneously documenting identity and activity in real-world habitats such as soil. The most notable obstacles are the size discrepancy between humans and microorganisms, incomplete understanding of microhabitat physicochemical characteristics, a large reservoir of inactive, but potentially responsive cells in environmental samples, and the related propensity for microbial comm...

SummaryBurkholderia cenocepacia is an opportunistic human pathogen that can aggressively colonize the cystic fibrosis lung. This organism has a LuxR/LuxI-type quorum sensing system that enables cell-cell communication via exchange of acyl homoserine lactones (AHLs). The CepR and CepI proteins constitute a global regulatory system, controlling expression of at least 40 genes, including those controlling swarming motility and biofilm formation. In this study, we isolated seven lacZ fusions in a clinical isolate of B. cenocepacia that are inducible by octanoyl-HSL. Induction of all of these genes requires CepR. The cepI promoter was tested for induction by a set of 33 synthetic autoinducers and analogues, and was most strongly induced by long-chain AHLs lacking 3-oxo substitutions. Expression of this promoter was inhibited by high concentrations of three different autoinducers, each having six-carbon acyl chains. When CepR protein was overproduced in Escherichia coli , it accumulated in a soluble form in the presence of octanoyl-HSL, but accumulated only as insoluble inclusion bodies in its absence. Purified CepR-OHL complexes bound to specific DNA sequences at the cepI and aidA promoters with high specificity. These binding sites included a 16-nucleotide imperfect dyad symmetry. Both CepR binding sites are centred approximately 44 nucleotides upstream of the respective transcription start sites.

SummaryThe ability of LuxR-type proteins to regulate transcription is controlled by bacterial pheromones, N-acylhomoserine lactones (AHLs). Most LuxR-family proteins require their cognate AHLs for activity, and at least some of them require AHLs for folding and protease resistance. However, a few members of this family are able to fold, dimerize, bind DNA, and regulate transcription in the absence of AHLs; moreover, these proteins are antagonized by their cognate AHLs. Complexes between some of these proteins and their DNA binding sites are disrupted by AHLs in vitro. All such proteins are fairly closely related within the larger LuxR family, indicating that they share a relatively recent common ancestor. The 3Ј ends of the genes encoding these receptors invariably overlap with the 3Ј ends of the cognate AHL synthase genes, suggesting additional antagonism at the level of mRNA synthesis, stability or translation.

Microorganisms commonly exhibit preferential glucose consumption and diauxic growth when cultured in mixtures of glucose and other sugars. Although various genetic perturbations have alleviated the effects of glucose repression on consumption of specific sugars, a broadly applicable mechanism remains unknown. Here, we report that a reduction in the rate of glucose phosphorylation alleviates the effects of glucose repression in Saccharomyces cerevisiae. Through adaptive evolution under a mixture of xylose and the glucose analog 2-deoxyglucose, we isolated a mutant strain capable of simultaneously consuming glucose and xylose. Genome sequencing of the evolved mutant followed by CRISPR/Cas9-based reverse engineering revealed that mutations in the glucose phosphorylating enzymes (Hxk1, Hxk2, Glk1) were sufficient to confer simultaneous glucose and xylose utilization. We then found that varying hexokinase expression with an inducible promoter led to the simultaneous utilization of glucose and xylose. Interestingly, no mutations in sugar transporters occurred during the evolution, and no specific transporter played an indispensable role in simultaneous sugar utilization. Additionally, we demonstrated that slowing glucose consumption also enabled simultaneous utilization of glucose and galactose. These results suggest that the rate of intracellular glucose phosphorylation is a decisive factor for metabolic regulations of mixed sugars.The baker's yeast Saccharomyces cerevisiae has long served as a model for studying glucose repression, the multi-layer process by which glucose is consumed before all other carbon sources 1,2 . A wide variety of interconnected mechanisms contribute to yeast's ability to sense, respond, and optimize internal metabolism to preferentially consume glucose 3,4 . Transcriptional repressors such as Mig1, Cat8, and the Ssn6/Tup1 complex prevent transcription of glucose-repressed genes, such as those involved in gluconeogenesis and metabolism of alternative carbon sources [5][6][7][8] . The activities of these repressors are then mediated by kinases and phosphatases such as Snf1 and Glc7/Reg1, respectively 6,9,10 . Beyond these intracellular sensing mechanisms, membrane sensors such as Snf3 and Rgt2 allow yeast to sense extracellular sugar concentrations and internalize signals 11 . In sum, the S. cerevisiae glucose repression pathway is a complex network of signals and regulations comprising significant amounts of research and a continuously growing base of knowledge.Recently, a new layer of glucose repression of galactose consumption has been reported to be linked to the kinetic properties of sugar transporters 12 . Because sugars compete for cellular uptake, relative transport efficiency between two sugars will depend on extracellular sugar concentrations as well as transporter affinities (K m values) for each sugar. Consequently, it was reported that the extracellular sugar concentrations coupled with transporter substrate affinity determine the intracellular sugar concentrations 12 . As GAL ge...

The attKLM operon encodes a lactonase (AttM) that hydrolyzes acylhomoserine lactone autoinducers, as well as two putative dehydrogenases (AttK and AttL). Here we show that AttK, AttL, and AttM collectively covert gamma-butyrolactone to succinate. Two metabolic intermediates, gamma-hydroxybutyrate and succinic semialdehyde, inactivated the AttJ repressor in vitro and induced attKLM transcription in vivo.N-Acyl-homoserine lactones (AHLs) are utilized by a variety of proteobacteria as signal molecules that mediate cell-cell chemical communication. These signals are thought to provide information about cell population density, a phenomenon termed autoinduction or, more recently, quorum sensing (8,18,20). In general, quorum-sensing bacteria synthesize AHLs by a LuxI-type AHL synthase (12), and in most cases these AHL molecules are freely diffusible across the cell envelope. High population densities cause the accumulation of AHLs, which interact with a cognate LuxR-type AHL-dependent transcription factor (13,19,24,25).AHLs can be metabolized by a variety of bacteria, a phenomenon sometimes referred to as quorum quenching (23). AHL metabolism has attracted a great deal of interest, partly for possible therapeutic applications.