Editing DNA (section)

=== DNA-binding proteins ===
{{further|DNA-binding protein}}
[[File:Nucleosome1.png|thumb|260px|left|Interaction of DNA (in orange) with [[histone]]s (in blue). These proteins' basic amino acids bind to the acidic phosphate groups on DNA.]]

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 [[histone]]s, while in prokaryotes multiple types of proteins are involved.<ref>{{cite journal | vauthors = Sandman K, Pereira SL, Reeve JN | s2cid = 21101836 | title = Diversity of prokaryotic chromosomal proteins and the origin of the nucleosome | journal = Cellular and Molecular Life Sciences | volume = 54 | issue = 12 | pages = 1350–64 | date = December 1998 | pmid = 9893710 | doi = 10.1007/s000180050259 | pmc = 11147202 }}</ref><ref>{{cite journal | vauthors = Dame RT | title = The role of nucleoid-associated proteins in the organization and compaction of bacterial chromatin | journal = Molecular Microbiology | volume = 56 | issue = 4 | pages = 858–70 | date = May 2005 | pmid = 15853876 | doi = 10.1111/j.1365-2958.2005.04598.x | s2cid = 26965112 | doi-access = free }}</ref> 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 bond]]s to the acidic sugar-phosphate backbone of the DNA, and are thus largely independent of the base sequence.<ref>{{cite journal | vauthors = Luger K, Mäder AW, Richmond RK, Sargent DF, Richmond TJ | title = Crystal structure of the nucleosome core particle at 2.8 A resolution | journal = Nature | volume = 389 | issue = 6648 | pages = 251–60 | date = September 1997 | pmid = 9305837 | doi = 10.1038/38444 | bibcode = 1997Natur.389..251L | s2cid = 4328827 }}</ref> Chemical modifications of these basic amino acid residues include [[methylation]], [[phosphorylation]], and [[acetylation]].<ref>{{cite journal | vauthors = Jenuwein T, Allis CD | title = Translating the histone code | journal = Science | volume = 293 | issue = 5532 | pages = 1074–80 | date = August 2001 | pmid = 11498575 | doi = 10.1126/science.1063127 | s2cid = 1883924 | url = http://www.gs.washington.edu/academics/courses/braun/55104/readings/jenuwein.pdf | url-status=live | archive-url = https://web.archive.org/web/20170808142426/http://www.gs.washington.edu/academics/courses/braun/55104/readings/jenuwein.pdf | archive-date = 8 August 2017 | df = dmy-all }}</ref> These chemical changes alter the strength of the interaction between the DNA and the histones, making the DNA more or less accessible to [[transcription factor]]s and changing the rate of transcription.<ref>{{cite book | vauthors = Ito T | title = Protein Complexes that Modify Chromatin | chapter = Nucleosome Assembly and Remodeling | series = Current Topics in Microbiology and Immunology | volume = 274 | pages = 1–22 | year = 2003 | pmid = 12596902 | doi = 10.1007/978-3-642-55747-7_1 | isbn = 978-3-540-44208-0 }}</ref> Other non-specific DNA-binding proteins in chromatin include the high-mobility group proteins, which bind to bent or distorted DNA.<ref>{{cite journal | vauthors = Thomas JO | title = HMG1 and 2: architectural DNA-binding proteins | journal = Biochemical Society Transactions | volume = 29 | issue = Pt 4 | pages = 395–401 | date = August 2001 | pmid = 11497996 | doi = 10.1042/BST0290395 }}</ref> These proteins are important in bending arrays of nucleosomes and arranging them into the larger structures that make up chromosomes.<ref>{{cite journal | vauthors = Grosschedl R, Giese K, Pagel J | title = HMG domain proteins: architectural elements in the assembly of nucleoprotein structures | journal = Trends in Genetics | volume = 10 | issue = 3 | pages = 94–100 | date = March 1994 | pmid = 8178371 | doi = 10.1016/0168-9525(94)90232-1 }}</ref>

A distinct group of DNA-binding proteins is 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.<ref>{{cite journal | vauthors = Iftode C, Daniely Y, Borowiec JA | title = Replication protein A (RPA): the eukaryotic SSB | journal = Critical Reviews in Biochemistry and Molecular Biology | volume = 34 | issue = 3 | pages = 141–80 | year = 1999 | pmid = 10473346 | doi = 10.1080/10409239991209255 }}</ref> These binding proteins seem to stabilize single-stranded DNA and protect it from forming [[stem-loop]]s or being degraded by [[nuclease]]s.

[[File:Lambda repressor 1LMB.png|thumb|upright=1.1|The lambda repressor [[helix-turn-helix]] transcription factor bound to its DNA target<ref>{{Cite web| vauthors = Beamer LJ, Pabo CO |title=RCSB PDB – 1LMB: Refined 1.8 Å crystal structure of the lambda repressor-operator complex |url=https://www.rcsb.org/structure/1LMB|access-date=2023-03-27|website=www.rcsb.org|language=en-US}}</ref>]]
In contrast, other proteins have evolved to bind to particular DNA sequences. The most intensively studied of these are the various [[transcription factor]]s, which are proteins that regulate transcription. Each transcription factor binds to one particular set of DNA sequences and activates or inhibits the transcription of genes that have these sequences close to 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.<ref>{{cite journal | vauthors = Myers LC, Kornberg RD | title = Mediator of transcriptional regulation | journal = Annual Review of Biochemistry | volume = 69 | pages = 729–49 | year = 2000 | pmid = 10966474 | doi = 10.1146/annurev.biochem.69.1.729 }}</ref> Alternatively, transcription factors can bind [[enzyme]]s that modify the histones at the promoter. This changes the accessibility of the DNA template to the polymerase.<ref>{{cite journal | vauthors = Spiegelman BM, Heinrich R | title = Biological control through regulated transcriptional coactivators | journal = Cell | volume = 119 | issue = 2 | pages = 157–67 | date = October 2004 | pmid = 15479634 | doi = 10.1016/j.cell.2004.09.037 | doi-access = free }}</ref>

As these DNA targets can occur throughout an organism's genome, changes in the activity of one type of transcription factor can affect thousands of genes.<ref>{{cite journal | vauthors = Li Z, Van Calcar S, Qu C, Cavenee WK, Zhang MQ, Ren B | title = A global transcriptional regulatory role for c-Myc in Burkitt's lymphoma cells | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 14 | pages = 8164–69 | date = July 2003 | pmid = 12808131 | pmc = 166200 | doi = 10.1073/pnas.1332764100 | bibcode = 2003PNAS..100.8164L | doi-access = free }}</ref> Consequently, 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.<ref name="Pabo1984" />