Question: What is the principle of the PAR-CLIP methodology?

Context:
We developed a cell-based crosslinking approach to determine at high resolution and transcriptome-wide the binding sites of cellular RBPs and miRNPs.
To gain insight into the complexity of snoRNA processing and the functional relevance of snoRNA-derived small RNAs, we sequence long and short RNAs, small RNAs that co-precipitate with the Argonaute 2 protein and RNA fragments obtained in photoreactive nucleotide-enhanced crosslinking and immunoprecipitation (PAR-CLIP) of core snoRNA-associated proteins.
A large amount of miRNA-target interactions (MTIs) have been identified by the crosslinking and immunoprecipitation (CLIP) and the photoactivatable-ribonucleoside-enhanced CLIP (PAR-CLIP) along with the next-generation sequencing (NGS)
PAR-CLIP shows high efficiency of RNA co-immunoprecipitation, but it also lead to T to C conversion in miRNA-RNA-protein crosslinking regions
A recent method, PAR-CLIP, uses photoreactive nucleosides to crosslink RBPs to target RNAs in cells prior to immunoprecipitation.
In this article, we review crosslinking and immunoprecipitation (CLIP) methods adapted for large-scale identification of target RNA-binding sites and the respective RNA recognition elements.
CLIP methods have the potential to detect hundreds of thousands of binding sites in single experiments although the separation of signal from noise can be challenging.
We focus on photoactivatable ribonucleoside-enhanced CLIP, which relies on the intracellular incorporation of photoactivatable ribonucleoside analogs into nascent transcripts, and yields characteristic sequence changes upon crosslinking that facilitate the separation of signal from noise.
Photo-Activatable Ribonucleoside-enhanced CrossLinking and ImmunoPrecipitation (PAR-CLIP) method was recently developed for global identification of RNAs interacting with proteins.
The strength of this versatile method results from induction of specific T to C transitions at sites of interaction.
PAR-CLIP reveals a collection of RNAs bound to a protein whereas SILAC-based RNA pull-downs identify a group of proteins bound to an RNA. 
Here we present a step-by-step protocol and guidelines for the computational analysis for the large-scale identification of miRNA target sites in cultured cells by photoactivatable ribonucleoside enhanced crosslinking and immunoprecipitation (PAR-CLIP) of AGO proteins.
In particular, PAR-CLIP utilizes a photoactivatable nucleoside for more efficient crosslinking.
rosslinking and immunoprecipitation (CLIP) protocols have made it possible to identify transcriptome-wide RNA-protein interaction sites
In this issue of Molecular Cell, Mukherjee et al. (2011) and Lebedeva et al. (2011) identify transcriptome-wide HuR-RNA interactions using PAR-CLIP, unveiling HuR's nuclear role in pre-mRNA processing.
ross-linking and immunoprecipitation (CLIP) is increasingly used to map transcriptome-wide binding sites of RNA-binding proteins.
We developed a method for CLIP data analysis, and applied it to compare CLIP with photoactivatable ribonucleoside-enhanced CLIP (PAR-CLIP) and to uncover how differences in cross-linking and ribonuclease digestion affect the identified sites.
In order to increase the specificity and positional resolution, a strategy referred to as CLIP (UV cross-linking and immunoprecipitation) was introduced.
Recently, PAR-CLIP was introduced that uses photoreactive ribonucleoside analogs for cross-linking
AR-CliP--a method to identify transcriptome-wide the binding sites of RNA binding proteins
We developed a powerful cell-based crosslinking approach to determine at high resolution and transcriptome-wide the binding sites of cellular RBPs and miRNPs that we term PAR-CliP (Photoactivatable-Ribonucleoside-Enhanced Crosslinking and Immunoprecipitation)
The method relies on the incorporation of photoreactive ribonucleoside analogs, such as 4-thiouridine (4-SU) and 6-thioguanosine (6-SG) into nascent RNA transcripts by living cells. 
One characteristic feature of cDNA libraries prepared by PAR-CliP is that the precise position of crosslinking can be identified by mutations residing in the sequenced cDNA. 

Answer: In particular, PAR-CLIP utilizes a photoactivatable nucleoside for more efficient crosslinking.  A recent method, PAR-CLIP, uses photoreactive nucleosides to crosslink RBPs to target RNAs in cells prior to immunoprecipitation.  One characteristic feature of cDNA libraries prepared by PAR-CliP is that the precise position of crosslinking can be identified by mutations residing in the sequenced cDNA.   
A powerful cell-based crosslinking approach to determine at high resolution and transcriptome-wide the binding sites of cellular RBPs and miRNPs was termed PAR-CliP (Photoactivatable-Ribonucleoside-Enhanced Crosslinking and Immunoprecipitation). PAR-CLIP shows high efficiency of RNA co-immunoprecipitation, but it also lead to T to C conversion in miRNA-RNA-protein crosslinking regions. It relies on the intracellular incorporation of photoactivatable ribonucleoside analogs into nascent transcripts, and yields characteristic sequence changes upon crosslinking that facilitate the separation of signal from noise.
AR-CliP--a method to identify transcriptome-wide the binding sites of RNA binding proteinsOne characteristic feature of cDNA libraries prepared by PAR-CliP is that the precise position of crosslinking can be identified by mutations residing in the sequenced cDNA. 
In particular, PAR-CLIP utilizes a photoactivatable nucleoside for more efficient crosslinking.  A recent method, PAR-CLIP, uses photoreactive nucleosides to crosslink RBPs to target RNAs in cells prior to immunoprecipitation.  
In particular, PAR-CLIP utilizes a photoactivatable nucleoside for more efficient crosslinking.  A recent method, PAR-CLIP, uses photoreactive nucleosides to crosslink RBPs to target RNAs in cells prior to immunoprecipitation.  One characteristic feature of cDNA libraries prepared by PAR-CliP is that the precise position of crosslinking can be identified by mutations residing in the sequenced cDNA.   Photo-Activatable Ribonucleoside-enhanced CrossLinking and ImmunoPrecipitation (PAR-CLIP) method was recently developed for global identification of RNAs interacting with proteins.  PAR-CLIP shows high efficiency of RNA co-immunoprecipitation, but it also lead to T to C conversion in miRNA-RNA-protein crosslinking regions