Search

Functional metagenomic analysis of soil metagenomes is a method for uncovering as-yet unidentified mechanisms for antibiotic resistance. Here we report an unconventional mode by which a response regulator derived from a soil metagenome confers resistance to the β-lactam antibiotic carbenicillin in Escherichia coli. A recombinant clone (βlr16) harboring a 5,169 bp DNA insert was selected from a metagenomic library previously constructed from a remote Alaskan soil. The βlr16 clone conferred specific resistance to carbenicillin, with limited increases in resistance to other tested antibiotics, including other β-lactams (penicillins and cephalosporins), rifampin, ciprofloxacin, erythromycin, chloramphenicol, nalidixic acid, fusidic acid, and gentamicin. Resistance was more pronounced at 24°C than at 37°C. Zone-of-inhibition assays suggested that the mechanism of carbenicillin resistance was not due to antibiotic inactivation. The DNA insert did not encode any genes known to confer antibiotic resistance, but did have two putative open reading frames (ORFs) that were annotated as a metallopeptidase and a two-component response regulator. Transposon mutagenesis and subcloning of the two ORFs followed by phenotypic assays showed that the response regulator gene was necessary and sufficient to confer the resistance phenotype. Quantitative reverse transcriptase PCR showed that the response regulator suppressed expression of the ompF porin gene, independently of the small RNA regulator micF, and enhanced expression of the acrD, mdtA, and mdtB efflux pump genes. This work demonstrates that antibiotic resistance can be achieved by the modulation of gene regulation by heterologous DNA. Functional analyses such as these can be important for making discoveries in antibiotic resistance gene biology and ecology.

We report the screening of 16,000 synthetic compounds for induction and inhibition of quorum sensing in a Pseudomonas putida N-acylated l-homoserine lactone (AHL) sensor strain engineered with the LasR transcriptional activator. LasR controls virulence gene expression in the opportunistic pathogen Pseudomonas aeruginosa and is of significant interest as a therapeutic target. Nine compounds that inhibit and 14 compounds that induce LasR activity were identified in our high-throughput screen.

Bacillus thuringiensis is the leading biopesticide used to control insect pests worldwide. Although they have a long record of safe use, under certain conditions commercial strains of B. thuringiensis have the ability to produce numerous putative enterotoxins that have been associated with food poisoning attributed to Bacillus cereus. Therefore, we designed a strategy to delete the genes encoding these toxins. B. thuringiensis strain VBTS 2477 contained genes encoding NHE, CytK-2 and three homologues of haemolysin BL (HBL, HBL(a1) and HBL(a2)). This is the first report, to our knowledge, of a strain of B. cereus or B. thuringiensis containing three sets of hbl operons. The genes encoding HBL(a1) and HBL(a2) were 96-97 % identical to each other and 76-84 % identical to those encoding HBL. The hbl(a2) operon was detected by PCR amplification only after hbl(a1) was deleted. We used sequential gene replacement to replace the wild-type copies of the NHE and three HBL operons with copies that contained internal deletions that span the three genes in each operon. The insecticidal activity of the quadruple-enterotoxin-deficient mutant was similar to that of the wild-type strain against larvae of Trichoplusia ni, Spodoptera exigua and Plutella xylostella. This demonstrates that the genes for enterotoxins can be deleted, eliminating the possibility of enterotoxin production without compromising the insecticidal efficacy of a strain of B. thuringiensis.

Trees in the genus Populus can provide substantial commercial and ecological benefits, including sustainable alternatives to traditional forestry. Realization of this potential requires intensive management, but damage by defoliating insects can severely limit productivity in such systems. Two approaches to limiting these losses include cultivation of poplar varieties with inherent resistance to pests and application of microbial pesticides. Little is known about the interaction between host resistance and the ability of poplars to support the efficacy of biocontrol agents. The research described here was conducted to survey the effect of hybrid poplar clones on gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), a pest on these trees. We assessed the effect of various poplar clones on larval performance and susceptibility to Bacillus thuringiensis subsp. kurstaki. Larvae were reared from hatching on the foliage of 25 hybrid poplar clones and we monitored larval survival, development time, and weight at fourth instar. Eight of these clones showed high resistance against gypsy moth. The remaining clones showed high variation in their effect on larval performance. We evaluated the susceptibility of third-instar larvae to B. thuringiensis subsp. kurstaki when reared on the 17 remaining clones. There was a significant effect of poplar clone on time to death after ingestion of B. thuringiensis subsp. kurstaki. The susceptibility of gypsy moth larvae to B. thuringiensis on various clones was not correlated with the effects of these clones on larval performance in the absence of B. thuringiensis, suggesting this interaction is more complex than merely reflecting higher mortality to previously stressed larvae.

Acidobacterium capsulatum is an acid-tolerant, encapsulated, Gram-negative member of the ubiquitous, but poorly understood Acidobacteria phylum. Little is known about the genetics and regulatory mechanisms of A. capsulatum. To begin to address this gap, we identified the gene encoding the A. capsulatum major sigma factor, rpoD, which encodes a 597-amino acid protein with a predicted sequence highly similar to the major sigma factors of Solibacter usitatus Ellin6076 and Geobacter sulfurreducens PCA. Purified hexahistidine-tagged RpoD migrates at approximately 70 kDa under SDS-PAGE conditions, which is consistent with the predicted MW of 69.2 kDa, and the gene product is immunoreactive with monoclonal antibodies specific for either bacterial RpoD proteins or the N-terminal histidine tag. A. capsulatum RpoD restored normal growth to E. coli strain CAG20153 under conditions that prevent expression of the endogenous rpoD. These results indicate we have cloned the gene encoding the A. capsulatum major sigma factor and the gene product is active in E. coli.

The goal of this study was to design and evaluate a rapid screen to identify metagenomic clones that produce biologically active small molecules. We built metagenomic libraries with DNA from soil on the floodplain of the Tanana River in Alaska. We extracted DNA directly from the soil and cloned it into fosmid and bacterial artificial chromosome vectors, constructing eight metagenomic libraries that contain 53,000 clones with inserts ranging from 1 to 190 kb. To identify clones of interest, we designed a high throughput "intracellular" screen, designated METREX, in which metagenomic DNA is in a host cell containing a biosensor for compounds that induce bacterial quorum sensing. If the metagenomic clone produces a quorum-sensing inducer, the cell produces green fluorescent protein (GFP) and can be identified by fluorescence microscopy or captured by fluorescence-activated cell sorting. Our initial screen identified 11 clones that induce and two that inhibit expression of GFP. The intracellular screen detected quorum-sensing inducers among metagenomic clones that a traditional overlay screen would not. One inducing clone carries a LuxI homologue that directs the synthesis of an N-acyl homoserine lactone quorum-sensing signal molecule. The LuxI homologue has 62% amino acid sequence identity to its closest match in GenBank, AmfI from Pseudomonas fluorescens, and is on a 78-kb insert that contains 67 open reading frames. Another inducing clone carries a gene with homology to homocitrate synthase. Our results demonstrate the power of an intracellular screen to identify functionally active clones and biologically active small molecules in metagenomic libraries.

Zwittermicin A represents a new chemical class of antibiotic and has diverse biological activities, including suppression of oomycete diseases of plants and potentiation of the insecticidal activity of Bacillus thuringiensis. To identify genes involved in zwittermicin A production, we generated 4,800 transposon mutants of B. cereus UW101C and screened them for zwittermicin A accumulation. Nine mutants did not produce detectable zwittermicin A, and one mutant produced eightfold more than the parent strain. The DNA flanking the transposon insertions in six of the nine nonproducing mutants contains significant sequence similarity to genes involved in peptide and polyketide antibiotic biosynthesis. The mutant that overproduced zwittermicin A contained a transposon insertion immediately upstream from a gene that encodes a deduced protein that is a member of the MarR family of transcriptional regulators. Three genes identified by the mutant analysis mapped to a region that was previously shown to carry the zwittermicin A self-resistance gene, zmaR, and a biosynthetic gene (E. A. Stohl, J. L. Milner, and J. Handelsman, Gene 237:403-411, 1999). Further sequencing of this region revealed genes proposed to encode zwittermicin A precursor biosynthetic enzymes, in particular, those involved in the formation of the aminomalonyl- and hydroxymalonyl-acyl carrier protein intermediates. Additionally, nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) homologs are present, suggesting that zwittermicin A is synthesized by a mixed NRPS/PKS pathway.

The recognition that most microorganisms in the environment cannot be cultured by standard methods stimulated the development of metagenomics, which is the genomic analysis of uncultured microorganisms. Two types of analysis have been used to obtain information from metagenomic libraries: a function-driven approach, in which metagenomic libraries are initially screened for an expressed trait, and a sequence-driven approach, in which libraries are initially screened for particular DNA sequences. New antibiotics and enzymes are among the early discoveries from metagenomics. Future refinement of methods that enrich for genes of particular function will accelerate the rate of discovery of useful molecules.

The goal of this study was to identify genes in Bacillus cereus, a bacterium commonly associated with plant seeds and roots, that are affected by compounds originating from a host plant, tomato, or another rhizosphere resident, Pseudomonas aureofaciens. We constructed a B. cereus chromosomal DNA library in a promoter-trap plasmid, pAD123, which contains a promoterless version of the green fluorescent protein (GFP) gene, gfpmut3a. The library was screened by using fluorescence-activated cell sorting for clones showing a change in GFP expression in response to either tomato seed exudate or culture supernatant of P. aureofaciens strain 30-84. We identified two clones carrying genes that were induced by the presence of tomato seed exudate and nine clones carrying genes that were repressed by P. aureofaciens culture supernatant. A clone chosen for further study contained an open reading frame, designated lipA, that encodes a deduced protein with a lipoprotein signal peptide sequence similar to lipoproteins in B. subtilis. Expression of gusA under control of the lipA promoter increased twofold when cells were exposed to tomato seed exudate and in a concentration-dependent manner when exposed to a mixture of amino acids. When the wild type and a 10-fold excess of a lipA mutant were applied together to tomato seeds, 2 days after planting, the wild type displayed medium-dependent culturability, whereas the lipA mutant was unaffected. This study demonstrates the power of a promoter trap to identify genes in a gram-positive bacterium that are regulated by the biotic environment and resulted in the discovery of lipA, a plant-regulated gene in B. cereus.

RosR is a determinant of nodulation competitiveness and cell surface characteristics of Rhizobium etli and has sequence similarity to a family of transcriptional repressors. To understand how RosR affects these phenotypes, we mutagenized a rosR mutant derivative of R. etli strain CE3 with a mini-Tn5 that contains a promoterless gusA gene at one end, which acts as a transcriptional reporter. Using a mass-mating technique, we introduced rosR into each mutant in trans and screened for mutants that expressed different levels of beta-glucuronidase activity in the presence and absence of rosR. A screen of 18,000 mutants identified 52 insertions in genes negatively regulated by RosR and 1 insertion in a gene positively regulated by RosR. Nucleotide sequence analysis of the regions flanking the insertions suggests that RosR regulates genes of diverse function, including those involved in polysaccharide production and in carbohydrate metabolism and those in a region containing sequence similarity to virC1 and virD3 from Agrobacterium tumefaciens. Two of the mutants produced colonies with altered morphology and were more competitive in nodulation than was CE3DeltarosR, the rosR parent. One mutant that contained an insertion in a gene with similarity to exsH of Sinorhizobium meliloti did not nodulate the plant host Phaseolus vulgaris without rosR. These results indicate that RosR directly or indirectly influences expression of diverse genes in R. etli, some of which affect the cell surface and nodulation competitiveness.

ZmaR is a resistance determinant of unusual abundance in the environment and confers on gram-positive and gram-negative bacteria resistance to zwittermicin A, a novel broad-spectrum antibiotic produced by species of Bacillus. The ZmaR protein has no sequence similarity to proteins of known function; thus, the purpose of the present study was to determine the function of ZmaR in vitro. Cell extracts of E. coli containing zmaR inactivated zwittermicin A by covalent modification. Chemical analysis of inactivated zwittermicin A by 1H NMR, 13C NMR, and high- and low-resolution mass spectrometry demonstrated that the inactivated zwittermicin A was acetylated. Purified ZmaR protein inactivated zwittermicin A, and biochemical assays for acetyltransferase activity with [14C]acetyl coenzyme A demonstrated that ZmaR catalyzes the acetylation of zwittermicin A with acetyl coenzyme A as a donor group, suggesting that ZmaR may constitute a new class of acetyltransferases. Our results allow us to assign a biochemical function to a resistance protein that has no sequence similarity to proteins of known function, contributing fundamental knowledge to the fields of antibiotic resistance and protein function.

We previously described a Tn5 mutant of Rhizobium etli strain CE3, designated CE3003, that is decreased in nodulation competitiveness, reduced in competitive growth in the rhizosphere, and has a hydrophobic cell surface (R. S. Araujo, E. A. Robleto, and J. Handelsman, Appl. Environ. Microbiol., 60:1430-1436, 1994). To determine the molecular basis for the mutant phenotypes, we identified a 1.2-kb fragment of DNA derived from the parent that restored the wild-type phenotypes to the mutant. DNA sequence analysis indicated that this 1.2-kb fragment contained a single open reading frame that we designated rosR. The Tn5 insertion in CE3003 was within rosR. We constructed a derivative of CE3 that contained a deletion in rosR, and this mutant was phenotypically indistinguishable from CE3003 in cell surface and competitive characteristics. Based on the nucleotide sequence, the deduced RosR amino acid sequence is 80% identical to that of the Ros protein from Agrobacterium tumefaciens and the MucR protein from Rhizobium meliloti. Both Ros and MucR are transcriptional repressors that contain a putative zinc-finger DNA-binding domain. This study defines a gene, rosR, that is homologous to a family of transcriptional regulators and contributes to nodulation competitiveness of R. etli. Moreover, we established that a single gene affects nodulation competitiveness, competitive growth in the rhizosphere, and cell surface hydrophobicity.

Zwittermicin A is a novel aminopolyol antibiotic produced by Bacillus cereus that is active against diverse bacteria and lower eukaryotes (L.A. Silo-Suh, B.J. Lethbridge, S.J. Raffel, H. He, J. Clardy, and J. Handelsman, Appl. Environ. Microbiol. 60:2023-2030, 1994). To identify a determinant for resistance to zwittermicin A, we constructed a genomic library from B. cereus UW85, which produces zwittermicin A, and screened transformants of Escherichia coli DH5alpha, which is sensitive to zwittermicin A, for resistance to zwittermicin A. Subcloning and mutagenesis defined a genetic locus, designated zmaR, on a 1.2-kb fragment of DNA that conferred zwittermicin A resistance on E. coli. A DNA fragment containing zmaR hybridized to a corresponding fragment of genomic DNA from B. cereus UW85. Corresponding fragments were not detected in mutants of B. cereus UW85 that were sensitive to zwittermicin A, and the plasmids carrying zmaR restored resistance to the zwittermicin A-sensitive mutants, indicating that zmaR was deleted in the zwittermicin A-sensitive mutants and that zmaR is functional in B. cereus. Sequencing of the 1.2-kb fragment of DNA defined an open reading frame, designated ZmaR. Neither the nucleotide sequence nor the predicted protein sequence had significant similarity to sequences in existing databases. Cell extracts from an E. coli strain carrying zmaR contained a 43.5-kDa protein whose molecular mass and N-terminal sequence matched those of the protein predicted by the zmaR sequence. The results demonstrate that we have isolated a gene, zmaR, that encodes a zwIttermicin A resistance determinant that is functional in both B. cereus and E. coli.