ChIP-seq guidelines and practices of the ENCODE and modENCODE consortia

Chromatin immunoprecipitation (ChIP) followed by high-throughput DNA sequencing (ChIP-seq) has become a valuable and widely used approach for mapping the genomic location of transcription-factor binding and histone modifications in living cells. Despite its widespread use, there are considerable differences in how these experiments are conducted, how the results are scored and evaluated for quality, and how the data and metadata are archived for public use. These practices affect the quality and utility of any global ChIP experiment. Through our experience in performing ChIP-seq experiments, the ENCODE and modENCODE consortia have developed a set of working standards and guidelines for ChIP experiments that are updated routinely. The current guidelines address antibody validation, experimental replication, sequencing depth, data and metadata reporting, and data quality assessment. We discuss how ChIP quality, assessed in these ways, affects different uses of ChIP-seq data. All data sets used in the analysis have been deposited for public viewing and downloading at the ENCODE (http://encodeproject.org/ENCODE/) and modENCODE (http://www.modencode.org/) portals.

A cell-based method for screening RNA-protein interactions: identification of constitutive transport element-interacting proteins

We have developed a mammalian cell-based screening platform to identify proteins that assemble into RNA-protein complexes. Based on Tat-mediated activation of the HIV LTR, proteins that interact with an RNA target elicit expression of a GFP reporter and are captured by fluorescence activated cell sorting. This "Tat-hybrid" screening platform was used to identify proteins that interact with the Mason Pfizer monkey virus (MPMV) constitutive transport element (CTE), a structured RNA hairpin that mediates the transport of unspliced viral mRNAs from the nucleus to the cytoplasm. Several hnRNP-like proteins, including hnRNP A1, were identified and shown to interact with the CTE with selectivity in the reporter system comparable to Tap, a known CTE-binding protein. In vitro gel shift and pull-down assays showed that hnRNP A1 is able to form a complex with the CTE and Tap and that the RGG domain of hnRNP A1 mediates binding to Tap. These results suggest that hnRNP-like proteins may be part of larger export-competent RNA-protein complexes and that the RGG domains of these proteins play an important role in directing these binding events. The results also demonstrate the utility of the screening platform for identifying and characterizing new components of RNA-protein complexes.

An encyclopedia of mouse DNA elements (Mouse ENCODE)

To complement the human Encyclopedia of DNA Elements (ENCODE) project and to enable a broad range of mouse genomics efforts, the Mouse ENCODE Consortium is applying the same experimental pipelines developed for human ENCODE to annotate the mouse genome.

Characterization of enhancer function from genome-wide analyses

There has been a recent surge in the use of genome-wide methodologies to identify and annotate the transcriptional regulatory elements in the human genome. Here we review some of these methodologies and the conceptual insights about transcription regulation that have been gained from the use of genome-wide studies. It has become clear that the binding of transcription factors is itself a highly regulated process, and binding does not always appear to have functional consequences. Numerous properties have now been associated with regulatory elements that may be useful in their identification. Several aspects of enhancer function have been shown to be more widespread than was previously appreciated, including the highly combinatorial nature of transcription factor binding, the postinitiation regulation of many target genes, and the binding of enhancers at early stages to maintain their competence during development. Going forward, the integration of multiple genome-wide data sets should become a standard approach to elucidate higher-order regulatory interactions.

A strategy for identifying noncoding RNAs using whole-genome tiling arrays

Whole-genome tiling arrays are powerful tools for detecting and characterizing novel RNA transcripts. Here, we describe a complete method combining elements of molecular and computational biology to identify small noncoding RNA (sRNA) transcripts. We focus on the key features of this approach, which include size-fractionation of input RNA, direct detection of array hybridization with antibodies that recognize RNA:DNA hybrids, and correlation-based computational methods for automated sRNA identification and boundary determination.

Small non-coding RNAs in Caulobacter crescentus

Small non-coding RNAs (sRNAs) are active in many bacterial cell functions, including regulation of the cell's response to environmental challenges. We describe the identification of 27 novel Caulobacter crescentus sRNAs by analysis of RNA expression levels assayed using a tiled Caulobacter microarray and a protocol optimized for detection of sRNAs. The principal analysis method involved identification of sets of adjacent probes with unusually high correlation between the individual intergenic probes within the set, suggesting presence of a sRNA. Among the validated sRNAs, two are candidate transposase gene antisense RNAs. The expression of 10 of the sRNAs is regulated by either entry into stationary phase, carbon starvation, or rich versus minimal media. The expression of four of the novel sRNAs changes as the cell cycle progresses. One of these shares a promoter motif with several genes expressed at the swarmer-to-stalked cell transition; while another appears to be controlled by the CtrA global transcriptional regulator. The probe correlation analysis approach reported here is of general use for large-scale sRNA identification for any sequenced microbial genome.

A simple motif for protein recognition in DNA secondary structures

DNA in a single-stranded form (ssDNA) exists transiently within the cell and comprises the telomeres of linear chromosomes and the genomes of some DNA viruses. As with RNA, in the single-stranded state, some DNA sequences are able to fold into complex secondary and tertiary structures that may be recognized by proteins and participate in gene regulation. To better understand how such DNA elements might fold and interact with proteins, and to compare recognition features to those of a structured RNA, we used in vitro selection to identify ssDNAs that bind an RNA-binding peptide from the HIV Rev protein with high affinity and specificity. The large majority of selected binders contain a non-Watson-Crick G.T base-pair and an adjacent C:G base-pair and both are essential for binding. This GT motif can be presented in different DNA contexts, including a nearly perfect duplex and a branched three-helix structure, and appears to be recognized in large part by arginine residues separated by one turn of an alpha-helix. Interestingly, a very similar GT motif is necessary also for protein binding and function of a well-characterized model ssDNA regulatory element from the proenkephalin promoter.

Localized Influence of 2‘-Hydroxyl Groups and Helix Geometry on Protein Recognition in the RNA Major Groove

The local geometry of a DNA helix can influence protein recognition, but the sequence-specific features that contribute to helix structure are not fully understood, and even less is known about how RNA helix geometry may affect protein recognition. To begin to understand how local or global helix structure may influence binding in an RNA model system, we generated a series of DNA analogues of HIV and BIV TAR RNAs in which ribose sugars were systematically substituted in and around the known binding sites for argininamide and a BIV Tat arginine-rich peptide, respectively, and measured their corresponding binding affinities. For each TAR interaction, binding occurs in the RNA major groove with high specificity, whereas binding to the all-DNA analogue is weak and nonspecific. Relatively few substitutions are needed to convert either DNA analogue of TAR into a high-affinity binder, with the ribose requirements being restricted largely to regions that directly contact the ligand. Substitutions at individual positions show up to 70-fold differences in binding affinity, even at adjacent base pairs, while two base pairs at the core of the BIV Tat peptide-RNA interface are largely unaffected by deoxyribose substitution. These results suggest that the helix geometries and unique conformational features required for binding are established locally and are relatively insulated from effects more than one base pair away. It seems plausible that arginine-rich peptides are able to adapt to a mosaic helical architecture in which segments as small as single base steps may be considered as modular recognition units.