Transcript analysis ofEscherichia coliK-12 insertion element IS5

Investigating How Genomic Contexts Impact IS5 Transposition Within the Escherichia coli Genome

Insertions of the transposable element IS5 into its target sites in response to stressful environmental conditions, DNA structures, and DNA-binding proteins are well studied, but how the genomic contexts near IS5's native loci impact its transpositions is largely unknown. Here, by examining the roles of all 11 copies of IS5 within the genome of E. coli strain BW25113 in transposition, we reveal that the most significant copy of IS5 is one nested within and oriented in the same direction as the nmpC gene, while two other copies of IS5 harboring point mutations are hardly transposed. Transposition activity is heavily reliant on the upstream nmpC promoter that drives IS5 transposase gene ins5A, with more transpositions resulting from greater promoter activity. The IS5 element at nmpC but not at other loci transcribed detectable amounts of ins5A mRNA. By increasing expression of the ins5CB operon harbored in IS5, we demonstrate that Ins5B and Ins5C appear to exert a stimulatory role in IS5 transposition, suggesting that the downstream genomic regions near the native loci are involved in overall IS5 transposition as well. Using a strain that carries IS5 only at the nmpC locus, we confirm that IS5 primarily uses a copy/paste mechanism for transposition, although we cannot rule out the cut/paste mechanism.

The effect of DNA-binding proteins on insertion sequence element transposition upstream of the bgl operon in Escherichia coli

The bglGFB operon in Escherichia coli K-12 strain BW25113, encoding the proteins necessary for the uptake and metabolism of β-glucosides, is normally not expressed. Insertion of either IS1 or IS5 upstream of the bgl promoter activates expression of the operon only when the cell is starving in the presence of a β-glucoside, drastically increasing transcription and allowing the cell to survive and grow using this carbon source. Details surrounding the exact mechanism and regulation of the IS insertional event remain unclear. In this work, the role of several DNA-binding proteins in how they affect the rate of insertion upstream of bgl are examined via mutation assays and protocols measuring transcription. Both Crp and IHF exert a positive effect on insertional Bgl+ mutations when present, active, and functional in the cell. Our results characterize IHF's effect in conjunction with other mutations, show that IHF's effect on IS insertion into bgl also affects other operons, and indicate that it may exert its effect by binding to and altering the DNA conformation of IS1 and IS5 in their native locations, rather than by directly influencing transposase gene expression. In contrast, the cAMP-CRP complex acts directly upon the bgl operon by binding upstream of the promoter, presumably altering local DNA into a conformation that enhances IS insertion.

Enhanced production of nonanedioic acid from nonanoic acid by engineered Escherichia coli

In this study, whole-cell biotransformation was conducted to produce nonanedioic acid from nonanoic acid by expressing the alkane hydroxylating system (AlkBGT) from Pseudomonas putida GPo1 in Escherichia coli. Following adaptive laboratory evolution, an efficient E. coli mutant strain, designated as MRE, was successfully obtained, demonstrating the fastest growth (27-fold higher) on nonanoic acid as the sole carbon source compared to the wild-type strain. Additionally, the MRE strain was engineered to block nonanoic acid degradation by deleting fadE. The resulting strain exhibited a 12.8-fold increase in nonanedioic acid production compared to the wild-type strain. Six mutations in acrR, P_crp , dppA, P_fadD , e14, and yeaR were identified in the mutant MRE strain, which was characterized using genomic modifications and RNA-sequencing. The acquired mutations were found to be beneficial for rapid growth and nonanedioic acid production.

CRISPR-interference-based modulation of mobile genetic elements in bacteria

Spontaneous mutagenesis of synthetic genetic constructs by mobile genetic elements frequently results in the rapid loss of engineered functions. Previous efforts to minimize such mutations required the exceedingly time-consuming manipulation of bacterial chromosomes and the complete removal of insertional sequences (ISes). To this aim, we developed a single plasmid-based system (pCRIS) that applies CRISPR-interference to inhibit the transposition of bacterial ISes. pCRIS expresses multiple guide RNAs to direct inactivated Cas9 (dCas9) to simultaneously silence IS1, IS3, IS5 and IS150 at up to 38 chromosomal loci in Escherichia coli, in vivo. As a result, the transposition rate of all four targeted ISes dropped to negligible levels at both chromosomal and episomal targets. Most notably, pCRIS, while requiring only a single plasmid delivery performed within a single day, provided a reduction of IS-mobility comparable to that seen in genome-scale chromosome engineering projects. The fitness cost of multiple IS-knockdown, detectable in flask-and-shaker systems was readily outweighed by the less frequent inactivation of the transgene, as observed in green fluorescent protein (GFP)-overexpression experiments. In addition, global transcriptomics analysis revealed only minute alterations in the expression of untargeted genes. Finally, the transposition-silencing effect of pCRIS was easily transferable across multiple E. coli strains. The plasticity and robustness of our IS-silencing system make it a promising tool to stabilize bacterial genomes for synthetic biology and industrial biotechnology applications.

Construction of plasmid-free Escherichia coli for the production of arabitol-free xylitol from corncob hemicellulosic hydrolysate

High costs and low production efficiency are a serious constraint to bio-based xylitol production. For industrial-scale production of xylitol, a plasmid-free Escherichia coli for arabitol-free xylitol production from corncob hemicellulosic hydrolysate has been constructed. Instead of being plasmid and inducer dependent, this strain relied on multiple-copy integration of xylose reductase (XR) genes into the chromosome, where their expression was controlled by the constitutive promoter P43. In addition, to minimize the flux from L-arabinose to arabitol, two strategies including low XR total activity and high selectivity of XR has been adopted. Arabitol was significantly decreased using plasmid-free strain which had lower XR total activity and an eight point-mutations of XR with a 27-fold lower enzyme activity toward L-arabinose was achieved. The plasmid-free strain in conjunction with this mutant XR can completely eliminate arabitol formation in xylitol production. In fed-batch fermentation, this plasmid-free strain produced 143.8 g L(-1) xylitol at 1.84 g L(-1) h(-1) from corncob hemicellulosic hydrolysate. From these results, we conclude that this route by plasmid-free E. coli has potential to become a commercially viable process for xylitol production.

Contributions of mutations in acrR and marR genes to organic solvent tolerance in Escherichia coli

The AcrAB-TolC efflux pump is involved in maintaining intrinsic organic solvent tolerance in Escherichia coli. Mutations in regulatory genes such as marR, soxR, and acrR are known to increase the expression level of the AcrAB-TolC pump. To identify these mutations in organic solvent tolerant E. coli, eight cyclohexane-tolerant E. coli JA300 mutants were isolated and examined by DNA sequencing for mutations in marR, soxR, and acrR. Every mutant carried a mutation in either marR or acrR. Among all mutants, strain CH7 carrying a nonsense mutation in marR (named marR109) and an insertion of IS5 in acrR, exhibited the highest organic solvent-tolerance levels. To clarify the involvement of these mutations in improving organic solvent tolerance, they were introduced into the E. coli JA300 chromosome by site-directed mutagenesis using λ red-mediated homologous recombination. Consequently, JA300 mutants carrying acrR::IS5, marR109, or both were constructed and named JA300 acrRIS, JA300 marR, or JA300 acrRIS marR, respectively. The organic solvent tolerance levels of these mutants were increased in the following order: JA300 < JA300 acrRIS < JA300 marR < JA300 acrRIS marR. JA300 acrRIS marR formed colonies on an agar plate overlaid with cyclohexane and p-xylene (6:4 vol/vol mixture). The organic solvent-tolerance level and AcrAB-TolC efflux pump-expression level in JA300 acrRIS marR were similar to those in CH7. Thus, it was shown that the synergistic effects of mutations in only two regulatory genes, acrR and marR, can significantly increase organic solvent tolerance in E. coli.

Visualizing evolution in real time to determine the molecular mechanisms of n-butanol tolerance in Escherichia coli

Toxicity of products or feedstock components poses a challenge in the biocatalyst-based production of fuels and chemicals. The genetic determinants that are involved in increased resistance to an inhibitor form the adaptive landscape for the phenotype; so in order to engineer more robust biocatalysts, a better understanding of the adaptive landscape is required. Here, we used an adaptive laboratory evolution method called visualizing evolution in real time (VERT) to help map out part of the adaptive landscape of Escherichia coli tolerance to the biofuel n-butanol. VERT enables identification of adaptive events (population expansions triggered by adaptive mutants) via visualization of the relative proportions of different fluorescently-labeled cells. Knowledge of the occurrence of adaptive events allows for a more systematic isolation of adaptive mutants while simultaneously reducing the number of missed adaptive mutants (and the underlying adaptive mechanisms) that result from clonal interference during the course of in vitro evolution. Based on the evolutionary dynamics observed, clonal interference was found to play a significant role in shaping the population structure of E. coli during exposure to n-butanol, and VERT helped to facilitate the isolation of adaptive mutants from the population. We further combined adaptive laboratory evolution with genome shuffling to significantly enhance the desired n-butanol tolerance phenotype. Subsequent transcriptome analysis of the isolated adaptive mutants revealed different mechanisms of n-butanol resistance in different lineages. In one fluorescently-marked subpopulation, members of the Fur regulon were upregulated; which was not observed in the other subpopulation. In addition, genome sequencing of several adaptive mutants revealed the genetic basis for some of the observed transcriptome profiles. We further elucidated the potential role of the iron-related gene in n-butanol tolerance via overexpression and deletion studies and hypothesized that the upregulation of the iron-related genes indirectly led to modifications in the outer membrane, which contributed to enhanced n-butanol tolerance.

A proteogenomic analysis of Shigella flexneri using 2D LC-MALDI TOF/TOF

BACKGROUND

New strategies for high-throughput sequencing are constantly appearing, leading to a great increase in the number of completely sequenced genomes. Unfortunately, computational genome annotation is out of step with this progress. Thus, the accurate annotation of these genomes has become a bottleneck of knowledge acquisition.

RESULTS

We exploited a proteogenomic approach to improve conventional genome annotation by integrating proteomic data with genomic information. Using Shigella flexneri 2a as a model, we identified total 823 proteins, including 187 hypothetical proteins. Among them, three annotated ORFs were extended upstream through comprehensive analysis against an in-house N-terminal extension database. Two genes, which could not be translated to their full length because of stop codon 'mutations' induced by genome sequencing errors, were revised and annotated as fully functional genes. Above all, seven new ORFs were discovered, which were not predicted in S. flexneri 2a str.301 by any other annotation approaches. The transcripts of four novel ORFs were confirmed by RT-PCR assay. Additionally, most of these novel ORFs were overlapping genes, some even nested within the coding region of other known genes.

CONCLUSIONS

Our findings demonstrate that current Shigella genome annotation methods are not perfect and need to be improved. Apart from the validation of predicted genes at the protein level, the additional features of proteogenomic tools include revision of annotation errors and discovery of novel ORFs. The complementary dataset could provide more targets for those interested in Shigella to perform functional studies.

In vivo selection of carbapenem-resistant Klebsiella pneumoniae by OmpK36 loss during meropenem treatment

We recovered a carbapenem-resistant Klebsiella pneumoniae isolate H224 under in vivo meropenem selection pressure. Insertional inactivation of a major porin gene, ompK36, by IS5 element might play a role in acquiring carbapenem resistance in this strain harboring plasmid-borne DHA-1 AmpC beta-lactamase.