is greatly extended compared with its usual 7,8 Molecular biology form . When the complementary DNA strand pairs with the RecA-coated presynaptic single-stranded DNA, it also adopts this Demystifying the D-loop extended conformation and is stabilized mainly by base-pairing interactions with in DNA recombination the pre synaptic DNA. This provides a way to identify a sequence match that has the correct 7 Upasana Roy & Eric. C. Greene nucleo tide-pairing interactions . However, many aspects of this process remained Homologous recombination is a mechanism for DNA repair unclear. In particular, there was a lack of that enables the exchange of genetic information between structural information on how RecA-bound DNA molecules. Structural analysis reveals how the protein DNA first interacts with a double-stranded DNA molecule and then opens the strands up RecA orchestrates this process. See p.801 to form a D-loop. Yang et al. used using high-resolution cryo-electron microscopy to reveal the struc- Our chromosomes are subjected to a constant quickly because many double-stranded DNA ture of D-loop intermediates. Structural studies barrage of DNA-damaging agents that can lead sequences might need to be sampled until the of RecA have proved notoriously difficult, to the loss of genome integrity1,2. Homologous correct match is found. because RecA can form filaments of different recombination is an evolutionarily conserved The filaments formed by RecA enzymes lengths bound to DNA. To overcome this prob- type of DNA repair that can fix a particularly stretch presynaptic single-stranded DNA lem, the authors fused nine RecA proteins dangerous form of damage in which both by approximately 50% relative to its normal together and mutated the amino-acid residues strands of a DNA molecule break3,4. The crucial length7. The DNA is not stretched uniformly, at the ends of this ‘mini-filament’ to prevent any need for homologous recombination is however. The DNA bound within a so-called further addition of RecA molecules. The use of reflected in the cancers and cancer-prone primary DNA-binding site of RecA is organized these short, well-defined RecA mini-filaments syndromes that are associated with defects into triplets of nucleotide bases (one triplet enabled the authors to uncover some key in proteins, such as BRCA1 and BRCA2, that per RecA molecule) that retain the dimen- missing information about the process of function in recombination5. On page 801, Yang sions of normal double-stranded DNA. But recombination. Their findings illuminate the et al.6 report structural data that shed light on the DNA backbone (the chain of sugar and choreography of protein–DNA interactions how the protein RecA facilitates a key early phosphate groups) between these triplets that occur as RecA mediates D-loop formation. step in the process of homologous recombi- nation in bacteria. During homologous recombination a Generation of Strand exchange Newly single-stranded and D-loop synthesized DNA (Fig. 1a), DNA at the site of a double-strand Homologous DNA DNA overhang formation break undergoes an enzymatic processing 5’ 3’ step that generates single-stranded over- 3’ 5’ hangs called presynaptic single-stranded 5’ 3’ DNA. This presynaptic DNA then ‘invades’ 3’ 5’ DNA break an identical section of a double-stranded Presynaptic Displaced single-stranded DNA strand DNA molecule (termed homologous DNA) and pairs with the matching complemen- b 3’ 5’ tary strand, which results in displacement of the non-complementary strand of the CTD double-stranded DNA3,4. This pairing event is called strand exchange, and it generates a DNA 3’ 5’ configuration referred to as a displacement loop (D-loop), which is an essential intermedi- RecA ate structure needed for the events that lead 3,4 to DNA repair . Yang and colleagues provide Primary Secondary detailed data for D-loop structures that yield DNA-binding site DNA-binding site new insights into the mechanism of homolo- gous recombination. Figure 1 | The structure of a DNA D-loop. a, Broken DNA is repaired using an identical (homologous) The complex DNA transactions that take double-stranded DNA molecule as a template. The broken DNA is processed to generate single-stranded place during recombination are mediated by overhangs termed presynaptic DNA. An overhang invades homologous DNA to pair with its complementary members of the RecA family of recombinase strand (light blue), thereby displacing the non-complementary strand (dark blue). This strand-exchange event generates a displacement loop (D-loop) structure. DNA synthesis then occurs, leading to DNA repair. enzymes. During this process, RecA pro- b, Yang et al.6 present structural data that illuminates early events during D-loop formation. RecA proteins teins form long filaments on the presynaptic bind to presynaptic DNA to form a filament of DNA and RecA. Double-stranded DNA initially binds to RecA’s single-stranded DNA. This RecA-bound DNA carboxy-terminal domain (CTD). It is then guided further inwards towards the centre of the filament, and the then invades a double-stranded DNA molecule two strands of its DNA are separated. The complementary strand pairs with the presynaptic DNA in RecA’s and separates the two strands. Whether primary DNA-binding site (which holds three paired nucleotides), and the non-complementary strand binds this opened DNA matches the sequence of RecA’s secondary DNA-binding site (which binds to five unpaired nucleotides). When the DNA sequences the RecA-coated DNA is then determined. match in the primary site, strand separation of the double-stranded DNA continues, and D-loop growth This process must happen extremely occurs in a direction corresponding to the 3′-to-5′ orientation of the presynaptic DNA. Nature | Vol 586 | 29 October 2020 | 677 ©2020 Spri nger Nature Li mited. All rights reserved. ©2020 Spri nger Nature Li mited. All rights reserved. News & views The structural data suggest that at the start non-homologous sequences that don’t match Upasana Roy and Eric C. Greene are in the of the process (Fig. 1b), the carboxy-terminal the presynaptic DNA strand be tested and Department of Biochemistry and Molecular domain (CTD) region of RecA binds to double- rejected? Non-homologous sequences vastly Biophysics, Columbia University Irving Medical stranded DNA and guides it towards the pri- outnumber homologous ones, and homol- Center, New York, New York 10032, USA. mary DNA-binding site near the centre of the ogous recombination could not succeed e-mails: [email protected]; RecA-bound DNA filament. A small protein loop if too much time were spent sampling the [email protected] there helps to open up the double-stranded wrong sequences. How much complemen- DNA. Once open, the displaced non-comple- tarity between sequences of nucleotides is mentary strand binds to a region along the necessary for stable pairing interactions to surface of the RecA–DNA filament called the occur, and what happens when sequences secondary DNA-binding site. This site was are not perfectly matched? Understanding already known to exist, but its location on the the exact nature of these interactions could protein surface had not been fully defined. The shed light on what determines whether a non-displaced complementary DNA strand is sequence is sufficiently homologous to positioned in a way that enables it to pair with support RecA-mediated recombination. 1. Hoeijmakers, J. H. J. Nature 411, 366–374 (2001). the presynaptic single-stranded DNA. The version of RecA found in eukaryotes 2. Hoeijmakers, J. H. J. N. Engl. J. Med. 361, 1475–1485 (2009). The continued opening of the double- (organisms that have a nucleus in their cells) 3. Mehta, A. & Haber, J. E. Cold Spring Harb. Perspect. Biol. stranded DNA then proceeds preferentially is known as Rad51. Understanding recombina- 6, a016428 (2014). in a 3′-to-5′ direction with respect to the ori- tion in eukaryotes will require determination 4. Kowalczykowski, S. C. Cold Spring Harb. Perspect. Biol. 7, a016410 (2015). entation of the presynaptic DNA. However, of the structures and mechanisms used by 5. Prakash, R., Zhang, Y., Feng, W. & Jasin, M. Cold Spring if the newly interacting strands don’t match, other accessory proteins that are also involved Harb. Perspect. Biol. 7, a016600 (2015). the probability of further opening of the dou- in recombination, because Rad51 is highly 6. Yang, H., Zhou, C., Dhar, A. & Pavletich, N. P. Nature 586, 9,10 801–806 (2020). ble-stranded DNA at each successive RecA dependent on such protein co-factors . 7. Chen, Z., Yang, H. & Pavletich, N. P. Nature 453, 489–494 protein is substantially reduced. When the The advent of cryo-electron microscopy as a (2008). interacting strands match, the D-loop contin- tool for defining high-resolution structures 8. Xu, J. et al. Nature Struct. Mol. Biol. 24, 40–46 (2017). 9. San Filippo, J., Sung, P. & Klein, H. Annu. Rev. Biochem. 77, ues to be extended. In this case, the structure is of complex recombination intermediates 229–257 (2008). further stabilized by binding of the displaced offers the potential to address these and many 10. Heyer, W.-D. Cold Spring Harb. Perspect. Biol. 7, a016501 strand to the secondary DNA-binding site, as other intriguing puzzles about homologous (2015). well as by base-pairing interactions between recombination. This article was published online on 14 October 2020. the complementary strand and the presynap- tic single-stranded DNA. Catalysis Yang and colleagues also discovered that this process has a surprising feature — there is an asymmetry between the DNA inter actions Titanium atoms pair up in the primary DNA-binding site and the sec- ondary DNA-binding site. Three nucleotides of the stretched single-stranded DNA are in industrial catalyst bound to each RecA protein within the primary site, whereas each RecA protein binds to five Bert M.