CRISPR-Cas9-Based Toolkit for Clostridium botulinum Group II Spore and Sporulation Research.

The spores of Clostridium botulinum Group II strains pose a significant threat to the safety of modern packaged foods due to the risk of their survival in pasteurization and their ability to germinate into neurotoxigenic cultures at refrigeration temperatures. Moreover, spores are the infectious agents in wound botulism, infant botulism, and intestinal toxemia in adults. The identification of factors that contribute to spore formation is, therefore, essential to the development of strategies to control related health risks. Accordingly, development of a straightforward and versatile gene manipulation tool and an efficient sporulation-promoting medium is pivotal. Our strategy was to employ CRISPR-Cas9 and homology-directed repair (HDR) to replace targeted genes with mutant alleles incorporating a unique 24-nt "bookmark" sequence that could act as a single guide RNA (sgRNA) target for Cas9. Following the generation of the sporulation mutant, the presence of the bookmark allowed rapid generation of a complemented strain, in which the mutant allele was replaced with a functional copy of the deleted gene using CRISPR-Cas9 and the requisite sgRNA. Then, we selected the most appropriate medium for sporulation studies in C. botulinum Group II strains by measuring the efficiency of spore formation in seven different media. The most effective medium was exploited to confirm the involvement of a candidate gene in the sporulation process. Using the devised sporulation medium, subsequent comparisons of the sporulation efficiency of the wild type (WT), mutant and "bookmark"-complemented strain allowed the assignment of any defective sporulation phenotype to the mutation made. As a strain generated by complementation with the WT gene in the original locus would be indistinguishable from the parental strain, the gene utilized in complementation studies was altered to contain a unique "watermark" through the introduction of silent nucleotide changes. The mutagenesis system and the devised sporulation medium provide a solid basis for gaining a deeper understanding of spore formation in C. botulinum, a prerequisite for the development of novel strategies for spore control and related food safety and public health risk management.

Generation of a fully erythromycin-sensitive strain of Clostridioides difficile using a novel CRISPR-Cas9 genome editing system.

Understanding the molecular pathogenesis of Clostridioides difficile has relied on the use of ermB-based mutagens in erythromycin-sensitive strains. However, the repeated subcultures required to isolate sensitive variants can lead to the acquisition of ancillary mutations that affect phenotype, including virulence. CRISPR-Cas9 allows the direct selection of mutants, reducing the number of subcultures and thereby minimising the likelihood of acquiring additional mutations. Accordingly, CRISPR-Cas9 was used to sequentially remove from the C. difficile 630 reference strain (NCTC 13307) two ermB genes and pyrE. The genomes of the strains generated (630Δerm* and 630Δerm*ΔpyrE, respectively) contained no ancillary mutations compared to the NCTC 13307 parental strain, making these strains the preferred option where erythromycin-sensitive 630 strains are required. Intriguingly, the cas9 gene of the plasmid used contained a proximal frameshift mutation. Despite this, the frequency of mutant isolation was high (96% and 89% for ermB and pyrE, respectively) indicating that a functional Cas9 is still being produced. Re-initiation of translation from an internal AUG start codon would produce a foreshortened protein lacking a RuvCI nucleolytic domain, effectively a 'nickase'. The mutation allowed cas9 to be cloned downstream of the strong Pthl promoter. It may find application elsewhere where the use of strong, constitutive promoters is preferred.

RiboCas: A Universal CRISPR-Based Editing Tool for Clostridium.

Members of the genus Clostridium represent a diverse assemblage of species exhibiting both medical and industrial importance. Deriving both a greater understanding of their biology, while at the same time enhancing their exploitable properties, requires effective genome editing tools. Here, we demonstrate the first implementation in the genus of theophylline-dependent, synthetic riboswitches exhibiting a full set of dynamic ranges, also suitable for applications where tight control of gene expression is required. Their utility was highlighted by generating a novel riboswitch-based editing tool-RiboCas-that overcomes the main obstacles associated with CRISPR/Cas9 systems, including low transformation efficiencies and excessive Cas9 toxicity. The universal nature of the tool was established by obtaining chromosomal modifications in C. pasteurianum, C. difficile, and C. sporogenes, as well as by carrying out the first reported example of CRISPR-targeted gene disruption in C. botulinum. The high efficiency (100% mutant generation) and ease of application of RiboCas make it suitable for use in a diverse range of microorganisms.

Exploring the Nonconserved Sequence Space of Synthetic Expression Modules in Bacillus subtilis.

Increasing protein expression levels is a key step in the commercial production of enzymes. Predicting promoter activity and translation initiation efficiency based solely on consensus sequences have so far met with mixed results. Here, we addressed this challenge using a "brute-force" approach by designing and synthesizing a large combinatorial library comprising ∼12 000 unique synthetic expression modules (SEMs) for Bacillus subtilis. Using GFP fluorescence as a reporter of gene expression, we obtained a dynamic expression range that spanned 5 orders of magnitude, as well as a maximal 13-fold increase in expression compared with that of the already strong veg expression module. Analyses of the synthetic modules indicated that sequences at the 5'-end of the mRNA were the most important contributing factor to the differences in expression levels, presumably by preventing formation of strong secondary mRNA structures that affect translation initiation. When the gfp coding region was replaced by the coding region of the xynA gene, encoding the industrially relevant B. subtilis xylanase enzyme, only a 3-fold improvement in xylanase production was observed. Moreover, the correlation between GFP and xylanase expression levels was weak. This suggests that the differences in expression levels between the gfp and xynA constructs were due to differences in 5'-end mRNA folding and consequential differences in the rates of translation initiation. Our data show that the use of large libraries of SEMs, in combination with high-throughput technologies, is a powerful approach to improve the production of a specific protein, but that the outcome cannot necessarily be extrapolated to other proteins.

Enzyme-catalyzed oxidation of 5-hydroxymethylfurfural to furan-2,5-dicarboxylic acid.

Furan-2,5-dicarboxylic acid (FDCA) is a biobased platform chemical for the production of polymers. In the past few years, numerous multistep chemical routes have been reported on the synthesis of FDCA by oxidation of 5-hydroxymethylfurfural (HMF). Recently we identified an FAD-dependent enzyme which is active towards HMF and related compounds. This oxidase has the remarkable capability of oxidizing [5-(hydroxymethyl)furan-2-yl]methanol to FDCA, a reaction involving four consecutive oxidations. The oxidase can produce FDCA from HMF with high yield at ambient temperature and pressure. Examination of the underlying mechanism shows that the oxidase acts on alcohol groups only and depends on the hydration of aldehydes for the oxidation reaction required to form FDCA.