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Identification of the main promoter directing cereulide biosynthesis in emetic Bacillus cereus and its application for real-time monitoring of ces gene expression in foods.

ABSTRACT: Cereulide, the emetic Bacillus cereus toxin, is synthesized by cereulide synthetase via a nonribosomal peptide synthetase (NRPS) mechanism. Previous studies focused on the identification, structural organization, and biochemical characterization of the ces gene locus encoding cereulide synthetase; however, detailed information about the transcriptional organization of the ces genes was lacking. The present study shows that the cesPTABCD genes are transcribed as a 23-kb polycistronic transcript, while cesH, encoding a putative hydrolase, is transcribed from its own promoter. Transcription initiation was mapped by primer extension and rapid amplification of cDNA ends (RACE). Deletion analysis of promoter elements revealed a main promoter located upstream of the cesP coding sequence, encoding a 4'-phosphopantetheinyl transferase. This promoter drives transcription of cesPTABCD. In addition, intracistronic promoter regions in proximity to the translational start sites of cesB and cesT were identified but were only weakly active under the chosen assay conditions. The identified main promoter was amplified from the emetic reference strain B. cereus F4810/72 and fused to luciferase genes in order to study promoter activity in complex environments and to establish a biomonitoring system to assess cereulide production in different types of foods. ces promoter activity was strongly influenced by the food matrix and varied by 5 orders of magnitude. The amount of cereulide toxin extracted from spiked foods correlated well with the bioluminescence data, thus illustrating the potential of the established reporter system for monitoring of ces gene expression in complex matrices.

SUBMITTER: Dommel MK

PROVIDER: S-EPMC2820966 | biostudies-literature | 2010 Feb

REPOSITORIES: biostudies-literature

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Identification of the main promoter directing cereulide biosynthesis in emetic Bacillus cereus and its application for real-time monitoring of ces gene expression in foods.

Dommel Monica K MK Frenzel Elrike E Strasser Bernd B Blöchinger Claudia C Scherer Siegfried S Ehling-Schulz Monika M

Applied and environmental microbiology 20091228 4

Cereulide, the emetic Bacillus cereus toxin, is synthesized by cereulide synthetase via a nonribosomal peptide synthetase (NRPS) mechanism. Previous studies focused on the identification, structural organization, and biochemical characterization of the ces gene locus encoding cereulide synthetase; however, detailed information about the transcriptional organization of the ces genes was lacking. The present study shows that the cesPTABCD genes are transcribed as a 23-kb polycistronic transcript, ...[more]

PMID: 20038713

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Project description:BackgroundCereulide, a depsipeptide structurally related to valinomycin, is responsible for the emetic type of gastrointestinal disease caused by Bacillus cereus. Recently, it has been shown that this toxin is produced by a nonribosomal peptide synthetase (NRPS), but its exact genetic organization and biochemical synthesis is unknown.ResultsThe complete sequence of the cereulide synthetase (ces) gene cluster, which encodes the enzymatic machinery required for the biosynthesis of cereulide, was dissected. The 24 kb ces gene cluster comprises 7 CDSs and includes, besides the typical NRPS genes like a phosphopantetheinyl transferase and two CDSs encoding enzyme modules for the activation and incorporation of monomers in the growing peptide chain, a CDS encoding a putative hydrolase in the upstream region and an ABC transporter in the downstream part. The enzyme modules responsible for incorporation of the hydroxyl acids showed an unusual structure while the modules responsible for the activation of the amino acids Ala and Val showed the typical domain organization of NRPS. The ces gene locus is flanked by genetic regions with high homology to virulence plasmids of B. cereus, Bacillus thuringiensis and Bacillus anthracis. PFGE and Southern hybridization showed that the ces genes are restricted to emetic B. cereus and indeed located on a 208 kb megaplasmid, which has high similarities to pXO1-like plasmids.ConclusionThe ces gene cluster that is located on a pXO1-like virulence plasmid represents, beside the insecticidal and the anthrax toxins, a third type of B. cereus group toxins encoded on megaplasmids. The ces genes are restricted to emetic toxin producers, but pXO1-like plasmids are also present in emetic-like strains. These data might indicate the presence of an ancient plasmid in B. cereus which has acquired different virulence genes over time. Due to the unusual structure of the hydroxyl acid incorporating enzyme modules of Ces, substantial biochemical efforts will be required to dissect the complete biochemical pathway of cereulide synthesis.

Project description:BACKGROUND: Cereulide is a cyclic dodecadepsipeptide ionophore, produced via non-ribosomal peptide synthetases (NRPS), which in rare cases can lead to human death. Early studies had shown that emetic toxin formation belongs to a homogeneous group of Bacillus cereus sensu stricto and the genetic determinants of cereulide (a 24-kb gene cluster of cesHPTABCD) are located on a 270-kb plasmid related to the Bacillus anthracis virulence plasmid pXO1. RESULTS: The whole genome sequences from seven emetic isolates, including two B. cereus sensu stricto and five Bacillus weihenstephanensis strains, were compared, and their inside and adjacent DNA sequences of the cereulide biosynthesis gene clusters were analyzed. The sequence diversity was observed, which classified the seven emetic isolates into three clades. Different genomic locations of the cereulide biosynthesis gene clusters, plasmid-borne and chromosome-borne, were also found. Potential mobile genetic elements (MGEs) were identified in the flanking sequences of the ces gene cluster in all three types. The most striking observation was the identification of a putative composite transposon, Tnces, consisting of two copies of ISces element (belonging to IS6 family) in opposite orientations flanking the ces gene cluster in emetic B. weihenstephanensis. The mobility of this element was tested by replacing the ces gene cluster by a KmR gene marker and performing mating-out transposition assays in Escherichia coli. The results showed that Tnces::km transposes efficiently (1.04 × 10(-3) T/R) and produces 8-bp direct repeat (DR) at the insertion sites. CONCLUSIONS: Cereulide biosynthesis gene clusters display sequence diversity, different genomic locations and association with MGEs, in which the transposition capacity of a resistant derivative of the composite transposon Tnces in E. coli was demonstrated. Further study is needed to look for appropriate genetic tools to analysis the transposition of Tnces in Bacillus spp. and the dynamics of other MGEs flanking the ces gene clusters.

Dataset Information

Identification of the main promoter directing cereulide biosynthesis in emetic Bacillus cereus and its application for real-time monitoring of ces gene expression in foods.

Publications

Identification of the main promoter directing cereulide biosynthesis in emetic Bacillus cereus and its application for real-time monitoring of ces gene expression in foods.

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