Targeted DNA Affinity Purification for Quantifying DNA-Binding Specificities of Transcription Factors.

Plant growth relies on flexible gene regulation to adapt to environmental changes. This process is ultimately controlled by transcription factors (TFs), which bind to specific DNA motifs, known as TF-binding sites (TFBS), located in the gene regulatory regions to regulate their expression. These interactions play crucial roles in plant development and responses to environmental cues, as well as in plant evolution and domestication, making both cis- (i.e., TFBS) and trans-regulatory factors as potential molecular targets in plant breeding for traits such as yield, quality, and stress resilience. These biotechnological approaches require precise knowledge of the target gene sets and TFBS specifically recognized by TFs. Recent advances in high-throughput sequencing techniques have enabled precise identification of TF target genes, especially thanks to methodologies that combine the main features of both in vitro and in vivo approaches. However, small scale and targeted approaches are still needed to evaluate the relative contribution of specific nucleotide positions in TF recognition. In this chapter, we describe a modified version of DNA Affinity Purification sequencing (DAP-seq) that replaces genomic DNA with a PCR-generated library of TFBS variants. This approach, termed targeted-DAP, allows a flexible and quantitative analysis of TF-binding using next-generation sequencing. Additionally, expressing TFs in Escherichia coli provides an economical source of proteins, enabling scalable and cost-effective analysis of DNA-binding specificities. We showed the benefits of this technique to demonstrate the contribution of the genomic context around the TFBS for specific recognition of a bHLH TF. Development of targeted DAP-seq would be of interest for the evaluation of nucleotide variation-either allelic or generated by CRISPR/Cas-within TFBS in TF recognition with predictable consequences on plant phenotypes.