Structural proteins that bind DNA are well-understood examples of non-specific DNA-protein interactions. Within chromosomes, DNA is held in complexes with structural proteins. These proteins organize the DNA into a compact structure called chromatin. In eukaryotes, this structure involves DNA binding to a complex of small basic proteins called histones. In prokaryotes, multiple types of proteins are involved. The histones form a disk-shaped complex called a nucleosome, which contains two complete turns of double-stranded DNA wrapped around its surface. These non-specific interactions are formed through basic residues in the histones making ionic bonds to the acidic sugar-phosphate backbone of the DNA, and are therefore largely independent of the base sequence. Chemical modifications of these basic amino acid residues include methylation, phosphorylation and acetylation. These chemical changes alter the strength of the interaction between the DNA and the histones, making the DNA more or less accessible to transcription factors and changing the rate of transcription. Other non-specific DNA-binding proteins in chromatin include the high-mobility group proteins, which bind to bent or distorted DNA. Biophysical studies show that these architectural HMG proteins bind, bend and loop DNA to perform its biological functions. These proteins are important in bending arrays of nucleosomes and arranging them into the larger structures that form chromosomes.
Proteins that specifically bind single-stranded DNA
A distinct group of DNA-binding proteins are the DNA-binding proteins that specifically bind single-stranded DNA. In humans, replication protein A is the best-understood member of this family and is used in processes where the double helix is separated, including DNA replication, recombination and DNA repair. These binding proteins seem to stabilize single-stranded DNA and protect it from forming stem-loops or being degraded by nucleases.
Binding to specific DNA sequences
In contrast, other proteins have evolved to bind to specific DNA sequences. The most intensively studied of these are the various transcription factors, which are proteins that regulate transcription. Each transcription factor binds to one specific set of DNA sequences and activates or inhibits the transcription of genes that have these sequences near their promoters. The transcription factors do this in two ways. Firstly, they can bind the RNA polymerase responsible for transcription, either directly or through other mediator proteins; this locates the polymerase at the promoter and allows it to begin transcription. Alternatively, transcription factors can bind enzymes that modify the histones at the promoter. This alters the accessibility of the DNA template to the polymerase. These DNA targets can occur throughout an organism's genome. Thus, changes in the activity of one type of transcription factor can affect thousands of genes. Thus, these proteins are often the targets of the signal transduction processes that control responses to environmental changes or cellular differentiation and development. The specificity of these transcription factors' interactions with DNA come from the proteins making multiple contacts to the edges of the DNA bases, allowing them to read the DNA sequence. Most of these base-interactions are made in the major groove, where the bases are most accessible. Mathematical descriptions of protein-DNA binding taking into account sequence-specificity, and competitive and cooperative binding of proteins of different types are usually performed with the help of the lattice models. Computational methods to identify the DNA binding sequence specificity have been proposed to make a good use of the abundant sequence data in the post-genomic era.
Protein–DNA interactions
Protein–DNA interactions occur when a protein binds a molecule of DNA, often to regulate the biological function of DNA, usually the expression of a gene. Among the proteins that bind to DNA are transcription factors that activate or repress gene expression by binding to DNA motifs and histones that form part of the structure of DNA and bind to it less specifically. Also proteins that repair DNA such as uracil-DNA glycosylase interact closely with it. In general, proteins bind to DNA in the major groove; however, there are exceptions. Protein–DNA interaction are of mainly two types, either specific interaction, or non-specific interaction. Recent single-molecule experiments showed that DNA binding proteins undergo of rapid rebinding in order to bind in correct orientation for recognizing the target site.
There are many in vitro and in vivo techniques which are useful in detecting DNA-Protein Interactions. The following lists some methods currently in use: Electrophoretic mobility shift assay is a widespread technique to identify protein–DNA interactions. DNase footprinting assay can be used to identify the specific site of binding of a protein to DNA. Chromatin immunoprecipitation is used to identify the sequence of the DNA fragments which bind to a known transcription factor. This technique when combined with high throughput sequencing is known as ChIP-Seq and when combined with microarrays it is known as ChIP-chip. Yeast one-hybrid System is used to identify which protein binds to a particular DNA fragment. Bacterial one-hybrid system is used to identify which protein binds to a particular DNA fragment. Structure determination using X-ray crystallography has been used to give a highly detailed atomic view of protein–DNA interactions.
Manipulating the interactions
The protein–DNA interactions can be modulated using stimuli like ionic strength of the buffer, macromolecular crowding, temperature, pH and electric field. This can lead to reversible dissociation/association of the protein–DNA complex.
