An enChIP system for the analysis of bacterial genome functions

Objective

The engineered DNA-binding molecule-mediated chromatin immunoprecipitation (enChIP) technology enables purification of specific genomic regions interacting with their associated molecules. In enChIP, the locus to be purified is first tagged with engineered DNA-binding molecules. An example of such engineered DNA-binding molecules to tag the locus of interest is the clustered regularly interspaced short palindromic repeats (CRISPR) system, consisting of a catalytically-inactive form of Cas9 (dCas9) and guide RNA (gRNA). Subsequently, the tagged locus is subjected to affinity purification for identification of interacting molecules. In our previous studies, we developed enChIP systems for analysis of mammalian genome functions. Here, we developed an enChIP system to analyze bacterial genome functions.

Results

We generated a plasmid inducibly expressing Streptococcus pyogenes dCas9 fused to a 3xFLAG-tag (3xFLAG-dCas9) in bacteria. Inducible expression of 3xFLAG-dCas9 in Escherichia coli was confirmed by immunoblot analysis. We were able to purify specific genomic regions of E. coli preserving their molecular interactions. The system is potentially useful for analysis of interactions between specific genomic regions and their associated molecules in bacterial cells to understand genome functions such as transcription, DNA repair, and DNA recombination.

Transgenic mouse lines expressing the 3xFLAG-dCas9 protein for enChIP analysis

We developed the engineered DNA-binding molecule-mediated chromatin immunoprecipitation (enChIP) technology to isolate specific genomic regions while retaining their molecular interactions. In enChIP, the locus of interest is tagged with an engineered DNA-binding molecule, such as a modified form of the clustered regularly interspaced short palindromic repeats (CRISPR) system containing a guide RNA (gRNA) and a catalytically inactive form of Cas9 (dCas9). The locus is then affinity-purified to enable identification of associated molecules. In this study, we generated transgenic mice expressing 3xFLAG-tagged Streptococcus pyogenes dCas9 (3xFLAG-dCas9) and retrovirally transduced gRNA into primary CD4+ T cells from these mice for enChIP. Using this approach, we achieved high yields of enChIP at the targeted genomic region. Our novel transgenic mouse lines provide a valuable tool for enChIP analysis in primary mouse cells.

enChIP systems using different CRISPR orthologues and epitope tags

Objective

Previously, we developed the engineered DNA-binding molecule-mediated chromatin immunoprecipitation (enChIP) technology, which isolates specific genomic regions while preserving their molecular interactions. In enChIP, the locus of interest is tagged with engineered DNA-binding molecules such as the clustered regularly interspaced short palindromic repeats (CRISPR) system, consisting of a catalytically inactive form of Cas9 (dCas9) and guide RNA, followed by affinity purification of the tagged locus to allow identification of associated molecules. In our previous studies, we used a 3xFLAG-tagged CRISPR system from Streptococcus pyogenes (S. pyogenes). In this study, to increase the flexibility of enChIP, we used the CRISPR system from Staphylococcus aureus (S. aureus) along with different epitope tags.

Results

We generated a plasmid expressing S. aureus dCas9 (Sa-dCas9) fused to a nuclear localization signal (NLS) and a 3xFLAG-tag (Sa-dCas9-3xFLAG). The yields of enChIP using Sa-dCas9-3xFLAG were comparable to those using S. pyogenes dCas9 fused with an NLS and a 3xFLAG-tag (3xFLAG-Sp-dCas9). We also generated another enChIP system using Sp-dCas9 fused with an NLS and a 2xAM-tag (Sp-dCas9-2xAM). We obtained high enChIP yields using this system as well. Our findings indicate that these tools will increase the flexibility of enChIP analysis.

In vitro Engineered DNA-binding Molecule-mediated Chromatin Immunoprecipitation (in vitro enChIP) Using CRISPR Ribonucleoproteins in Combination with Next-generation Sequencing (in vitro enChIP-Seq) for the Identification of Chromosomal Interactions

We have developed locus-specific chromatin immunoprecipitation (locus-specific ChIP) technologies consisting of insertional ChIP (iChIP) and engineered DNA-binding molecule-mediated ChIP (enChIP). Locus-specific ChIP is a method to isolate a genomic region of interest from cells while it also identifies what binds to this region using mass spectrometry (for protein) or next generation sequencing (for RNA or DNA) as described in Fujita et al. (2016a). Recently, we identified genomic regions that physically interact with a locus using an updated form of enChIP, in vitro enChIP, in combination with NGS (in vitro enChIP-Seq) ( Fujita et al., 2017a ). Here, we describe a protocol on in vitro enChIP to isolate a target locus for identification of genomic regions that physically interact with the locus.

Biochemical Analysis of Genome Functions Using Locus-Specific Chromatin Immunoprecipitation Technologies

To isolate specific genomic regions that retain their molecular interactions, allowing direct identification of chromatin-bound molecules, we developed two locus-specific chromatin immunoprecipitation (locus-specific ChIP) technologies, insertional ChIP (iChIP) and engineered DNA-binding molecule-mediated ChIP (enChIP) using the clustered regularly interspaced short palindromic repeats (CRISPR) system or transcription activator-like (TAL) proteins. Essentially, a locus-specific ChIP consists of locus-tagging and affinity purification and can be combined with downstream analyses to identify molecules associated with the target genomic regions. In this review, we discuss the applications of locus-specific ChIP to analyze the genome functions, including transcription and epigenetic regulation.

Isolation of Specific Genomic Regions and Identification of Their Associated Molecules by Engineered DNA-Binding Molecule-Mediated Chromatin Immunoprecipitation (enChIP) Using the CRISPR System and TAL Proteins

Comprehensive understanding of genome functions requires identification of molecules (proteins, RNAs, genomic regions, etc.) bound to specific genomic regions of interest in vivo. To perform biochemical and molecular biological analysis of specific genomic regions, we developed engineered DNA-binding molecule-mediated chromatin immunoprecipitation (enChIP) to purify genomic regions of interest. In enChIP, specific genomic regions are tagged for biochemical purification using engineered DNA-binding molecules, such as transcription activator-like (TAL) proteins and a catalytically inactive form of the clustered regularly interspaced short palindromic repeats (CRISPR) system. enChIP is a comprehensive approach that emphasizes non-biased search using next-generation sequencing (NGS), microarrays, mass spectrometry (MS), and other methods. Moreover, this approach is not restricted to cultured cell lines and can be easily extended to organisms. In this review, we discuss applications of enChIP to elucidating the molecular mechanisms underlying genome functions.