T7 DNA helicase

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DNA primase/helicase
OrganismEnterobacteria phage T7

T7 DNA helicase (gp4) is a hexameric motor protein encoded by T7 phages that uses energy from dTTP hydrolysis to process unidirectionally along single stranded DNA, separating (helicase) the two strands as it progresses. It is also a primase, making short stretches of RNA that initiates DNA synthesis.[1] It forms a complex with T7 DNA polymerase. Its homologs are found in mitochrondria (as Twinkle) and chloroplasts.[2][3]

Crystal structure[edit]

The crystal structure was solved to 3.0 Å resolution in 2000,[4] as shown in the figure. In (A), notice that the separate subunits appear to be anchored through interactions between an alpha helix and an adjacent subunit. In (B), there are six sets of three loops. The red loop, known as loop II, contains three lysine residues and is thought to be involved in binding the ssDNA that is fed through the center of the enzyme.

Mechanism of sequential dTTP hydrolysis[edit]

Crampton et al. have proposed a mechanism for the ssDNA-dependent hydrolysis of dTTP by T7 DNA helicase as shown in the figure below.[5] In their model, protein loops located on each hexameric subunit, each of which contain three lysine residues, sequentially interact with the negatively charged phosphate backbone of ssDNA. This interaction presumably causes a conformational change in the actively bound subunit, providing for the efficient release of dTDP from its dTTP binding site. In the process of dTDP release, the ssDNA is transferred to the neighboring subunit, which undergoes a similar process. Previous studies have already suggested that ssDNA is able to bind to two hexameric subunits simultaneously.[6]

See also[edit]


  1. ^ Lee SJ, Richardson CC (October 2011). "Choreography of bacteriophage T7 DNA replication". Current Opinion in Chemical Biology. 15 (5): 580–6. doi:10.1016/j.cbpa.2011.07.024. PMC 3195405. PMID 21907611.
  2. ^ Spelbrink JN, Li FY, Tiranti V, Nikali K, Yuan QP, Tariq M, et al. (July 2001). "Human mitochondrial DNA deletions associated with mutations in the gene encoding Twinkle, a phage T7 gene 4-like protein localized in mitochondria". Nature Genetics. 28 (3): 223–31. doi:10.1038/90058. PMID 11431692.
  3. ^ Diray-Arce, J; Liu, B; Cupp, JD; Hunt, T; Nielsen, BL (4 March 2013). "The Arabidopsis At1g30680 gene encodes a homologue to the phage T7 gp4 protein that has both DNA primase and DNA helicase activities". BMC Plant Biology. 13: 36. doi:10.1186/1471-2229-13-36. PMC 3610141. PMID 23452619.
  4. ^ Singleton MR, Sawaya MR, Ellenberger T, Wigley DB (June 2000). "Crystal structure of T7 gene 4 ring helicase indicates a mechanism for sequential hydrolysis of nucleotides". Cell. 101 (6): 589–600. doi:10.1016/S0092-8674(00)80871-5. PMID 10892646.
  5. ^ Crampton DJ, Mukherjee S, Richardson CC (January 2006). "DNA-induced switch from independent to sequential dTTP hydrolysis in the bacteriophage T7 DNA helicase". Mol. Cell. 21 (2): 165–74. doi:10.1016/j.molcel.2005.11.027. PMID 16427007.
  6. ^ Yu X, Hingorani MM, Patel SS, Egelman EH (September 1996). "DNA is bound within the central hole to one or two of the six subunits of the T7 DNA helicase". Nat. Struct. Biol. 3 (9): 740–3. doi:10.1038/nsb0996-740. PMID 8784344.

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