Degradation, Promoter Recruitment and Transactivation Mediated by the Extreme N-Terminus of MHC Class II Transactivator CIITA Isoform III.

Multiple relationships between ubiquitin-proteasome mediated protein turnover and transcriptional activation have been well documented, but the underlying mechanisms are still poorly understood. One way to induce degradation is via ubiquitination of the N-terminal α-amino group of proteins. The major histocompatibility complex (MHC) class II transactivator CIITA is the master regulator of MHC class II gene expression and we found earlier that CIITA is a short-lived protein. Using stable and transient transfections of different CIITA constructs into HEK-293 and HeLa cell lines, we show here that the extreme N-terminal end of CIITA isoform III induces both rapid degradation and transactivation. It is essential that this sequence resides at the N-terminal end of the protein since blocking of the N-terminal end with an epitope-tag stabilizes the protein and reduces transactivation potential. The first ten amino acids of CIITA isoform III act as a portable degron and transactivation sequence when transferred as N-terminal extension to truncated CIITA constructs and are also able to destabilize a heterologous protein. The same is observed with the N-terminal ends of several known N-terminal ubiquitination substrates, such as Id2, Cdt1 and MyoD. Arginine and proline residues within the N-terminal ends contribute to rapid turnover. The N-terminal end of CIITA isoform III is responsible for efficient in vivo recruitment to the HLA-DRA promoter and increased interaction with components of the transcription machinery, such as TBP, p300, p400/Domino, the 19S ATPase S8, and the MHC-II promoter binding complex RFX. These experiments reveal a novel function of free N-terminal ends of proteins in degradation-dependent transcriptional activation.

Pharmacological inhibition of DNA-PK stimulates Cas9-mediated genome editing.

BACKGROUND: The ability to modify the genome of any cell at a precise location has drastically improved with the recent discovery and implementation of CRISPR/Cas9 editing technology. However, the capacity to introduce specific directed changes at given loci is hampered by the fact that the major cellular repair pathway that occurs following Cas9-mediated DNA cleavage is the erroneous non-homologous end joining (NHEJ) pathway. Homology-directed recombination (HDR) is far less efficient than NHEJ and makes screening of clones containing directed changes time-consuming and labor-intensive. METHODS: We investigated the possibility of pharmacologically inhibiting DNA-PKcs, a key player in NHEJ, using small molecule inhibitors (NU7441 and KU-0060648), to ameliorate the rates of HDR repair events. These compounds were tested in a sensitive reporter assay capable of simultaneously informing on NHEJ and HDR, as well as on an endogenous gene targeted by Cas9. RESULTS: We find that NU7441 and KU-0060648 reduce the frequency of NHEJ while increasing the rate of HDR following Cas9-mediated DNA cleavage. CONCLUSIONS: Our results identify two small molecules compatible for use with Cas9-editing technology to improve the frequency of HDR.

Protospacer adjacent motif (PAM)-distal sequences engage CRISPR Cas9 DNA target cleavage.

The clustered regularly interspaced short palindromic repeat (CRISPR)-associated enzyme Cas9 is an RNA-guided nuclease that has been widely adapted for genome editing in eukaryotic cells. However, the in vivo target specificity of Cas9 is poorly understood and most studies rely on in silico predictions to define the potential off-target editing spectrum. Using chromatin immunoprecipitation followed by sequencing (ChIP-seq), we delineate the genome-wide binding panorama of catalytically inactive Cas9 directed by two different single guide (sg) RNAs targeting the Trp53 locus. Cas9:sgRNA complexes are able to load onto multiple sites with short seed regions adjacent to (5')NGG(3') protospacer adjacent motifs (PAM). Yet among 43 ChIP-seq sites harboring seed regions analyzed for mutational status, we find editing only at the intended on-target locus and one off-target site. In vitro analysis of target site recognition revealed that interactions between the 5' end of the guide and PAM-distal target sequences are necessary to efficiently engage Cas9 nucleolytic activity, providing an explanation for why off-target editing is significantly lower than expected from ChIP-seq data.

A Torrent of data: mapping chromatin organization using 5C and high-throughput sequencing.

The study of three-dimensional genome organization is an exciting research area, which has benefited from the rapid development of high-resolution molecular mapping techniques over the past decade. These methods are derived from the chromosome conformation capture (3C) technique and are each aimed at improving some aspect of 3C. All 3C technologies use formaldehyde fixation and proximity-based ligation to capture chromatin contacts in cell populations and consider in vivo spatial proximity more or less inversely proportional to the frequency of measured interactions. The 3C-carbon copy (5C) method is among the most quantitative of these approaches. 5C is extremely robust and can be used to study chromatin organization at various scales. Here, we present a modified 5C analysis protocol adapted for sequencing with an Ion Torrent Personal Genome Machine™ (PGM™). We explain how Torrent 5C libraries are produced and sequenced. We also describe the statistical and computational methods we developed to normalize and analyze raw Torrent 5C sequence data. The Torrent 5C protocol should facilitate the study of in vivo chromatin architecture at high resolution because it benefits from high accuracy, greater speed, low running costs, and the flexibility of in-house next-generation sequencing.

Discovering genome regulation with 3C and 3C-related technologies.

It has been known for some time that eukaryotic genomic DNA is packaged in the form of highly organized chromatin in vivo. This organization is important not only to reduce the length of chromosomes during interphase but also because it represents a type of higher-order genome regulation mechanism. Indeed, spatial chromatin architecture is known to be important for transcription, DNA replication and repair. Chromosome structure can be observed at different scales and studied with a variety of complementary techniques. For example, microscopy can provide single cell information while technologies such as the chromosome conformation capture (3C) method and its derivatives can yield higher-resolution data from cell populations. In this review, we report on the biological questions addressed with 3C and 3C-related techniques and what has been uncovered to date. We also explore what these methods may further reveal about the regulation of genomic DNA activities.