Latest revision as of 01:35, 22 October 2021

colicin-specific sgRNA

Introduction

Vilnius-Lithuania iGEM 2021 project AmeByelooks at amebiasis holistically and comprehensively, therefore target E. histolytica from several angles: prevention and diagnostics. Our team's preventive solution consists of probiotics engineered to produce naringenin - an antiprotozoal compound. Two strains of genetically modified microorganisms were chosen as the main chassis - world-renowned Lactobacillus casei BL23 (Lactobacillus paracasei) and Escherichia coli Nissle 1917. Furthermore, the team made specific gene deletions to enhance naringenin production and adapted a novel toxin-antitoxin system to prevent GMO spreads into the environment. The diagnostic part includes a rapid, point of care, user-friendly diagnostic test identifying extraintestinal amebiasis. The main components of this test are aptamers specific to the E. histolytica secreted proteins. These single-stranded DNA sequences fold into tertiary structures for particular fit with target proteins.

Usage and Biology

CRISPR-Cas9 is a versatile genome-editing technique. In our approach to editing E. coli Nissle 1917 genome, we have used two plasmid based system enabling to combine of Lambda Red recombination and CRISPR-Cas9 as counterselection tools encoded in pCas and pTarget.

Mechanism of genome editing

pCas plasmid is used for Cas9, Lambda Red system expression, and plasmid curing of pTarget. Cas9 - the RNA-guided endonuclease - is expressed constitutively, while the expression of Lambda Red genes (Gam, Exo, Beta) is under the control of arabinose inducible promoter araBp. pTarget plasmid caries constitutively expressed single-guide RNA (sgRNA). This RNA molecule, as and in nature, is composed of 17-20 nt length guide RNA (gRNA) sequence complementary to the targeted DNA adjacent to the protospacer adjacent motif (PAM) present at the 3' end, and the scaffold for the Cas9 nuclease binding to sgRNA and forming the ribonucleoprotein complex (1). Although in nature sgRNA exists as two separate RNA molecules, in laboratory experiments they are usually combined into one single-guide RNA (sgRNA) obviating additional maturation steps. As both pCas and pTarget plasmids are in a cell, Cas9 nuclease and sgRNA are able to form ribonucleoprotein complex, scan DNA for PAM sequences and perform a double-strand break in a part of the DNA which is complementary to the gRNA and adjacent to the PAM sequence - NGG. However, if arabinose has been added to the cell culture and a double-stranded DNA repair template is present in the cell, the Lambda Red system performs homologous recombination. If this process is unsuccessful, the Cas9-sgRNA complex will cause a double-strand break and will cause cell death (2). This is employed as a counterselection in order to avoid the additional antibiotic as selection marker usage.

Table 1. sgRNA collection for E. coli Nissle 1917 genome editing.

Table 2. Parts collection for genomic insertion into E. Coli Nissle 1917 colicin gene.

colicin-specific sgRNA importance to our project

Particularly in our project, we needed to insert the metabolic pathway for naringenin production into the genomes of our target probiotic strains. We have chosen two genomic sites for genomic insertions, which would do not harm or negatively affect our probiotic bacteria growth or overall performance - a putative colicin-encoding and nupG genes. We have inserted sfGFP encoding gene under the control of constitutive slpA promoter into these sequences. SfGFP fluorescence was significantly higher in samples with insertion in nupG, however, this case also demonstrated higher fluorescence fluctuations in comparison with putative colicin samples.

Fig. 1. Transcriptional activity comparison of GFP transcription from its constructs insertion in nupG or colicin genes.

Genome editing efficiency

GFP insertion into colicin gene with this sgRNA have be have achieved with 89 % efficiency (fig. 2).

Fig. 2. GFP insertion into colicin gene results. Here are represented with XbaI digested cPCR products from chosen transformant colonies. If insertion is successful, 1 kbp and 148 bp products are expected. L - GeneRuler 1 bkp Ladder, 1 - negative control (wild type E. coli Nissle 1917), 2 - colicin wild type, 3 - colicin-GFP mutant, 4 - colicin-GFP mutant, 5 - colicin-GFP mutant, 6 - colicin-GFP mutant, 7 - colicin-GFP mutant, 8 - colicin-GFP mutant, 9 - colicin-GFP mutant, 10 - colicin-GFP mutant.

Sequence and Features

References

1. Doudna, J. A., & Charpentier, E. (2014). The new frontier of genome engineering with CRISPR-Cas9. Science, 346(6213).
2. Mali, P., Yang, L., Esvelt, K. M., Aach, J., Guell, M., DiCarlo, J. E., Norville, J. E., & Church, G. M. (2013). RNA-guided human genome engineering via Cas9. Science (New York, N.Y.), 339(6121), 823–826.