Abstract:
The invention relates to a new method of characterizing a target polynucleotide. The method uses a pore and an XPD helicase. The helicase controls the movement of the target polynucleotide through the pore.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/GB2012/053273, filed on Dec. 28, 2012, which claims priority to and benefit of U.S. Provisional Application No. 61/581,340, filed Dec. 29, 2011, the entire contents of each of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a new method of characterising a target polynucleotide. The method uses a pore and an XPD helicase. The helicase controls the movement of the target polynucleotide through the pore. 
     BACKGROUND OF THE INVENTION 
     There is currently a need for rapid and cheap polynucleotide (e.g. DNA or RNA) sequencing and identification technologies across a wide range of applications. Existing technologies are slow and expensive mainly because they rely on amplification techniques to produce large volumes of polynucleotide and require a high quantity of specialist fluorescent chemicals for signal detection. 
     Transmembrane pores (nanopores) have great potential as direct, electrical biosensors for polymers and a variety of small molecules. In particular, recent focus has been given to nanopores as a potential DNA sequencing technology. 
     When a potential is applied across a nanopore, there is a change in the current flow when an analyte, such as a nucleotide, resides transiently in the barrel for a certain period of time. Nanopore detection of the nucleotide gives a current change of known signature and duration. In the “Strand Sequencing” method, a single polynucleotide strand is passed through the pore and the identity of the nucleotides are derived. Strand Sequencing can involve the use of a nucleotide handling protein to control the movement of the polynucleotide through the pore. 
     SUMMARY OF THE INVENTION 
     The inventors have demonstrated that an XPD helicase can control the movement of a polynucleotide through a pore especially when a potential, such as a voltage, is applied. The helicase is capable of moving a target polynucleotide in a controlled and stepwise fashion against or with the field resulting from the applied voltage. Surprisingly, the helicase is capable of functioning at a high salt concentration which is advantageous for characterising the polynucleotide and, in particular, for determining its sequence using Strand Sequencing. This is discussed in more detail below. 
     Accordingly, the invention provides a method of characterising a target polynucleotide, comprising: 
     (a) contacting the target polynucleotide with a transmembrane pore and a XPD helicase such that the target polynucleotide moves through the pore and the XPD helicase controls the movement of the target polynucleotide through the pore; and 
     (b) taking one or more measurements as the polynucleotide moves with respect to the pore wherein the measurements are indicative of one or more characteristics of the target polynucleotide and thereby characterising the target polynucleotide. 
     The invention also provides:
         a method of forming a sensor for characterising a target polynucleotide, comprising forming a complex between a pore and an XPD helicase and thereby forming a sensor for characterising the target polynucleotide;       

     use of an XPD helicase to control the movement of a target polynucleotide through a pore; 
     a kit for characterising a target polynucleotide comprising (a) a pore and (b) an XPD helicase; and 
     an analysis apparatus for characterising target polynucleotides in a sample, comprising a plurality of pores and a plurality of an XPD helicase; 
     a method of characterising a target polynucleotide, comprising: 
     (a) contacting the target polynucleotide with a XPD helicase such that the XPD helicase controls the movement of the target polynucleotide; and 
     (b) taking one or more measurements as the XPD helicase controls the movement of the polynucleotide wherein the measurements are indicative of one or more characteristics of the target polynucleotide and thereby characterising the target polynucleotide; 
     use of a XPD helicase to control the movement of a target polynucleotide during characterisation of the polynucleotide; 
     use of a XPD helicase to control the movement of a target polynucleotide during sequencing of part or all of the polynucleotide; 
     an analysis apparatus for characterising target polynucleotides in a sample, characterised in that it comprises a XPD helicase; and 
     a kit for characterising a target polynucleotide comprising (a) an analysis apparatus for characterising target polynucleotides and (b) a XPD helicase. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1 . A) Example schematic of use of a helicase to control DNA movement through a nanopore. The arrows shown on the trans side show the direction of motion of the DNA. The arrows on the cis side show direction of motion of the helicase relative to the DNA. From left to right) A ssDNA substrate ( FIG. 1B ) with an annealed primer containing a cholesterol-tag is added to the cis side of the bilayer. The cholesterol tag binds to the bilayer, enriching the substrate at the bilayer surface. Helicase added to the cis compartment binds to the DNA. In the presence of divalent metal ions and NTP substrate, the helicase moves along the DNA. Under an applied voltage, the DNA substrate is captured by the nanopore via the leader section on the DNA. The DNA is pulled through the pore under the force of the applied potential until a helicase, bound to the DNA, contacts the top of the pore, preventing further uncontrolled DNA translocation. The helicase movement along the DNA in a 5′ to 3′ direction facilitates the controlled translocation of the threaded DNA through the pore with the applied field. The helicase facilitates translocation of the DNA through the nanopore, feeding it into the trans compartment. The last section of DNA to pass through the nanopore is the 3′ end. When the helicase has facilitated complete translocation of the DNA through the nanopore the helicase dissociates from the strand. B) One of the DNA substrate designs used in the Example. 
         FIG. 2 . Helicase is able to move DNA through a nanopore in a controlled fashion, producing stepwise changes in current as the DNA moves through the nanopore. Example helicase-DNA events (140 mV, 400 mM NaCl, Hepes pH 8.0, 0.6 nM 400 mer DNA, 100 nM XPD Mbu, 1 mM DTT, 1 mM ATP, 1 mM MgCl 2 ). Top) Section of current vs. time acquisition of XPD 400mer DNA events through an MspA B2 nanopore. The open-pore current is ˜95 pA. DNA is captured by the nanopore under the force of the applied potential (+140 mV). DNA with enzyme attached results in a long block (at ˜25 pA in this condition) that shows stepwise changes in current as the enzyme moves the DNA through the pore. Bottom) The bottom traces shows an enlargement of one of the helicase controlled DNA movement events, showing DNA-enzyme capture, stepwise current changes as the DNA is pulled through the pore. 
         FIG. 3 . A further example of helicase controlled DNA movement event. Bottom) An enlargement of a section of the event showing the stepwise changes in current from the different sections of DNA as the strand moves through the nanopore. 
         FIG. 4 . The helicase can control the movement of DNA in at least two modes of operation. The helicase moves along the DNA in the 5′-3′ direction, but the orientation of the DNA in the nanopore (dependent on which end of the DNA is captured) means that the enzyme can be used to either move the DNA out of the nanopore against the applied field, or move the DNA into the nanopore with the applied field. Left) When the 3′ end of the DNA is captured the helicase works against the direction of the field applied by the voltage, pulling the threaded DNA out of the nanopore and into the cis chamber. Right) When the DNA is captured 5′-down in the nanopore, the enzyme moves the DNA into the nanopore in the direction of the field into the trans side of the bilayer. 
         FIG. 5 . Fluorescence assay for testing enzyme activity. A) A custom fluorescent substrate was used to assay the ability of the helicase to displace hybridised dsDNA. 1) The fluorescent substrate strand (50 nM final) has a 5′ ssDNA overhang, and a 40 base section of hybridised dsDNA. The major upper strand has a carboxyfluorescein base at the 3′ end, and the hybridised complement has a black-hole quencher (BHQ-1) base at the 5′ end. When hybridised the fluorescence from the fluorescein is quenched by the local BHQ-1, and the substrate is essentially non-fluorescent. 1 μM of a capture strand that is complementary to the shorter strand of the fluorescent substrate is included in the assay. 2) In the presence of ATP (1 mM) and MgCl 2  (10 mM), helicase (150 nM) added to the substrate binds to the 5′ tail of the fluorescent substrate, moves along the major strand, and displaces the complementary strand as shown. 3) Once the complementary strand with BHQ-1 is fully displaced the fluorescein on the major strand fluoresces. 4) Excess of capture strand preferentially anneals to the complementary DNA to prevent re-annealing of initial substrate and loss of fluorescence. B) Graph of the initial rate of Mbu XPD helicase activity in buffer solutions (100 mM Hepes pH 8.0, 1 mM ATP, 10 mM MgCl 2 , 50 nM fluorescent substrate DNA, 1 μM capture DNA) containing different concentrations of KCl from 100 mM to 2 M. 
         FIG. 6 . The structure of the spacer iSp18 used in Example 1. 
     
    
    
     DESCRIPTION OF THE SEQUENCE LISTING 
     SEQ ID NO: 1 shows the codon optimised polynucleotide sequence encoding the MS-B1 mutant MspA monomer. This mutant lacks the signal sequence and includes the following mutations: D90N, D91N, D93N, D118R, D134R and E139K. 
     SEQ ID NO: 2 shows the amino acid sequence of the mature form of the MS-B1 mutant of the MspA monomer. This mutant lacks the signal sequence and includes the following mutations: D90N, D91N, D93N, D118R, D134R and E139K. 
     SEQ ID NO: 3 shows the polynucleotide sequence encoding one subunit of α-hemolysin-E111N/K147N (α-HL-NN; Stoddart et al., PNAS, 2009; 106(19): 7702-7707). 
     SEQ ID NO: 4 shows the amino acid sequence of one subunit of α-HL-NN. 
     SEQ ID NOs: 5 to 7 show the amino acid sequences of MspB, C and D. 
     SEQ ID NOs: 8 and 9 show the amino acid sequences of XPD motifs V and VI. 
     SEQ ID NOs: 10 to 62 show the amino acid sequences of the XPD helicases in Table 5. 
     SEQ ID NOs: 63 to 68 show the sequences used in the Examples. 
     DETAILED DESCRIPTION OF THE INVENTION 
     It is to be understood that different applications of the disclosed products and methods may be tailored to the specific needs in the art. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only, and is not intended to be limiting. 
     In addition as used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to “a pore” includes two or more such pores, reference to “a helicase” includes two or more such helicases, reference to “a polynucleotide” includes two or more such polynucleotides, and the like. 
     All publications, patents and patent applications cited herein, whether supra or infra, are hereby incorporated by reference in their entirety. 
     Methods of the Invention 
     The invention provides a method of characterising a target polynucleotide. The method comprises contacting the target polynucleotide with a transmembrane pore and an XPD helicase such that the target polynucleotide moves through the pore and the XPD helicase controls the movement of the target polynucleotide through the pore. One or more characteristics of the target polynucleotide are then measured as the polynucleotide moves with respect to the pore using standard methods known in the art. One or more characteristics of the target polynucleotide are preferably measured as the polynucleotide moves through the pore. Steps (a) and (b) are preferably carried out with a potential applied across the pore. As discussed in more detail below, the applied potential typically results in the formation of a complex between the pore and the helicase. The applied potential may be a voltage potential. Alternatively, the applied potential may be a chemical potential. An example of this is using a salt gradient across an amphiphilic layer. A salt gradient is disclosed in Holden et al., J Am Chem. Soc. 2007 Jul. 11; 129(27):8650-5. 
     In some instances, the current passing through the pore as the polynucleotide moves with respect to the pore is used to determine the sequence of the target polynucleotide. This is Strand Sequencing. 
     The method has several advantages. First, the inventors have surprisingly shown that XPD helicases have a surprisingly high salt tolerance and so the method of the invention may be carried out at high salt concentrations. In the context of Strand Sequencing, a charge carrier, such as a salt, is necessary to create a conductive solution for applying a voltage offset to capture and translocate the target polynucleotide and to measure the resulting sequence-dependent current changes as the polynucleotide moves with respect to the pore. Since the measurement signal is dependent on the concentration of the salt, it is advantageous to use high salt concentrations to increase the magnitude of the acquired signal. High salt concentrations provide a high signal to noise ratio and allow for currents indicative of the presence of a nucleotide to be identified against the background of normal current fluctuations. For Strand Sequencing, salt concentrations in excess of 100 mM are ideal, for example salt concentrations in excess of 400 mM, 600 mM or 800 mM. The inventors have surprisingly shown that XPD helicases will function effectively at very high salt concentrations such as, for example, 1 M. The invention encompasses helicases which function effectively at salt concentrations in excess of 1M, for example 2M. 
     Second, when a voltage is applied, XPD helicases can surprisingly move the target polynucleotide in two directions, namely with or against the field resulting from the applied voltage. Hence, the method of the invention may be carried out in one of two preferred modes. Different signals are obtained depending on the direction the target polynucleotide moves with respect to the pore, ie in the direction of or against the field. This is discussed in more detail below. 
     Third, XPD helicases typically move the target polynucleotide through the pore one nucleotide at a time. XPD helicases can therefore function like a single-base ratchet. This is of course advantageous when sequencing a target polynucleotide because substantially all, if not all, of the nucleotides in the target polynucleotide may be identified using the pore. 
     Fourth, XPD helicases are capable of controlling the movement of single stranded polynucleotides and double stranded polynucleotides. This means that a variety of different target polynucleotides can be characterised in accordance with the invention. 
     Fifth, XPD helicases appear very resistant to the field resulting from applied voltages. The inventors have seen very little movement of the polynucleotide under an “unzipping” condition. Unzipping conditions will typically be in the absence of nucleotides, for example the absence of ATP. When the helicase is operating in unzipping mode it acts like a brake preventing the target sequence from moving through the pore too quickly under the influence of the applied voltage. This is important because it means that there are no complications from unwanted “backwards” movements when moving polynucleotides against the field resulting from an applied voltage. 
     Sixth, XPD helicases are easy to produce and easy to handle. Their use therefore contributed to a straightforward and less expensive method of sequencing. 
     The method of the invention is for characterising a target polynucleotide. A polynucleotide, such as a nucleic acid, is a macromolecule comprising two or more nucleotides. The polynucleotide or nucleic acid may comprise any combination of any nucleotides. The nucleotides can be naturally occurring or artificial. One or more nucleotides in the target polynucleotide can be oxidized or methylated. One or more nucleotides in the target polynucleotide may be damaged. One or more nucleotides in the target polynucleotide may be modified, for instance with a label or a tag. The target polynucleotide may comprise one or more spacers. 
     A nucleotide typically contains a nucleobase, a sugar and at least one phosphate group. The nucleobase is typically heterocyclic. Nucleobases include, but are not limited to, purines and pyrimidines and more specifically adenine, guanine, thymine, uracil and cytosine. The sugar is typically a pentose sugar. Nucleotide sugars include, but are not limited to, ribose and deoxyribose. The nucleotide is typically a ribonucleotide or deoxyribonucleotide. The nucleotide typically contains a monophosphate, diphosphate or triphosphate. Phosphates may be attached on the 5′ or 3′ side of a nucleotide. 
     Nucleotides include, but are not limited to, adenosine monophosphate (AMP), guanosine monophosphate (GMP), thymidine monophosphate (TMP), uridine monophosphate (UMP), cytidine monophosphate (CMP), cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), deoxyadenosine monophosphate (dAMP), deoxyguanosine monophosphate (dGMP), deoxythymidine monophosphate (dTMP), deoxyuridine monophosphate (dUMP) and deoxycytidine monophosphate (dCMP). The nucleotides are preferably selected from AMP, TMP, GMP, CMP, UMP, dAMP, dTMP, dGMP or dCMP. 
     A nucleotide may be abasic (i.e. lack a nucleobase). 
     The polynucleotide may be single stranded or double stranded. At least a portion of the polynucleotide is preferably double stranded. 
     The polynucleotide can be a nucleic acid, such as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The target polynucleotide can comprise one strand of RNA hybridized to one strand of DNA. The polynucleotide may be any synthetic nucleic acid known in the art, such as peptide nucleic acid (PNA), glycerol nucleic acid (GNA), threose nucleic acid (TNA), locked nucleic acid (LNA) or other synthetic polymers with nucleotide side chains. 
     The whole or only part of the target polynucleotide may be characterised using this method. The target polynucleotide can be any length. For example, the polynucleotide can be at least 10, at least 50, at least 100, at least 150, at least 200, at least 250, at least 300, at least 400 or at least 500 nucleotide pairs in length. The polynucleotide can be 1000 or more nucleotide pairs, 5000 or more nucleotide pairs in length or 100000 or more nucleotide pairs in length. 
     The target polynucleotide is present in any suitable sample. The invention is typically carried out on a sample that is known to contain or suspected to contain the target polynucleotide. Alternatively, the invention may be carried out on a sample to confirm the identity of one or more target polynucleotides whose presence in the sample is known or expected. 
     The sample may be a biological sample. The invention may be carried out in vitro on a sample obtained from or extracted from any organism or microorganism. The organism or microorganism is typically archaean, prokaryotic or eukaryotic and typically belongs to one the five kingdoms: plantae, animalia, fungi, monera and protista. The invention may be carried out in vitro on a sample obtained from or extracted from any virus. The sample is preferably a fluid sample. The sample typically comprises a body fluid of the patient. The sample may be urine, lymph, saliva, mucus or amniotic fluid but is preferably blood, plasma or serum. Typically, the sample is human in origin, but alternatively it may be from another mammal animal such as from commercially farmed animals such as horses, cattle, sheep or pigs or may alternatively be pets such as cats or dogs. Alternatively a sample of plant origin is typically obtained from a commercial crop, such as a cereal, legume, fruit or vegetable, for example wheat, barley, oats, canola, maize, soya, rice, bananas, apples, tomatoes, potatoes, grapes, tobacco, beans, lentils, sugar cane, cocoa, cotton. 
     The sample may be a non-biological sample. The non-biological sample is preferably a fluid sample. Examples of a non-biological sample include surgical fluids, water such as drinking water, sea water or river water, and reagents for laboratory tests. 
     The sample is typically processed prior to being assayed, for example by centrifugation or by passage through a membrane that filters out unwanted molecules or cells, such as red blood cells. The sample may be measured immediately upon being taken. The sample may also be typically stored prior to assay, preferably below −70° C. 
     A transmembrane pore is a structure that crosses the membrane to some degree. It permits ions, such as hydrated ions, driven by an applied potential to flow across or within the membrane. The transmembrane pore typically crosses the entire membrane so that ions may flow from one side of the membrane to the other side of the membrane. However, the transmembrane pore does not have to cross the membrane. It may be closed at one end. For instance, the pore may be a well in the membrane along which or into which ions may flow. 
     Any membrane may be used in accordance with the invention. Suitable membranes are well-known in the art. The membrane is preferably an amphiphilic layer. An amphiphilic layer is a layer formed from amphiphilic molecules, such as phospholipids, which have both at least one hydrophilic portion and at least one lipophilic or hydrophobic portion. The amphiphilic layer may be a monolayer or a bilayer. The amphiphilic layer is typically a planar lipid bilayer or a supported bilayer. 
     The amphiphilic layer is typically a lipid bilayer. Lipid bilayers are models of cell membranes and serve as excellent platforms for a range of experimental studies. For example, lipid bilayers can be used for in vitro investigation of membrane proteins by single-channel recording. Alternatively, lipid bilayers can be used as biosensors to detect the presence of a range of substances. The lipid bilayer may be any lipid bilayer. Suitable lipid bilayers include, but are not limited to, a planar lipid bilayer, a supported bilayer or a liposome. The lipid bilayer is preferably a planar lipid bilayer. Suitable lipid bilayers are disclosed in International Application No. PCT/GB08/000,563 (published as WO 2008/102121), International Application No. PCT/GB08/004,127 (published as WO 2009/077734) and International Application No. PCT/GB2006/001057 (published as WO 2006/100484). 
     Methods for forming lipid bilayers are known in the art. Suitable methods are disclosed in the Example. Lipid bilayers are commonly formed by the method of Montal and Mueller (Proc. Natl. Acad. Sci. USA., 1972; 69: 3561-3566), in which a lipid monolayer is carried on aqueous solution/air interface past either side of an aperture which is perpendicular to that interface. 
     The method of Montal &amp; Mueller is popular because it is a cost-effective and relatively straightforward method of forming good quality lipid bilayers that are suitable for protein pore insertion. Other common methods of bilayer formation include tip-dipping, painting bilayers and patch-clamping of liposome bilayers. 
     In a preferred embodiment, the lipid bilayer is formed as described in International Application No. PCT/GB08/004,127 (published as WO 2009/077734). 
     In another preferred embodiment, the membrane is a solid state layer. A solid-state layer is not of biological origin. In other words, a solid state layer is not derived from or isolated from a biological environment such as an organism or cell, or a synthetically manufactured version of a biologically available structure. Solid state layers can be formed from both organic and inorganic materials including, but not limited to, microelectronic materials, insulating materials such as Si 3 N 4 , Al 2 O 3 , and SiO, organic and inorganic polymers such as polyamide, plastics such as Teflon® or elastomers such as two-component addition-cure silicone rubber, and glasses. The solid state layer may be formed from monatomic layers, such as graphene, or layers that are only a few atoms thick. Suitable graphene layers are disclosed in International Application No. PCT/US2008/010637 (published as WO 2009/035647). 
     The method is typically carried out using (i) an artificial amphiphilic layer comprising a pore, (ii) an isolated, naturally-occurring lipid bilayer comprising a pore, or (iii) a cell having a pore inserted therein. The method is typically carried out using an artificial amphiphilic layer, such as an artificial lipid bilayer. The layer may comprise other transmembrane and/or intramembrane proteins as well as other molecules in addition to the pore. Suitable apparatus and conditions are discussed below. The method of the invention is typically carried out in vitro. 
     The polynucleotide may be coupled to the membrane. This may be done using any known method. If the membrane is an amphiphilic layer, such as a lipid bilayer (as discussed in detail above), the polynucleotide is preferably coupled to the membrane via a polypeptide present in the membrane or a hydrophobic anchor present in the membrane. The hydrophobic anchor is preferably a lipid, fatty acid, sterol, carbon nanotube or amino acid. 
     The polynucleotide may be coupled directly to the membrane. The polynucleotide is preferably coupled to the membrane via a linker. Preferred linkers include, but are not limited to, polymers, such as polynucleotides, polyethylene glycols (PEGs) and polypeptides. If a polynucleotide is coupled directly to the membrane, then some data will be lost as the characterising run cannot continue to the end of the polynucleotide due to the distance between the membrane and the helicase. If a linker is used, then the polynucleotide can be processed to completion. If a linker is used, the linker may be attached to the polynucleotide at any position. The linker is preferably attached to the polynucleotide at the tail polymer. 
     The coupling may be stable or transient. For certain applications, the transient nature of the coupling is preferred. If a stable coupling molecule were attached directly to either the 5′ or 3′ end of a polynucleotide, then some data will be lost as the characterising run cannot continue to the end of the polynucleotide due to the distance between the bilayer and the helicase&#39;s active site. If the coupling is transient, then when the coupled end randomly becomes free of the bilayer, then the polynucleotide can be processed to completion. Chemical groups that form stable or transient links with the membrane are discussed in more detail below. The polynucleotide may be transiently coupled to an amphiphilic layer, such as a lipid bilayer using cholesterol or a fatty acyl chain. Any fatty acyl chain having a length of from 6 to 30 carbon atoms, such as hexadecanoic acid, may be used. 
     In preferred embodiments, the polynucleotide is coupled to an amphiphilic layer. Coupling of polynucleotides to synthetic lipid bilayers has been carried out previously with various different tethering strategies. These are summarised in Table 1 below. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Attachment 
                 Type of 
                   
               
               
                 group 
                 coupling 
                 Reference 
               
               
                   
               
             
             
               
                 Thiol 
                 Stable 
                 Yoshina-Ishii, C. and S. G. Boxer (2003). 
               
               
                   
                   
                 “Arrays of mobile tethered vesicles on 
               
               
                   
                   
                 supported lipid bilayers.” J Am Chem Soc   
               
               
                   
                   
                 125(13): 3696-7. 
               
               
                 Biotin 
                 Stable 
                 Nikolov, V., R. Lipowsky, et al. (2007). 
               
               
                   
                   
                 “Behavior of giant vesicles with anchored DNA 
               
               
                   
                   
                 molecules.”  Biophys J  92(12): 4356-68 
               
               
                 Cholestrol 
                 Transient 
                 Pfeiffer, I. and F. Hook (2004). “Bivalent 
               
               
                   
                   
                 cholesterol-based coupling of oligonucletides 
               
               
                   
                   
                 to lipid membrane assemblies.”  J Am Chem   
               
               
                   
                   
                   Soc  126(33): 10224-5 
               
               
                 Lipid 
                 Stable 
                 van Lengerich, B., R. J. Rawle, et al. 
               
               
                   
                   
                 “Covalent attachment of lipid vesicles 
               
               
                   
                   
                 to a fluid-supported bilayer allows observation 
               
               
                   
                   
                 of DNA-mediated vesicle interactions.” 
               
               
                   
                   
                   Langmuir  26(11): 8666-72 
               
               
                   
               
             
          
         
       
     
     Polynucleotides may be functionalized using a modified phosphoramidite in the synthesis reaction, which is easily compatible for the addition of reactive groups, such as thiol, cholesterol, lipid and biotin groups. These different attachment chemistries give a suite of attachment options for polynucleotides. Each different modification group tethers the polynucleotide in a slightly different way and coupling is not always permanent so giving different dwell times for the polynucleotide to the bilayer. The advantages of transient coupling are discussed above. 
     Coupling of polynucleotides can also be achieved by a number of other means provided that a reactive group can be added to the polynucleotide. The addition of reactive groups to either end of DNA has been reported previously. A thiol group can be added to the 5′ of ssDNA using polynucleotide kinase and ATPγS (Grant, G. P. and P. Z. Qin (2007). “A facile method for attaching nitroxide spin labels at the 5′ terminus of nucleic acids.”  Nucleic Acids Res  35(10): e77). A more diverse selection of chemical groups, such as biotin, thiols and fluorophores, can be added using terminal transferase to incorporate modified oligonucleotides to the 3′ of ssDNA (Kumar, A., P. Tchen, et al. (1988) “Nonradioactive labeling of synthetic oligonucleotide probes with terminal deoxynucleotidyl transferase.”  Anal Biochem  169(2): 376-82). 
     Alternatively, the reactive group could be considered to be the addition of a short piece of DNA complementary to one already coupled to the bilayer, so that attachment can be achieved via hybridisation. Ligation of short pieces of ssDNA have been reported using T4 RNA ligase I (Troutt, A. B., M. G. McHeyzer-Williams, et al. (1992). “Ligation-anchored PCR: a simple amplification technique with single-sided specificity.”  Proc Natl Acad Sci USA  89(20): 9823-5). Alternatively either ssDNA or dsDNA could be ligated to native dsDNA and then the two strands separated by thermal or chemical denaturation. To native dsDNA, it is possible to add either a piece of ssDNA to one or both of the ends of the duplex, or dsDNA to one or both ends. Then, when the duplex is melted, each single strand will have either a 5′ or 3′ modification if ssDNA was used for ligation or a modification at the 5′ end, the 3′ end or both if dsDNA was used for ligation. If the polynucleotide is a synthetic strand, the coupling chemistry can be incorporated during the chemical synthesis of the polynucleotide. For instance, the polynucleotide can be synthesized using a primer a reactive group attached to it. 
     A common technique for the amplification of sections of genomic DNA is using polymerase chain reaction (PCR). Here, using two synthetic oligonucleotide primers, a number of copies of the same section of DNA can be generated, where for each copy the 5′ of each strand in the duplex will be a synthetic polynucleotide. By using an antisense primer that has a reactive group, such as a cholesterol, thiol, biotin or lipid, each copy of the target DNA amplified will contain a reactive group for coupling. 
     The transmembrane pore is preferably a transmembrane protein pore. A transmembrane protein pore is a protein structure that crosses the membrane to some degree. It permits ions driven by an applied potential to flow across or within the membrane. A transmembrane protein pore is typically a polypeptide or a collection of polypeptides that permits ions, such as analytes, to flow from one side of a membrane to the other side of the membrane. However, the transmembrane protein pore does not have to cross the membrane. It may be closed at one end. For instance, the transmembrane pore may form a well in the membrane along which or into which ions may flow. The transmembrane protein pore preferably permits analytes, such as nucleotides, to flow across or within the membrane, such as a lipid bilayer. The transmembrane protein pore allows a polynucleotide, such as DNA or RNA, to be moved through the pore. 
     The transmembrane protein pore may be a monomer or an oligomer. The pore is preferably made up of several repeating subunits, such as 6, 7, 8 or 9 subunits. The pore is preferably a hexameric, heptameric, octameric or nonameric pore. 
     The transmembrane protein pore typically comprises a barrel or channel through which the ions may flow. The subunits of the pore typically surround a central axis and contribute strands to a transmembrane β barrel or channel or a transmembrane α-helix bundle or channel. 
     The barrel or channel of the transmembrane protein pore typically comprises amino acids that facilitate interaction with analyte, such as nucleotides, polynucleotides or nucleic acids. These amino acids are preferably located near a constriction of the barrel or channel. The transmembrane protein pore typically comprises one or more positively charged amino acids, such as arginine, lysine or histidine, or aromatic amino acids, such as tyrosine or tryptophan. These amino acids typically facilitate the interaction between the pore and nucleotides, polynucleotides or nucleic acids. 
     Transmembrane protein pores for use in accordance with the invention can be derived from β-barrel pores or α-helix bundle pores. β-barrel pores comprise a barrel or channel that is formed from β-strands. Suitable β-barrel pores include, but are not limited to, β-toxins, such as α-hemolysin, anthrax toxin and leukocidins, and outer membrane proteins/porins of bacteria, such as  Mycobacterium smegmatis  porin (Msp), for example MspA, outer membrane porin F (OmpF), outer membrane porin G (OmpG), outer membrane phospholipase A and  Neisseria  autotransporter lipoprotein (NalP). α-helix bundle pores comprise a barrel or channel that is formed from α-helices. Suitable α-helix bundle pores include, but are not limited to, inner membrane proteins and α outer membrane proteins, such as WZA and ClyA toxin. The transmembrane pore may be derived from Msp or from α-hemolysin (α-HL). 
     The transmembrane protein pore is preferably derived from Msp, preferably from MspA. Such a pore will be oligomeric and typically comprises 7, 8, 9 or 10 monomers derived from Msp. The pore may be a homo-oligomeric pore derived from Msp comprising identical monomers. Alternatively, the pore may be a hetero-oligomeric pore derived from Msp comprising at least one monomer that differs from the others. Preferably the pore is derived from MspA or a homolog or paralog thereof. 
     A monomer derived from Msp comprises the sequence shown in SEQ ID NO: 2 or a variant thereof. SEQ ID NO: 2 is the MS-(B1)8 mutant of the MspA monomer. It includes the following mutations: D90N, D91N, D93N, D118R, D134R and E139K. A variant of SEQ ID NO: 2 is a polypeptide that has an amino acid sequence which varies from that of SEQ ID NO: 2 and which retains its ability to form a pore. The ability of a variant to form a pore can be assayed using any method known in the art. For instance, the variant may be inserted into an amphihpilic layer along with other appropriate subunits and its ability to oligomerise to form a pore may be determined. Methods are known in the art for inserting subunits into membranes, such as amphiphilic layers. For example, subunits may be suspended in a purified form in a solution containing a lipid bilayer such that it diffuses to the lipid bilayer and is inserted by binding to the lipid bilayer and assembling into a functional state. Alternatively, subunits may be directly inserted into the membrane using the “pick and place” method described in M. A. Holden, H. Bayley. J. Am. Chem. Soc. 2005, 127, 6502-6503 and International Application No. PCT/GB2006/001057 (published as WO 2006/100484). 
     Over the entire length of the amino acid sequence of SEQ ID NO: 2, a variant will preferably be at least 50% homologous to that sequence based on amino acid identity. More preferably, the variant may be at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% and more preferably at least 95%, 97% or 99% homologous based on amino acid identity to the amino acid sequence of SEQ ID NO: 2 over the entire sequence. There may be at least 80%, for example at least 85%, 90% or 95%, amino acid identity over a stretch of 100 or more, for example 125, 150, 175 or 200 or more, contiguous amino acids (“hard homology”). 
     Standard methods in the art may be used to determine homology. For example the UWGCG Package provides the BESTFIT program which can be used to calculate homology, for example used on its default settings (Devereux et al (1984)  Nucleic Acids Research  12, p 387-395). The PILEUP and BLAST algorithms can be used to calculate homology or line up sequences (such as identifying equivalent residues or corresponding sequences (typically on their default settings)), for example as described in Altschul S. F. (1993) J Mol Evol 36:290-300; Altschul, S. F et al (1990) J Mol Biol 215:403-10. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (www.ncbi.nlm.nih.gov/). 
     SEQ ID NO: 2 is the MS-(B1)8 mutant of the MspA monomer. The variant may comprise any of the mutations in the MspB, C or D monomers compared with MspA. The mature forms of MspB, C and D are shown in SEQ ID NOs: 5 to 7. In particular, the variant may comprise the following substitution present in MspB: A138P. The variant may comprise one or more of the following substitutions present in MspC: A96G, N102E and A138P. The variant may comprise one or more of the following mutations present in MspD: Deletion of G1, L2V, E5Q, L8V, D13G, W21A, D22E, K47T, I49H, I68V, D91G, A96Q, N102D, S103T, V104I, S136K and G141A. The variant may comprise combinations of one or more of the mutations and substitutions from Msp B, C and D. The variant preferably comprises the mutation L88N. The variant of SEQ ID NO: 2 has the mutation L88N in addition to all the mutations of MS-B1 and is called MS-B2. The pore used in the invention is preferably MS-(B2)8. 
     Amino acid substitutions may be made to the amino acid sequence of SEQ ID NO: 2 in addition to those discussed above, for example up to 1, 2, 3, 4, 5, 10, 20 or 30 substitutions. Conservative substitutions replace amino acids with other amino acids of similar chemical structure, similar chemical properties or similar side-chain volume. The amino acids introduced may have similar polarity, hydrophilicity, hydrophobicity, basicity, acidity, neutrality or charge to the amino acids they replace. Alternatively, the conservative substitution may introduce another amino acid that is aromatic or aliphatic in the place of a pre-existing aromatic or aliphatic amino acid. Conservative amino acid changes are well-known in the art and may be selected in accordance with the properties of the 20 main amino acids as defined in Table 2 below. Where amino acids have similar polarity, this can also be determined by reference to the hydropathy scale for amino acid side chains in Table 3. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Chemical properties of amino acids 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Ala 
                 aliphatic, hydrophobic, neutral 
                 Met 
                 hydrophobic, neutral 
               
               
                 Cys 
                 polar, hydrophobic, neutral 
                 Asn 
                 polar, hydrophilic, 
               
               
                   
                   
                   
                 neutral 
               
               
                 Asp 
                 polar, hydrophilic, charged (−) 
                 Pro 
                 hydrophobic, neutral 
               
               
                 Glu 
                 polar, hydrophilic, charged (−) 
                 Gln 
                 polar, hydrophilic, 
               
               
                   
                   
                   
                 neutral 
               
               
                 Phe 
                 aromatic, hydrophobic, neutral 
                 Arg 
                 polar, hydrophilic, 
               
               
                   
                   
                   
                 charged (+) 
               
               
                 Gly 
                 aliphatic, neutral 
                 Ser 
                 polar, hydrophilic, 
               
               
                   
                   
                   
                 neutral 
               
               
                 His 
                 aromatic, polar, hydrophilic, 
                 Thr 
                 polar, hydrophilic, 
               
               
                   
                   
                   
                 neutral charged (+) 
               
               
                 Ile 
                 aliphatic, hydrophobic, neutral 
                 Val 
                 aliphatic, hydrophobic, 
               
               
                   
                   
                   
                 neutral 
               
               
                 Lys 
                 polar, hydrophilic, charged(+) 
                 Trp 
                 aromatic, hydrophobic, 
               
               
                   
                   
                   
                 neutral 
               
               
                 Leu 
                 aliphatic, hydrophobic, neutral 
                 Tyr 
                 aromatic, polar, 
               
               
                   
                   
                   
                 hydrophobic 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Hydropathy scale 
               
             
          
           
               
                   
                 Side Chain 
                 Hydropathy 
               
               
                   
                   
               
             
          
           
               
                   
                 Ile 
                 4.5 
               
               
                   
                 Val 
                 4.2 
               
               
                   
                 Leu 
                 3.8 
               
               
                   
                 Phe 
                 2.8 
               
               
                   
                 Cys 
                 2.5 
               
               
                   
                 Met 
                 1.9 
               
               
                   
                 Ala 
                 1.8 
               
               
                   
                 Gly 
                 −0.4 
               
               
                   
                 Thr 
                 −0.7 
               
               
                   
                 Ser 
                 −0.8 
               
               
                   
                 Trp 
                 −0.9 
               
               
                   
                 Tyr 
                 −1.3 
               
               
                   
                 Pro 
                 −1.6 
               
               
                   
                 His 
                 −3.2 
               
               
                   
                 Glu 
                 −3.5 
               
               
                   
                 Gln 
                 −3.5 
               
               
                   
                 Asp 
                 −3.5 
               
               
                   
                 Asn 
                 −3.5 
               
               
                   
                 Lys 
                 −3.9 
               
               
                   
                 Arg 
                 −4.5 
               
               
                   
                   
               
             
          
         
       
     
     One or more amino acid residues of the amino acid sequence of SEQ ID NO: 2 may additionally be deleted from the polypeptides described above. Up to 1, 2, 3, 4, 5, 10, 20 or 30 residues may be deleted, or more. 
     Variants may include fragments of SEQ ID NO: 2. Such fragments retain pore forming activity. Fragments may be at least 50, 100, 150 or 200 amino acids in length. Such fragments may be used to produce the pores. A fragment preferably comprises the pore forming domain of SEQ ID NO: 2. Fragments must include one of residues 88, 90, 91, 105, 118 and 134 of SEQ ID NO: 2. Typically, fragments include all of residues 88, 90, 91, 105, 118 and 134 of SEQ ID NO: 2. 
     One or more amino acids may be alternatively or additionally added to the polypeptides described above. An extension may be provided at the amino terminal or carboxy terminal of the amino acid sequence of SEQ ID NO: 2 or polypeptide variant or fragment thereof. The extension may be quite short, for example from 1 to 10 amino acids in length. Alternatively, the extension may be longer, for example up to 50 or 100 amino acids. A carrier protein may be fused to an amino acid sequence according to the invention. Other fusion proteins are discussed in more detail below. 
     As discussed above, a variant is a polypeptide that has an amino acid sequence which varies from that of SEQ ID NO: 2 and which retains its ability to form a pore. A variant typically contains the regions of SEQ ID NO: 2 that are responsible for pore formation. The pore forming ability of Msp, which contains a β-barrel, is provided by β-sheets in each subunit. A variant of SEQ ID NO: 2 typically comprises the regions in SEQ ID NO: 2 that form β-sheets. One or more modifications can be made to the regions of SEQ ID NO: 2 that form β-sheets as long as the resulting variant retains its ability to form a pore. A variant of SEQ ID NO: 2 preferably includes one or more modifications, such as substitutions, additions or deletions, within its α-helices and/or loop regions. 
     The monomers derived from Msp may be modified to assist their identification or purification, for example by the addition of histidine residues (a hist tag), aspartic acid residues (an asp tag), a streptavidin tag or a flag tag, or by the addition of a signal sequence to promote their secretion from a cell where the polypeptide does not naturally contain such a sequence. An alternative to introducing a genetic tag is to chemically react a tag onto a native or engineered position on the pore. An example of this would be to react a gel-shift reagent to a cysteine engineered on the outside of the pore. This has been demonstrated as a method for separating hemolysin hetero-oligomers (Chem. Biol. 1997 July; 4(7):497-505). 
     The monomer derived from Msp may be labelled with a revealing label. The revealing label may be any suitable label which allows the pore to be detected. Suitable labels include, but are not limited to fluorescent molecules, radioisotopes, e.g.  125 I,  35 S, enzymes, antibodies, antigens, polynucleotides and ligands such as biotin. 
     The monomer derived from Msp may also be produced using D-amino acids. For instance, the monomer derived from Msp may comprise a mixture of L-amino acids and D-amino acids. This is conventional in the art for producing such proteins or peptides. 
     The monomer derived from Msp contains one or more specific modifications to facilitate nucleotide discrimination. The monomer derived from Msp may also contain other non-specific modifications as long as they do not interfere with pore formation. A number of non-specific side chain modifications are known in the art and may be made to the side chains of the monomer derived from Msp. Such modifications include, for example, reductive alkylation of amino acids by reaction with an aldehyde followed by reduction with NaBH 4 , amidination with methylacetimidate or acylation with acetic anhydride. 
     The monomer derived from Msp can be produced using standard methods known in the art. The monomer derived from Msp may be made synthetically or by recombinant means. For example, the pore may be synthesized by in vitro translation and transcription (IVTT). Suitable methods for producing pores are discussed in International Application Nos. PCT/GB09/001,690 (published as WO 2010/004273), PCT/GB09/001,679 (published as WO 2010/004265) or PCT/GB10/000,133 (published as WO 2010/086603). Methods for inserting pores into membranes are discussed. 
     The transmembrane protein pore is also preferably derived from α-hemolysin (α-HL). The wild type α-HL pore is formed of seven identical monomers or subunits (i.e. it is heptameric). The sequence of one monomer or subunit of α-hemolysin-NN is shown in SEQ ID NO: 4. The transmembrane protein pore preferably comprises seven monomers each comprising the sequence shown in SEQ ID NO: 4 or a variant thereof. Amino acids 1, 7 to 21, 31 to 34, 45 to 51, 63 to 66, 72, 92 to 97, 104 to 111, 124 to 136, 149 to 153, 160 to 164, 173 to 206, 210 to 213, 217, 218, 223 to 228, 236 to 242, 262 to 265, 272 to 274, 287 to 290 and 294 of SEQ ID NO: 4 form loop regions. Residues 113 and 147 of SEQ ID NO: 4 form part of a constriction of the barrel or channel of α-HL. 
     In such embodiments, a pore comprising seven proteins or monomers each comprising the sequence shown in SEQ ID NO: 4 or a variant thereof are preferably used in the method of the invention. The seven proteins may be the same (homoheptamer) or different (heteroheptamer). 
     A variant of SEQ ID NO: 4 is a protein that has an amino acid sequence which varies from that of SEQ ID NO: 4 and which retains its pore forming ability. The ability of a variant to form a pore can be assayed using any method known in the art. For instance, the variant may be inserted into an amphiphilic layer, such as a lipid bilayer, along with other appropriate subunits and its ability to oligomerise to form a pore may be determined. Methods are known in the art for inserting subunits into amphiphilic layers, such as lipid bilayers. Suitable methods are discussed above. 
     The variant may include modifications that facilitate covalent attachment to or interaction with the helicase. The variant preferably comprises one or more reactive cysteine residues that facilitate attachment to the helicase. For instance, the variant may include a cysteine at one or more of positions 8, 9, 17, 18, 19, 44, 45, 50, 51, 237, 239 and 287 and/or on the amino or carboxy terminus of SEQ ID NO: 4. Preferred variants comprise a substitution of the residue at position 8, 9, 17, 237, 239 and 287 of SEQ ID NO: 4 with cysteine (A8C, T9C, N17C, K237C, S239C or E287C). The variant is preferably any one of the variants described in International Application No. PCT/GB09/001,690 (published as WO 2010/004273), PCT/GB09/001,679 (published as WO 2010/004265) or PCT/GB10/000,133 (published as WO 2010/086603). 
     The variant may also include modifications that facilitate any interaction with nucleotides. 
     The variant may be a naturally occurring variant which is expressed naturally by an organism, for instance by a  Staphylococcus  bacterium. Alternatively, the variant may be expressed in vitro or recombinantly by a bacterium such as  Escherichia coli . Variants also include non-naturally occurring variants produced by recombinant technology. Over the entire length of the amino acid sequence of SEQ ID NO: 4, a variant will preferably be at least 50% homologous to that sequence based on amino acid identity. More preferably, the variant polypeptide may be at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% and more preferably at least 95%, 97% or 99% homologous based on amino acid identity to the amino acid sequence of SEQ ID NO: 4 over the entire sequence. There may be at least 80%, for example at least 85%, 90% or 95%, amino acid identity over a stretch of 200 or more, for example 230, 250, 270 or 280 or more, contiguous amino acids (“hard homology”). Homology can be determined as discussed above. 
     Amino acid substitutions may be made to the amino acid sequence of SEQ ID NO: 4 in addition to those discussed above, for example up to 1, 2, 3, 4, 5, 10, 20 or 30 substitutions. Conservative substitutions may be made as discussed above. 
     One or more amino acid residues of the amino acid sequence of SEQ ID NO: 4 may additionally be deleted from the polypeptides described above. Up to 1, 2, 3, 4, 5, 10, 20 or 30 residues may be deleted, or more. 
     Variants may be fragments of SEQ ID NO: 4. Such fragments retain pore-forming activity. Fragments may be at least 50, 100, 200 or 250 amino acids in length. A fragment preferably comprises the pore-forming domain of SEQ ID NO: 4. Fragments typically include residues 119, 121, 135. 113 and 139 of SEQ ID NO: 4. 
     One or more amino acids may be alternatively or additionally added to the polypeptides described above. An extension may be provided at the amino terminus or carboxy terminus of the amino acid sequence of SEQ ID NO: 4 or a variant or fragment thereof. The extension may be quite short, for example from 1 to 10 amino acids in length. Alternatively, the extension may be longer, for example up to 50 or 100 amino acids. A carrier protein may be fused to a pore or variant. 
     As discussed above, a variant of SEQ ID NO: 4 is a subunit that has an amino acid sequence which varies from that of SEQ ID NO: 4 and which retains its ability to form a pore. A variant typically contains the regions of SEQ ID NO: 4 that are responsible for pore formation. The pore forming ability of α-HL, which contains a β-barrel, is provided by β-strands in each subunit. A variant of SEQ ID NO: 4 typically comprises the regions in SEQ ID NO: 4 that form β-strands. The amino acids of SEQ ID NO: 4 that form β-strands are discussed above. One or more modifications can be made to the regions of SEQ ID NO: 4 that form β-strands as long as the resulting variant retains its ability to form a pore. Specific modifications that can be made to the β-strand regions of SEQ ID NO: 4 are discussed above. 
     A variant of SEQ ID NO: 4 preferably includes one or more modifications, such as substitutions, additions or deletions, within its α-helices and/or loop regions. Amino acids that form α-helices and loops are discussed above. 
     The variant may be modified to assist its identification or purification as discussed above. 
     Pores derived from α-HL can be made as discussed above with reference to pores derived from Msp. 
     In some embodiments, the transmembrane protein pore is chemically modified. The pore can be chemically modified in any way and at any site. The transmembrane protein pore is preferably chemically modified by attachment of a molecule to one or more cysteines (cysteine linkage), attachment of a molecule to one or more lysines, attachment of a molecule to one or more non-natural amino acids, enzyme modification of an epitope or modification of a terminus. Suitable methods for carrying out such modifications are well-known in the art. The transmembrane protein pore may be chemically modified by the attachment of any molecule. For instance, the pore may be chemically modified by attachment of a dye or a fluorophore. 
     Any number of the monomers in the pore may be chemically modified. One or more, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10, of the monomers is preferably chemically modified as discussed above. 
     The reactivity of cysteine residues may be enhanced by modification of the adjacent residues. For instance, the basic groups of flanking arginine, histidine or lysine residues will change the pKa of the cysteines thiol group to that of the more reactive S −  group. The reactivity of cysteine residues may be protected by thiol protective groups such as dTNB. These may be reacted with one or more cysteine residues of the pore before a linker is attached. 
     The molecule (with which the pore is chemically modified) may be attached directly to the pore or attached via a linker as disclosed in International Application Nos. PCT/GB09/001,690 (published as WO 2010/004273), PCT/GB09/001,679 (published as WO 2010/004265) or PCT/GB10/000,133 (published as WO 2010/086603). 
     Any XPD helicase may be used in accordance with the invention. XPD helicases are also known as Rad3 helicases and the two terms can be used interchangeably. 
     The structures of XPD helicases are known in the art (Cell. 2008 May 30; 133(5):801-12. Structure of the DNA repair helicase XPD. Liu H, Rudolf J, Johnson K A, McMahon S A, Oke M, Carter L, McRobbie A M, Brown S E, Naismith J H, White M F). The XPD helicase typically comprises the amino acid motif X1-X2-X3-G-X4-X5-X6-E-G (hereinafter called XPD motif V; SEQ ID NO: 8). X1, X2, X5 and X6 are independently selected from any amino acid except D, E, K and R. X1, X2, X5 and X6 are independently selected from G, P, A, V, L, I, M, C, F, Y, W, H, Q, N, S and T. X1, X2, X5 and X6 are preferably not charged. X1, X2, X5 and X6 are preferably not H. X1 is more preferably V, L, I, S or Y. X5 is more preferably V, L, I, N or F. X6 is more preferably S or A. X3 and X4 may be any amino acid residue. X4 is preferably K, R or T. 
     The XPD helicase typically comprises the amino acid motif Q-Xa-Xb-G-R-Xc-Xd-R-(Xe) 3 -Xf-(Xg) 7 -D-Xh-R (hereinafter called XPD motif VI; SEQ ID NO: 9). Xa, Xe and Xg may be any amino acid residue. Xb, Xc and Xd are independently selected from any amino acid except D, E, K and R. Xb, Xc and Xd are typically independently selected from G, P, A, V, L, I, M, C, F, Y, W, H, Q, N, S and T. Xb, Xc and Xd are preferably not charged. Xb, Xc and Xd are preferably not H. Xb is more preferably V, A, L, I or M. Xc is more preferably V, A, L, I, M or C. Xd is more preferably I, H, L, F, M or V. Xf may be D or E. (Xg) 7  is X g1 , X g2 , X g3 , X g4 , X g5 , X g6  and X g7 . X g2  is preferably G, A, S or C. X g5  is preferably F, V, L, I, M, A, W or Y. X g6  is preferably L, F, Y, M, I or V. X g7  is preferably A, C, V, L, I, M or S. 
     The XPD helicase preferably comprises XPD motifs V and VI. The most preferred XPD motifs V and VI are shown in Table 5 below. 
     The XPD helicase preferably further comprises an iron sulphide (FeS) core between two Walker A and B motifs (motifs I and II). An FeS core typically comprises an iron atom coordinated between the sulphide groups of cysteine residues. The FeS core is typically tetrahedral. 
     The XPD helicase is preferably one of the helicases shown in Table 4 below or a variant thereof. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Preferred XPD helicases 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 YP 566221.1 
                 Rad3-related DNA helicases [ Methanococcoides burtonii  DSM 
               
               
                 2 
                 YP 003727831.1 
                 DEAD 2 domain-containing protein [ Methanohalobium   
               
               
                 3 
                 YP 004617026.1 
                 DEAD 2 domain-containing protein [ Methanosalsum zhilinae   
               
               
                 4 
                 YP 003541314.1 
                 DEAD/DEAH box helicase [ Methanohalophilus mahii D SM 
               
               
                 5 
                 NP 633611.1 
                 DNA repair helicase [ Methanosarcina mazei  Go1] 
               
               
                 6 
                 NP 615308.1 
                 DNA helicase RepD [ Methanosarcina acetivorans  C2A] 
               
               
                 7 
                 YP 304917.1 
                 DNA helicase RepD [ Methanosarcina barkeri  str.  Fusaro ] 
               
               
                 8 
                 YP 686711.1 
                 putative ATP-dependent DNA-repair helicase [uncultured 
               
               
                 9 
                 YP 003355887.1 
                 DNA repair helicase [ Methanocella paludicola  SANAE] 
               
               
                 10 
                 ZP 08042768.1 
                 DEAD 2 domain protein [ Haladaptatus paucihalophilus   
               
               
                 11 
                 YP 003401909.1 
                 DEAD 2 domain protein [ Haloterrigena turkmenica  DSM 5511] 
               
               
                 12 
                 YP 004035231.1 
                 DNA helicase, rad3 [ Halogeometricum borinquense  DSM 
               
               
                 13 
                 YP 003534113.1 
                 helicase [ Haloferax volcanii  DS2] &gt;gb|ADE05091.1|helicase 
               
               
                 14 
                 YP 003176661.1 
                 DEAD 2 domain protein [ Halomicrobium mukohataei  DSM 
               
               
                 15 
                 YP 326312.1 
                 DNA repair helicase-like protein [ Natronomonas pharaonis   
               
               
                 16 
                 YP 004595879.1 
                 DEAD 2 domain-containing protein [ Halopiger xanaduensis   
               
               
                 17 
                 ZP 08967673.1 
                 DEAD 2 domain protein [ Natronobacterium gregoryi  SP2] 
               
               
                 18 
                 YP 003735388.1 
                 DEAD 2 domain-containing protein [ Halalkalicoccus jeotgali   
               
               
                 19 
                 YP 003479905.1 
                 DEAD 2 domain-containing protein [ Natrialba magadii  ATCC 
               
               
                 20 
                 YP 003130325.1 
                 DEAD 2 domain protein [ Halorhabdus utahensis  DSM 12940] 
               
               
                 21 
                 YP 002567268.1 
                 DEAD 2 domain protein [ Halorubrum lacusprofundi  ATCC 
               
               
                 22 
                 ZP 08561264.1 
                 DEAD 2 domain protein [ Halorhabdus tiamatea  SARL4B] 
               
               
                 23 
                 YP 004794633.1 
                 helicase [ Haloarcula hispanica  ATCC 33960] &gt;gb|AEM55645.1| 
               
               
                 24 
                 YP 137192.1 
                 helicase [ Haloarcula marismortui  ATCC 43049] 
               
               
                 25 
                 ZP 08963818.1 
                 helicase c2 [ Natrinema pellirubrum  DSM 15624] 
               
               
                 26 
                 NP 281042.1 
                 helicase [ Halobacterium  sp. NRC-1] &gt;ref|YP 001690174.1| 
               
               
                 27 
                 YP 004808929.1 
                 DEAD 2 domain-containing protein [halophilic archaeon 
               
               
                 28 
                 CCC41858.1 
                 DNA repair helicase Rad3 [ Haloquadratum walsbyi  C23] 
               
               
                 29 
                 YP 659380.1 
                 DNA repair helicase-like protein [ Haloquadratum walsbyi  DSM 
               
               
                 30 
                 YP 686810.1 
                 putative ATP-dependent helicase [uncultured methanogenic 
               
               
                 31 
                 ZP 09027753.1 
                 Helicase-like, DEXD box c2 type [ Halobacterium  sp. DL1] 
               
               
                 32 
                 ZP 08042627.1 
                 helicase [ Haladaptatus paucihalophilus  DX253] 
               
               
                 33 
                 YP 002565485.1 
                 helicase c2 [ Halorubrum lacusprofundi  ATCC 49239] 
               
               
                 34 
                 YP 003737184.1 
                 helicase [ Halalkalicoccus jeotgali  B3] &gt;gb|ADJ15392.1|helicase 
               
               
                 35 
                 YP 003480051.1 
                 helicase c2 [ Natrialba magadii  ATCC 43099] &gt;gb|ADD05489.1| 
               
               
                 36 
                 YP 004808332.1 
                 helicase c2 [ halophilic archaeon  DL31] &gt;gb|AEN05959.1| 
               
               
                 37 
                 ZP 08967636.1 
                 helicase c2 [ Natronobacterium gregoryi  SP2] &gt;gb|EHA70214.1| 
               
               
                 38 
                 ZP 09028751.1 
                 type III restriction protein res subunit [ Halobacterium  sp. DL1] 
               
               
                 39 
                 YP 326140.1 
                 DNA repair helicase [ Natronomonas pharaonis  DSM 2160] 
               
               
                 40 
                 YP 004595921.1 
                 helicase c2 [ Halopiger xanaduensis  SH-6] &gt;gb|AEH36042.1| 
               
               
                 41 
                 ZP 08963989.1 
                 helicase c2 [ Natrinema pellirubrum  DSM 15624] 
               
               
                 42 
                 ZP 08560622.1 
                 helicase [ Halorhabdus tiamatea  SARL4B] &gt;ref|ZP 08560903.1| 
               
               
                 43 
                 YP 003405301.1 
                 helicase c2 [ Haloterrigena turkmenica  DSM 5511] 
               
               
                 44 
                 ZP 08558438.1 
                 helicase c2 [ Halorhabdus tiamatea  SARL4B] &gt;gb|EGM36622.1| 
               
               
                 45 
                 YP 003131362.1 
                 helicase c2 [ Halorhabdus utahensis  DSM 12940] 
               
               
                 46 
                 EHF09015.1 
                 DEAD 2 domain protein [ Methanolinea tarda N OBI-1] 
               
               
                 47 
                 YP 001030540.1 
                 queuine tRNA-ribosyltransferase [ Methanocorpusculum   
               
               
                 48 
                 YP 135387.1 
                 helicase [ Haloarcula marismortui  ATCC 43049] 
               
               
                 49 
                 YP 004795935.1 
                 helicase [ Haloarcula hispanica  ATCC 33960] &gt;gb|AEM56947.1| 
               
               
                 50 
                 YP 003178787.1 
                 helicase c2 [ Halomicrobium mukohataei  DSM 12286] 
               
               
                 51 
                 YP 003895110.1 
                 DEAD 2 domain-containing protein [ Methanoplanus   
               
               
                 52 
                 ABQ75766.1 
                 DNA repair helicase Rad3 [uncultured haloarchaeon] 
               
               
                 53 
                 YP 003535402.1 
                 helicase [ Haloferax volcanii  DS2] &gt;gb|ADE04644.1|helicase 
               
               
                 54 
                 YP 502434.1 
                 helicase c2 [ Methanospirillum hungatei  JF-1] &gt;gb|ABD40715.1| 
               
               
                 55 
                 CCC40051.1 
                 DNA repair helicase Rad3 [ Haloquadratum walsbyi  C23] 
               
               
                 56 
                 YP 657741.1 
                 DNA repair helicase Rad3 [ Haloquadratum walsbyi  DSM 
               
               
                 57 
                 YP 004424267.1 
                 DNA repair helicase rad3 [ Pyrococcus  sp. NA2] 
               
               
                 58 
                 NP 578662.1 
                 DNA repair helicase rad3, putative [ Pyrococcus furiosus  DSM 
               
               
                 59 
                 BAB59155.1 
                 DNA repair helicase [ Thermoplasma volcanium  GSS1] 
               
               
                 60 
                 NP 110532.1 
                 Rad3-related DNA helicase [ Thermoplasma volcanium  GSS1] 
               
               
                 61 
                 YP 004036893.1 
                 DNA helicase, rad3 [ Halogeometricum borinquense  DSM 
               
               
                 62 
                 CBH38285.1 
                 putative helicase [uncultured archaeon] 
               
               
                 63 
                 AAU82137.1 
                 conserved hypothetical protein [uncultured archaeon 
               
               
                 64 
                 YP 844078.1 
                 helicase c2 [ Methanosaeta thermophila  PT] &gt;gb|ABK15438.1| 
               
               
                 65 
                 NP 127020.1 
                 DNA repair helicase rad3 [ Pvrococcus abyssi  GE5] 
               
               
                 66 
                 YP 001404278.1 
                 DEAD 2 domain-containing protein [ Candidatus   
               
               
                 67 
                 NP 280229.1 
                 helicase [ Halobacterium  sp. NRC-1] &gt;ref|YP 001689339.1| 
               
               
                 68 
                 YP 002994616.1 
                 ERCC2/XPD/Rad3-related DNA repair helicase [ Thermococcus   
               
               
                 69 
                 NP 393536.1 
                 Rad3-related DNA helicase [ Thermoplasma acidophilum  DSM 
               
               
                 70 
                 YP 004071335.1 
                 DNA repair Rad3-like helicase [ Thermococcus barophilus  MP] 
               
               
                 71 
                 YP 002307425.1 
                 ERCC2/XPD/Rad3-related DNA repair helicase [ Thermococcus   
               
               
                 72 
                 YP 002466730.1 
                 DEAD 2 domain protein [ Methanosphaerula palustris  E1-9c] 
               
               
                 73 
                 ZP 04876197.1 
                 DEAD 2 family [ Aciduliprofundum boonei  T469] 
               
               
                 74 
                 YP 004623247.1 
                 DNA repair helicase rad3 [ Pyrococcus yayanosii  CH1] 
               
               
                 75 
                 YP 002583205.1 
                 DNA repair helicase Rad3 [ Thermococcus  sp. AM4] 
               
               
                 76 
                 YP 004763640.1 
                 ERCC2/XPD/Rad3-related DNA repair helicase [ Thermococcus   
               
               
                 77 
                 ZP 08857944.1 
                 hypothetical protein HMPREF9022 03601 [ Erysipelotrichaceae   
               
               
                 78 
                 ZP 04875144.1 
                 DEAD 2 family [ Aciduliprofundum boonei  T469] 
               
               
                 79 
                 ZP 07832784.1 
                 DEAD2 domain protein [ Clostridium  sp. HGF2] 
               
               
                 80 
                 Q9HM14.1 
                 RecName: Full = ATP-dependent DNA helicase Ta0057 
               
               
                 81 
                 XP 001736798.1 
                 regulator of telomere elongation helicase 1 rtel1 [ Entamoeba   
               
               
                 82 
                 YP 004038234.1 
                 DEAD 2 domain-containing protein [ Halogeometricum   
               
               
                 83 
                 EET89549.1 
                 DEAD 2 domain protein [ Candidatus Micrarchaeum   
               
               
                 84 
                 NP 142644.1 
                 hypothetical protein PH0697 [ Pyrococcus horikoshii  OT3] 
               
               
                 85 
                 EGQ39936.1 
                 Rad3-related DNA helicase [ Candidatus Nanosalinarum  sp. 
               
               
                 86 
                 NP 348298.1 
                 Rad3-related DNA helicase [ Clostridium acetobutylicum  ATCC 
               
               
                 87 
                 ZP 05426745.1 
                 helicase [ Enterococcus faecalis  T2] &gt;gb|EET99653.1|helicase 
               
               
                 88 
                 EFU12338.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX1341] 
               
               
                 89 
                 ZP 07559866.1 
                 DEAD 2 protein [ Enterococcus faecalis TX0860]   
               
               
                 90 
                 ZP 02638747.1 
                 putative ATP-dependent helicase [ Clostridium perfringens  CPE 
               
               
                 91 
                 EGQ43484.1 
                 Rad3-related DNA helicase [ Candidatus Nanosalina  sp. 
               
               
                 92 
                 YP 695853.1 
                 putative ATP-dependent helicase [ Clostridium perfringens   
               
               
                 93 
                 XP 651401.1 
                 DNA repair helicase [ Entamoeba histolytica  HM-1: IMSS] 
               
               
                 94 
                 YP 004094073.1 
                 helicase c2 [ Bacillus cellulosilyticus  DSM 2522] 
               
               
                 95 
                 ZP 02635303.1 
                 putative ATP-dependent helicase [ Clostridium perfringens  B str. 
               
               
                 96 
                 YP 002459406.1 
                 DEAD/DEAH box helicase [ Desulfitobacterium hafniense   
               
               
                 97 
                 ZP 02631448.1 
                 putative ATP-dependent helicase [ Clostridium perfringens  E str. 
               
               
                 98 
                 YP 183197.1 
                 ERCC2/XPD/Rad3-related DNA repair helicase [ Thermococcus   
               
               
                 99 
                 XP 003386532.1 
                 PREDICTED: TFIIH basal transcription factor complex helicase 
               
               
                 100 
                 YP 698539.1 
                 ATP-dependent helicase [ Clostridium perfringens  SM101] 
               
               
                 101 
                 ZP 06870390.1 
                 ATP-dependent helicase [ Fusobacterium nucleatum  subsp. 
               
               
                 102 
                 YP 003851460.1 
                 DEAD 2 domain-containing protein [ Thermoanaerobacterium   
               
               
                 103 
                 YP 001046704.1 
                 helicase c2 [ Methanoculleus marisnigri  JR1] &gt;gb|ABN56722.1| 
               
               
                 104 
                 ZP 01860521.1 
                 probable helicase protein [ Bacillus  sp. SG-1] &gt;gb|EDL64380.1| 
               
               
                 105 
                 YP 518026.1 
                 hypothetical protein DSY1793 [ Desulfitobacterium hafniense   
               
               
                 106 
                 NP 562113.1 
                 helicase protein [ Clostridium perfringens  str. 13] 
               
               
                 107 
                 ZP 02643466.1 
                 putative ATP-dependent helicase [ Clostridium perfringens   
               
               
                 108 
                 NP 603640.1 
                 ATP-dependent helicase DinG [ Fusobacterium nucleatum  subsp. 
               
               
                 109 
                 XP 003488158.1 
                 PREDICTED: Fanconi anemia group J protein-like [ Bombus   
               
               
                 110 
                 ZP 05562829.1 
                 helicase [ Enterococcus faecalis  DS5] &gt;ref|ZP 05572349.1| 
               
               
                 111 
                 AEA93627.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 112 
                 ZP 07571029.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX0411] 
               
               
                 113 
                 ZP 07550093.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX4248] 
               
               
                 114 
                 ZP 07758962.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX0470] 
               
               
                 115 
                 EFT88579.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX2141] 
               
               
                 116 
                 ZP 05472340.1 
                 helicase [ Anaerococcus vaginalis  ATCC 51170] 
               
               
                 117 
                 ZP 06629187.1 
                 putative helicase [ Enterococcus faecalis  R712] 
               
               
                 118 
                 EFT95025.1 
                 Dead 2 protein [ Enterococcus faecalis  TX0012] 
               
               
                 119 
                 ZP 03982997.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 120 
                 ZP 05595984.1 
                 DEAD 2 family protein [ Enterococcus faecalis  T11] 
               
               
                 121 
                 ZP 05566414.1 
                 DEAD 2 [ Enterococcus faecalis  Merz96] &gt;gb|EEU69371.1| 
               
               
                 122 
                 ZP 05592774.1 
                 helicase [ Enterococcus faecalis  AR01/DG] &gt;gb|EEU87568.1| 
               
               
                 123 
                 EFU14918.1 
                 DEAD 2 protein [ Enterococcus faecalis TX1342] 
               
               
                 124 
                 ZP 05599911.1 
                 helicase [ Enterococcus faecalis  X98] &gt;gb|EEU94705.1|helicase 
               
               
                 125 
                 ZP 05569608.1 
                 DEAD 2 [ Enterococcus faecalis  HIP11704] &gt;gb|EEU72565.1| 
               
               
                 126 
                 NP 814892.1 
                 helicase [ Enterococcus faecalis  V583] &gt;gb|AAO80962.1| 
               
               
                 127 
                 ZP 05423702.1 
                 helicase [ Enterococcus faecalis  T1] &gt;gb|EET96610.1|helicase 
               
               
                 128 
                 ZP 04439070.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 129 
                 ZP 05579939.1 
                 DNA repair helicase [ Enterococcus faecalis  Fly1] 
               
               
                 130 
                 ZP 05558182.1 
                 DEAD 2 protein [ Enterococcus faecalis  T8] &gt;gb|EEU26309.1| 
               
               
                 131 
                 ZP 04971104.1 
                 ATP-dependent helicase [ Fusobacterium nucleatum  subsp. 
               
               
                 132 
                 ZP 05577237.1 
                 DNA repair helicase [ Enterococcus faecalis  E1Sol] 
               
               
                 133 
                 ZP 03948465.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 134 
                 ZP 05502750.1 
                 helicase [ Enterococcus faecalis  T3] &gt;gb|EEU23116.1|helicase 
               
               
                 135 
                 ZP 04434884.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 136 
                 ZP 08170094.1 
                 DEAD2 domain protein [ Anaerococcus hydrogenalis  ACS-025- 
               
               
                 137 
                 EGG57994.1 
                 DEAD2 domain protein [ Enterococcus faecalis  TX1467] 
               
               
                 138 
                 EFT39574.1 
                 DEAD 2 protein [ Enterococcus faecalis TX2137] 
               
               
                 139 
                 ZP 05580877.1 
                 DEAD 2 domain-containing protein [ Enterococcus faecalis  D6] 
               
               
                 140 
                 ZP 07761503.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX0635] 
               
               
                 141 
                 ZP 05583934.1 
                 helicase [ Enterococcus faecalis  CH188] &gt;gb|EEU84905.1| 
               
               
                 142 
                 ZP 07568518.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX0109] 
               
               
                 143 
                 XP 001943653.1 
                 PREDICTED: TFIIH basal transcription factor complex helicase 
               
               
                 144 
                 XP 003397282.1 
                 PREDICTED: Fanconi anemia group J protein homolog 
               
               
                 145 
                 AAB62733.1 
                 RepD [ Dictyostelium discoideum]   
               
               
                 146 
                 XP 647302.1 
                 transcription factor IIH component [ Dictyostelium discoideum   
               
               
                 147 
                 ZP 04666640.1 
                 DEAD 2 domain-containing protein [ Clostridiales bacterium   
               
               
                 148 
                 ZP 05475592.1 
                 helicase [ Enterococcus faecalis  ATCC 4200] &gt;gb|EEU17449.1| 
               
               
                 149 
                 XP 002172261.1 
                 DNA repair helicase RAD3 [ Schizosaccharomyces iaponicus   
               
               
                 150 
                 YP 002959163.1 
                 DNA repair helicase, ERCC2/XPD/rad3/TFIIH helicase beta 
               
               
                 151 
                 ZP 08958775.1 
                 hypothetical protein HAL1 05693 [ Halomonas  sp. HAL1] 
               
               
                 152 
                 ZP 07555075.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX0855] 
               
               
                 153 
                 ZP 06600298.1 
                 ATP-dependent helicase [ Fusobacterium periodonticum  ATCC 
               
               
                 154 
                 ZP 07106135.1 
                 DEAD2 domain protein [ Enterococcus faecalis  TUSoD Ef11] 
               
               
                 155 
                 NP 593025.1 
                 transcription factor TFIIH complex subunit Rad15 
               
               
                 156 
                 XP 001605333.2 
                 PREDICTED: TFIIH basal transcription factor complex helicase 
               
               
                 157 
                 CAA45870.1 
                 rhp3+ [ Schizosaccharomyces pombe ] 
               
               
                 158 
                 CAA43022.1 
                 rad15 [ Schizosaccharomyces pomb e] 
               
               
                 159 
                 EGG04750.1 
                 hypothetical protein MELLADRAFT 108080 [ Melampsora   
               
               
                 160 
                 EEH20175.1 
                 TFIIH basal transcription factor complex helicase subunit 
               
               
                 161 
                 ZP 02430992.1 
                 hypothetical protein CLOSCI 01208 [ Clostridium scindens   
               
               
                 162 
                 ZP 08601936.1 
                 hypothetical protein HMPREF0993 01313 [ Lachnospiraceae   
               
               
                 163 
                 XP 780825.2 
                 PREDICTED: similar to TFIIH basal transcription factor 
               
               
                 164 
                 XP 003307004.1 
                 hypothetical protein PTT 20325 [ Pyrenophora teres  f.  teres  0-1] 
               
               
                 165 
                 ADD93161.1 
                 hypothetical protein [uncultured archaeon MedDCM-OCT-S05- 
               
               
                 166 
                 EFW43568.1 
                 nucleotide excision repair protein [ Capsaspora owczarzaki   
               
               
                 167 
                 ZP 03759096.1 
                 hypothetical protein CLOSTASPAR 03119 [ Clostridium   
               
               
                 168 
                 YP 003246012.1 
                 DEAD 2 domain-containing protein [ Paenibacillus  sp. 
               
               
                 169 
                 AAA85822.1 
                 ERCC2/XPD [ Xiphophorus maculatus ] 
               
               
                 170 
                 XP 002796308.1 
                 DNA repair helicase RAD3 [ Paracoccidioides brasiliensis  Pb01] 
               
               
                 171 
                 EFT92378.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX4244] 
               
               
                 172 
                 EGF80866.1 
                 hypothetical protein BATDEDRAFT 29908 [ Batrachochytrium   
               
               
                 173 
                 ZP 07897118.1 
                 DEAD 2 domain protein [ Paenibacillus vortex  V453] 
               
               
                 174 
                 CBQ68748.1 
                 probable RAD3-DNA helicase/ATPase [ Sporisorium reilianum   
               
               
                 175 
                 EGP86663.1 
                 hypothetical protein MYCGRDRAFT 43883 [ Mycosphaerella   
               
               
                 176 
                 EGW30235.1 
                 DNA helicase component of transcription factor b [ Spathaspora   
               
               
                 177 
                 ZP 03166460.1 
                 hypothetical protein RUMLAC 00106 [ Ruminococcus lactaris   
               
               
                 178 
                 YP 003152744.1 
                 DEAD 2 domain-containing protein [ Anaerococcus prevotii   
               
               
                 179 
                 ZP 06750962.1 
                 ATP-dependent helicase. DinG family [ Fusobacterium  sp. 
               
               
                 180 
                 ZP 07670993.1 
                 putative helicase [ Erysipelotrichaceae bacterium  3 1 53] 
               
               
                 181 
                 ZP 04572193.1 
                 ATP-dependent helicase DinG [ Fusobacterium  sp. 4 1 13] 
               
               
                 182 
                 ZP 06673551.1 
                 DNA helicase [ Enterococcus faecium E1039] &gt;gb|EFF33221.1|   
               
               
                 183 
                 EFU18615.1 
                 DEAD 2 protein [ Enterococcus faecalis  TX1346] 
               
               
                 184 
                 EGG20082.1 
                 transcription factor IIH component [ Dictyostelium fasciculatum ] 
               
               
                 185 
                 YP 002944692.1 
                 DEAD 2 domain-containing protein [ Variovorax paradoxus   
               
               
                 186 
                 EHA50547.1 
                 DNA repair helicase rad15 [ Magnaporthe oryzae  70-15] 
               
               
                 187 
                 EEQ84204.1 
                 DNA repair helicase RAD3 [ Ajellomyces dermatitidis  ER-3] 
               
               
                 188 
                 CCE41018.1 
                 hypothetical protein CPAR2 300070 [ Candida parapsilosis ] 
               
               
                 189 
                 ZP 05550498.1 
                 ATP-dependent helicase [ Fusobacterium  sp. 3 1 36A2] 
               
               
                 190 
                 XP 003292674.1 
                 hypothetical protein DICPUDRAFT 50564 [ Dictyostelium   
               
               
                 191 
                 YP 003821569.1 
                 helicase c2 [ Clostridium saccharolyticum  WM1] 
               
               
                 192 
                 XP 002550658.1 
                 DNA repair helicase RAD3 [ Candida tropicalis  MYA-3404] 
               
               
                 193 
                 ZP 08689728.1 
                 ATP-dependent helicase [ Fusobacterium  sp. 2 1 31] 
               
               
                 194 
                 XP 002499882.1 
                 predicted protein [ Micromonas  sp. RCC299] &gt;gb|ACO61140.1] 
               
               
                 195 
                 ZP 06747464.1 
                 ATP-dependent helicase, DinG family [ Fusobacterium  sp. 
               
               
                 196 
                 CBK74961.1 
                 Rad3-related DNA helicases [ Butyrivibrio fibrisolvens  16/4] 
               
               
                 197 
                 ABZ07948.1 
                 putative DEAD 2 [uncultured marine microorganism 
               
               
                 198 
                 EEH08589.1 
                 DNA repair helicase RAD3 [ Ajellomyces capsulatus  G186AR] 
               
               
                 199 
                 YP 003959319.1 
                 hypothetical protein ELI 1370 [ Eubacterium limosum  KIST612] 
               
               
                 200 
                 XP 001727494.1 
                 DNA repair helicase rad15 [ Aspergillus oryzae  RIB40] 
               
               
                 201 
                 NP 001133411.1 
                 TFIIH basal transcription factor complex helicase subunit 
               
               
                 202 
                 EGM49944.1 
                 DEAD2 domain protein [ Lactobacillus salivarius  GJ-24] 
               
               
                 203 
                 ADJ78619.1 
                 Superfamily II DNA and RNA helicase [ Lactobacillus salivarius   
               
               
                 204 
                 ZP 04009603.1 
                 superfamily II DNA/RNA helicase [ Lactobacillus salivarius   
               
               
                 205 
                 XP 003456443.1 
                 PREDICTED: TFIIH basal transcription factor complex helicase 
               
               
                 206 
                 ZP 05922403.1 
                 helicase [ Enterococcus faecium  TC 6] &gt;ref|ZP 06446725.1] 
               
               
                 207 
                 ZP 00604560.1 
                 DEAD 2 [ Enterococcus faecium  DO] &gt;ref|ZP 05657789.1] 
               
               
                 208 
                 ADY43014.1 
                 TFIIH basal transcription factor complex helicase XPD subunit, 
               
               
                 209 
                 ZP 06680717.1 
                 DNA helicase [ Enterococcus faecium  E1071] &gt;gb|EFF19797.1] 
               
               
                 210 
                 XP 001210680.1 
                 DNA repair helicase RAD3 [ Aspergillus terreus  NIH2624] 
               
               
                 211 
                 ZP 05664394.1 
                 DEAD 2 helicase [ Enterococcus faecium  1,231,501] 
               
               
                 212 
                 ZP 07872333.1 
                 helicase c2 domain-containing protein [ Listeria ivanovii  FSL F6- 
               
               
                 213 
                 XP 002375772.1 
                 TFIIH complex helicase Rad3, putative [ Aspergillus flavus   
               
               
                 214 
                 XP 002995821.1 
                 hypothetical protein NCER 101193 [ Nosema ceranae  BRL01] 
               
               
                 215 
                 EGE80033.1 
                 DNA repair helicase RAD3 [ Ajellomyces dermatitidis  ATCC 
               
               
                 216 
                 XP 001930685.1 
                 TFIIH basal transcription factor complex helicase subunit 
               
               
                 217 
                 ZP 05678362.1 
                 DEAD 2 helicase [ Enterococcus faecium  Com15] 
               
               
                 218 
                 ZP 08193463.1 
                 helicase c2 [ Clostridium papyrosolvens  DSM 2782] 
               
               
                 219 
                 XP 002432433.1 
                 TFIIH basal transcription factor complex helicase subunit, 
               
               
                 220 
                 ZP 08278913.1 
                 DEAD2 domain protein [ Paenibacillus  sp. HGF5] 
               
               
                 221 
                 EFY84841.1 
                 DNA repair helicase RAD3 [ Metarhizium acridum  CQMa 102] 
               
               
                 222 
                 EFY99240.1 
                 DNA repair helicase RAD3 [ Metarhizium anisopliae  ARSEF 
               
               
                 223 
                 XP 001638817.1 
                 predicted protein [ Nematostella vectensis ] &gt;gb|EDO46754.1] 
               
               
                 224 
                 ZP 08401953.1 
                 helicase c2 [ Rubrivivax benzoatilyticus  JA2] &gt;gb|EGJ10286.1] 
               
               
                 225 
                 YP 004886930.1 
                 hypothetical protein TEH 14390 [ Tetragenococcus halophilus   
               
               
                 226 
                 XP 001749760.1 
                 hypothetical protein [ Monosiga brevicollis  MX1] 
               
               
                 227 
                 YP 535245.1 
                 superfamily II DNA/RNA helicase [ Lactobacillus salivarius   
               
               
                 228 
                 XP 002126055.1 
                 PREDICTED: similar to ERCC2/XPD gene product [ Ciona   
               
               
                 229 
                 EGC43631.1 
                 DNA repair helicase RAD3 [ Ajellomyces capsulatus  H88] 
               
               
                 230 
                 ZP 07207225.1 
                 DEAD2 domain protein [ Lactobacillus salivarius  ACS-116-V- 
               
               
                 231 
                 EEH44570.1 
                 DNA repair helicase RAD3 [ Paracoccidioides brasiliensis  Pb18] 
               
               
                 232 
                 ZP 03981356.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 233 
                 EFX74423.1 
                 hypothetical protein DAPPUDRAFT 324413 [ Daphnia pulex ] 
               
               
                 234 
                 ZP 06696925.1 
                 DNA helicase [ Enterococcus faecium  E1679] &gt;gb|EFF27693.1] 
               
               
                 235 
                 YP 608233.1 
                 hypothetical protein PSEEN2644 [ Pseudomonas entomophila   
               
               
                 236 
                 ZP 08007354.1 
                 hypothetical protein HMPREF1013 03969 [ Bacillus  sp. 
               
               
                 237 
                 CBF88902.1 
                 TPA: 5′ to 3′ DNA helicase (Eurofung) [ Aspergillus nidulans   
               
               
                 238 
                 ZP 03952877.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 239 
                 EGU74151.1 
                 hypothetical protein FOXB 15338 [ Fusarium oxysporum   
               
               
                 240 
                 EER42275.1 
                 DNA repair helicase RAD3 [ Ajellomyces capsulatus  H143] 
               
               
                 241 
                 ZP 06682941.1 
                 DNA helicase [ Enterococcus faecium  E980] &gt;gb|EFF37253.1] 
               
               
                 242 
                 XP 001799767.1 
                 hypothetical protein SNOG 09475 [ Phaeosphaeria nodorum   
               
               
                 243 
                 ZP 06623685.1 
                 DEAD2 domain protein [ Enterococcus faecium  PC4.1] 
               
               
                 244 
                 XP 002850254.1 
                 DNA repair helicase RAD3 [ Arthroderma otae  CBS 113480] 
               
               
                 245 
                 ZP 03939707.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 246 
                 ZP 03942652.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 247 
                 ZP 05667230.1 
                 DEAD 2 helicase [ Enterococcus faecium  1,141,733] 
               
               
                 248 
                 ZP 08142092.1 
                 DEAD 2 domain-containing protein [ Pseudomonas  sp. TJI-51] 
               
               
                 249 
                 EGL98319.1 
                 DinG family ATP-dependent helicase [ Lactobacillus salivarius   
               
               
                 250 
                 EGX45813.1 
                 hypothetical protein AOL s00117g18 [ Arthrobotrys oligospora   
               
               
                 251 
                 YP 001374582.1 
                 bifunctional ATP-dependent DNA helicase/DNA polymerase III 
               
               
                 252 
                 XP 384471.1 
                 hypothetical protein FG04295.1 [ Gibberella zeae  PH-1] 
               
               
                 253 
                 XP 002610234.1 
                 hypothetical protein BRAFLDRAFT 286830 [ Branchiostoma   
               
               
                 254 
                 ZP 05103122.1 
                 hypothetical protein MDMS009 258 [ Methylophaga   
               
               
                 255 
                 XP 002177848.1 
                 xeroderma pigmentosum group D complementing protein 
               
               
                 256 
                 YP 901517.1 
                 DEAD 2 domain-containing protein [ Pelobacter propionicus   
               
               
                 257 
                 YP 004646156.1 
                 Rtel-1 [ Paenibacillus mucilaginosus  KNP414] &gt;gb|AEI46286.1] 
               
               
                 258 
                 EHE97994.1 
                 hypothetical protein HMPREF9469 03327 [ Clostridium   
               
               
                 259 
                 ZP 07913491.1 
                 ATP-dependent helicase DinG [ Fusobacterium gonidiaformans   
               
               
                 260 
                 XP 001264365.1 
                 TFIIH complex helicase Rad3, putative [ Neosartorya fischeri   
               
               
                 261 
                 CBK81746.1 
                 Rad3-related DNA helicases [ Coprococcus catus  GD/7] 
               
               
                 262 
                 NP 001104820.1 
                 FancJ-like protein [ Bombyx mori ] &gt;dbi|BAF94023.1|FancJ-like 
               
               
                 263 
                 EFN69563.1 
                 TFIIH basal transcription factor complex helicase subunit 
               
               
                 264 
                 EGX89191.1 
                 DNA repair helicase RAD3 [ Cordyceps militaris  CM01] 
               
               
                 265 
                 YP 003463379.1 
                 hypothetical protein lse 0136 [ Listeria seeligeri  serovar 1/2b str. 
               
               
                 266 
                 ZP 04851424.1 
                 DEAD 2 domain-containing protein [ Paenibacillus  sp. oral 
               
               
                 267 
                 ZP 06525127.1 
                 ATP-dependent helicase DinG [ Fusobacterium  sp. D11] 
               
               
                 268 
                 ZP 08598770.1 
                 ATP-dependent helicase, DinG family [ Fusobacterium  sp. 
               
               
                 269 
                 YP 004155261.1 
                 helicase c2 [ Variovorax paradoxus  EPS] &gt;gb|ADU37150.1| 
               
               
                 270 
                 CBX95892.1 
                 similar to TFIIH basal transcription factor complex helicase 
               
               
                 271 
                 EFQ33459.1 
                 DNA repair helicase [ Glomerella graminicola  M1.001] 
               
               
                 272 
                 ZP 07775478.1 
                 DEAD 2 [ Pseudomonas fluorescens  WH6] &gt;gb|EFQ63207.1] 
               
               
                 273 
                 ZP 05814223.1 
                 ATP-dependent helicase [ Fusobacterium  sp. 3 1 33] 
               
               
                 274 
                 ADD93162.1 
                 hypothetical protein [uncultured archaeon MedDCM-OCT-S05- 
               
               
                 275 
                 YP 004383205.1 
                 hypothetical protein MCON 0548 [ Methanosaeta concilii  GP6] 
               
               
                 276 
                 ZP 08687376.1 
                 ATP-dependent helicase DinG [ Fusobacterium mortiferum   
               
               
                 277 
                 XP 752761.1 
                 TFIIH complex helicase Rad3 [ Aspergillus fumigatus  Af293] 
               
               
                 278 
                 YP 004701945.1 
                 DEAD 2 domain-containing protein [ Pseudomonas putida  S16] 
               
               
                 279 
                 XP 505677.1 
                 YALI0F20746p [ Yarrowia lipolytica ] &gt;emb|CAG78486.1| 
               
               
                 280 
                 YP 004399302.1 
                 DEAD 2 domain-containing protein [ Lactobacillus buchneri   
               
               
                 281 
                 YP 003278262.1 
                 hypothetical protein CtCNB1 2220 [ Comamonas testosteroni   
               
               
                 282 
                 XP 003464723.1 
                 PREDICTED: TFIIH basal transcription factor complex helicase 
               
               
                 283 
                 XP 003048472.1 
                 predicted protein [ Nectria haematococca  mpVI 77-13-4] 
               
               
                 284 
                 XP 760298.1 
                 hypothetical protein UM04151.1 [ Ustilago maydis  521] 
               
               
                 285 
                 YP 004840690.1 
                 hypothetical protein LSA 03010 [ Lactobacillus sanfranciscensis   
               
               
                 286 
                 YP 002872713.1 
                 hypothetical protein PFLU3138 [ Pseudomonas fluorescens   
               
               
                 287 
                 YP 004853959.1 
                 putative ATP dependent helicase [ Listeria ivanovii  subsp. 
               
               
                 288 
                 XP 001324631.1 
                 helicase [ Trichomonas vaginalis  G3] &gt;gb|EAY12408.1| 
               
               
                 289 
                 ZP 08581874.1 
                 hypothetical protein HMPREF0404 01165 [ Fusobacterium  sp. 
               
               
                 290 
                 EFS01538.1 
                 helicase c2 domain-containing protein [ Listeria seeligeri  FSL 
               
               
                 291 
                 XP 001189997.1 
                 PREDICTED: similar to TFIIH basal transcription factor 
               
               
                 292 
                 XP 360589.2 
                 hypothetical protein MGG 03132 [ Magnaporthe oryzae  70-15] 
               
               
                 293 
                 EGR44274.1 
                 DNA excision repair helicase [ Trichoderma reesei  QM6a] 
               
               
                 294 
                 XP 002115878.1 
                 hypothetical protein TRIADDRAFT 30066 [ Trichoplax   
               
               
                 295 
                 YP 002140828.1 
                 DNA helicase, DEAD 2 domain-containing protein [ Geobacter   
               
               
                 296 
                 CBK93524.1 
                 DEAD 2 [ Eubacterium rectale  M104/1] 
               
               
                 297 
                 CBK90099.1 
                 DEAD 2 [ Eubacterium rectale  DSM 17629] 
               
               
                 298 
                 ZP 04574821.1 
                 ATP-dependent helicase DinG [ Fusobacterium  sp. 7 1] 
               
               
                 299 
                 XP 001500524.3 
                 PREDICTED: LOW QUALITY PROTEIN: TFIIH basal 
               
               
                 300 
                 EFR92206.1 
                 helicase c2 domain-containing protein [ Listeria innocua  FSL S4- 
               
               
                 301 
                 NP 967260.1 
                 ATP dependent helicase [ Bdellovibrio bacteriovorus  HD100] 
               
               
                 302 
                 ZP 06645172.1 
                 putative helicase [ Erysipelotrichaceae bacterium  5 2 54FAA] 
               
               
                 303 
                 YP 002937404.1 
                 putative ATP-dependent DNA-repair helicase [ Eubacterium   
               
               
                 304 
                 AEO61831.1 
                 hypothetical protein MYCTH 104059 [ Myceliophthora   
               
               
                 305 
                 XP 002723661.1 
                 PREDICTED: excision repair cross-complementing rodent 
               
               
                 306 
                 ZP 07928170.1 
                 helicase c2 [ Fusobacterium ulcerans  ATCC 49185] 
               
               
                 307 
                 EFS04602.1 
                 helicase c2 domain-containing protein [ Listeria seeligeri  FSL 
               
               
                 308 
                 ZP 05792445.1 
                 putative helicase [ Butyrivibrio crossotus  DSM 2876] 
               
               
                 309 
                 XP 002829448.1 
                 PREDICTED: LOW QUALITY PROTEIN: TFIIH basal 
               
               
                 310 
                 ZP 02358461.1 
                 DNA repair helicase [ Burkholderia oklahomensis  EO147] 
               
               
                 311 
                 AAB58296.1 
                 DNA helicase [ Mus musculus ] 
               
               
                 312 
                 XP 003245432.1 
                 PREDICTED: Fanconi anemia group J protein homolog isoform 
               
               
                 313 
                 XP 001943091.2 
                 PREDICTED: Fanconi anemia group J protein homolog isoform 
               
               
                 314 
                 YP 004232710.1 
                 DEAD 2 domain-containing protein [ Acidovorax avenae  subsp. 
               
               
                 315 
                 EGY20402.1 
                 DNA repair helicase RAD3 [ Verticillium dahliae  VdLs.17] 
               
               
                 316 
                 ZP 08812184.1 
                 DEAD 2 family protein [ Desulfosporosinus  sp. OT] 
               
               
                 317 
                 BAJ21097.1 
                 excision repair cross-complementing rodent repair deficiency, 
               
               
                 318 
                 BAE26794.1 
                 unnamed protein product [ Mus musculus ] 
               
               
                 319 
                 ZP 08130487.1 
                 putative helicase [ Clostridium  sp. D5] &gt;gb|EGB92074.1| 
               
               
                 320 
                 XP 868818.1 
                 hypothetical protein AN9436.2 [ Aspergillus nidulans  FGSC A4] 
               
               
                 321 
                 XP 001656074.1 
                 DNA repair helicase rad3/xp-d [ Aedes aegypti ] 
               
               
                 322 
                 ZP 07045589.1 
                 DeaD2 [ Comamonas testosteroni  S44] &gt;gb|EF160820.1|DeaD2 
               
               
                 323 
                 NP 031975.2 
                 TFIIH basal transcription factor complex helicase XPD subunit 
               
               
                 324 
                 YP 001353393.1 
                 Rad3-related DNA helicases [ Janthinobacterium  sp. Marseille] 
               
               
                 325 
                 NP 001166280.1 
                 excision repair cross-complementing rodent repair deficiency, 
               
               
                 326 
                 AAH34517.1 
                 Ercc2 protein [ Mus musculus ] 
               
               
                 327 
                 NP 001231320.1 
                 TFIIH basal transcription factor complex helicase XPD subunit 
               
               
                 328 
                 XP 002561536.1 
                 Pc16g12370 [ Penicillium chrysogenum  Wisconsin 54-1255] 
               
               
                 329 
                 AEO71602.1 
                 hypothetical protein THITE 2124189 [ Thielavia terrestris   
               
               
                 330 
                 YP 968571.1 
                 helicase c2 [ Acidovorax citrulli  AAC00-1] &gt;gb|ABM30797.1| 
               
               
                 331 
                 XP 001548663.1 
                 conserved hypothetical protein [ Botryotinia fuckeliana  B05.10] 
               
               
                 332 
                 YP 004752764.1 
                 DinG family ATP-dependent helicase [ Collimonas fungivorans   
               
               
                 333 
                 CCD46916.1 
                 similar to DNA repair helicase RAD3 [ Botryotinia fuckeliana ] 
               
               
                 334 
                 XP 001392821.1 
                 DNA repair helicase rad15 [ Aspergillus niger  CBS 513.88] 
               
               
                 335 
                 ZP 02466136.1 
                 putative ATP-dependent helicase [ Burkholderia thailandensis   
               
               
                 336 
                 YP 004230200.1 
                 helicase c2 [ Burkholderia  sp. CCGE1001] &gt;gb|ADX57140.1| 
               
               
                 337 
                 YP 002538024.1 
                 DEAD/DEAH box helicase [ Geobacter  sp. FRC-32] 
               
               
                 338 
                 EGJ23664.1 
                 Helicase c2 domain protein [ Listeria monocytogenes  str. Scott 
               
               
                 339 
                 ZP 02365524.1 
                 DNA repair helicase [ Burkholderia oklahomensis  C6786] 
               
               
                 340 
                 ZP 08637589.1 
                 hypothetical protein GME 12875 [ Halomonas  sp. TD01] 
               
               
                 341 
                 YP 012782.1 
                 hypothetical protein LMOf2365 0172 [ Listeria monocytogenes   
               
               
                 342 
                 YP 003967478.1 
                 helicase c2 [ Ilyobacter polytropus  DSM 2926] 
               
               
                 343 
                 ZP 05230929.1 
                 conserved hypothetical protein [ Listeria monocytogenes  FSL J1- 
               
               
                 344 
                 XP 311900.4 
                 AGAP002988-PA [ Anopheles gambiae str. PEST] 
               
               
                 345 
                 YP 848334.1 
                 helicase c2 domain-containing protein [ Listeria welshimeri   
               
               
                 346 
                 EGG19835.1 
                 DEAD/DEAH box helicase [ Dictyostelium fasciculatum ] 
               
               
                 347 
                 EFD93152.1 
                 DEAD 2 domain protein [ Candidatus Parvarchaeum   
               
               
                 348 
                 EFD92202.1 
                 DEAD 2 domain protein [ Candidatus Parvarchaeum   
               
               
                 349 
                 XP 003007283.1 
                 DNA repair helicase RAD3 [ Verticillium albo-atrum  VaMs.102] 
               
               
                 350 
                 ZP 07896115.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 351 
                 ZP 02078033.1 
                 hypothetical protein EUBDOL 01841 [ Eubacterium dolichum   
               
               
                 352 
                 XP 003082439.1 
                 DNA repair/transcription factor protein (ISS) [ Ostreococcus   
               
               
                 353 
                 ZP 06555278.1 
                 conserved hypothetical protein [ Listeria monocytogenes  FSL J2- 
               
               
                 354 
                 YP 002756898.1 
                 ATP dependent helicase [ Listeria monocytogenes serotype  4b 
               
               
                 355 
                 ZP 00230627.1 
                 conserved hypothetical protein [ Listeria monocytogenes  str. 4b 
               
               
                 356 
                 ZP 07707600.1 
                 DEAD 2 domain protein [ Bacillus  sp. m3-13] 
               
               
                 357 
                 ZP 02376616.1 
                 helicase c2 [ Burkholderia ubonensis  Bu] 
               
               
                 358 
                 YP 003305885.1 
                 helicase c2 [ Streptobacillus moniliformis  DSM 12112] 
               
               
                 359 
                 XP 001539110.1 
                 DNA repair helicase RAD3 [ Ajellomyces capsulatus  NAm1] 
               
               
                 360 
                 NP 469540.1 
                 hypothetical protein lin0195 [ Listeria innocua  Clip11262] 
               
               
                 361 
                 ZP 03777358.1 
                 hypothetical protein CLOHYLEM 04410 [ Clostridium   
               
               
                 362 
                 ZP 02501541.1 
                 hypothetical protein Bpse112 28444 [ Burkholderia   
               
               
                 363 
                 EGD01053.1 
                 helicase c2 [ Burkholderia  sp. TJI49] 
               
               
                 364 
                 YP 001513884.1 
                 DEAD 2 domain-containing protein [ Alkaliphilus oremlandii   
               
               
                 365 
                 YP 004029082.1 
                 DNA REPAIR HELICASE (RAD3/RAD15/XPD FAMILY) 
               
               
                 366 
                 EFR95270.1 
                 helicase c2 domain-containing protein [ Listeria innocua  FSL J1- 
               
               
                 367 
                 AAM45142.1 
                 excision repair cross-complementing rodent repair deficiency, 
               
               
                 368 
                 BAB23443.1 
                 unnamed protein product [ Mus musculus ] 
               
               
                 369 
                 ZP 02384949.1 
                 DNA repair helicase [ Burkholderia thailandensis  Bt4] 
               
               
                 370 
                 AAI10524.1 
                 Excision repair cross-complementing rodent repair deficiency, 
               
               
                 371 
                 NP 000391.1 
                 TFIIH basal transcription factor complex helicase XPD subunit 
               
               
                 372 
                 NP 001233519.1 
                 TFIIH basal transcription factor complex helicase subunit [ Pan   
               
               
                 373 
                 ZP 02414937.1 
                 Uvs006 [ Burkholderia pseudomallei  14] 
               
               
                 374 
                 CAA36463.1 
                 ercc2 gene product [ Homo sapiens ] 
               
               
                 375 
                 YP 002908815.1 
                 putative ATP-dependent helicase  Burkholderia glumae  BGR1] 
               
               
                 376 
                 ZP 03462972.1 
                 hypothetical protein BACPEC 02058 [[ Bacteroides ] 
               
               
                 377 
                 AEO05170.1 
                 hypothetical protein LMRG 02402 [ Listeria monocytogenes   
               
               
                 378 
                 ZP 00234550.1 
                 conserved hypothetical protein [ Listeria monocytogenes  str. 1/2a 
               
               
                 379 
                 NP 463690.1 
                 hypothetical protein lmo0157 [ Listeria monocytogenes  EGD-e] 
               
               
                 380 
                 YP 001321959.1 
                 DEAD 2 domain-containing protein [ Alkaliphilus   
               
               
                 381 
                 ZP 08090025.1 
                 hypothetical protein HMPREF9474 01776 [ Clostridium   
               
               
                 382 
                 CBL13167.1 
                 DEAD 2 [ Roseburia intestinalis  XB6B4] 
               
               
                 383 
                 ZP 02206869.1 
                 hypothetical protein COPEUT 01661 [ Coprococcus eutactus   
               
               
                 384 
                 XP 003510809.1 
                 PREDICTED: Fanconi anemia group J protein homolog 
               
               
                 385 
                 EGS23598.1 
                 hypothetical protein CTHT 0002930 [ Chaetomium   
               
               
                 386 
                 XP 970844.1 
                 PREDICTED: similar to  Xeroderma pigmentosum  D CG9433- 
               
               
                 387 
                 NP 001096787.1 
                 TFIIH basal transcription factor complex helicase XPD subunit 
               
               
                 388 
                 EFX06354.1 
                 tfiih complex helicase [ Grosmannia clavigera  kw1407] 
               
               
                 389 
                 ZP 05591145.1 
                 DNA repair helicase [ Burkholderia thailandensis  E264] 
               
               
                 390 
                 ABM06129.1 
                 excision repair cross-complementing rodent repair deficiency, 
               
               
                 391 
                 YP 001075221.1 
                 putative ATP-dependent helicase [ Burkholderia pseudomallei   
               
               
                 392 
                 ZP 08274812.1 
                 DinG family ATP-dependent helicase [ Oxalobacteraceae   
               
               
                 393 
                 YP 439745.1 
                 DNA repair helicase [ Burkholderia thailandensis  E264] 
               
               
                 394 
                 ZP 02406424.1 
                 hypothetical protein BpseD 29478 [ Burkholderia pseudomallei   
               
               
                 395 
                 ZP 01893211.1 
                 DEAD 2 [ Marinobacter algicola  DG893] &gt;gb|EDM48600.1| 
               
               
                 396 
                 YP 337604.1 
                 Uvs006 [ Burkholderia pseudomallei  1710b] &gt;gb|ABA53062.1| 
               
               
                 397 
                 ZP 01739164.1 
                 hypothetical protein MELB17 23815 [ Marinobacter  sp. ELB17] 
               
               
                 398 
                 YP 110868.1 
                 hypothetical protein BPSS0856 [ Burkholderia pseudomallei   
               
               
                 399 
                 CBL41994.1 
                 Rad3-related DNA helicases [butyrate-producing bacterium 
               
               
                 400 
                 ZP 02459185.1 
                 hypothetical protein Bpseu9 28802 [ Burkholderia pseudomallei   
               
               
                 401 
                 YP 001062257.1 
                 putative ATP-dependent helicase [ Burkholderia pseudomallei   
               
               
                 402 
                 ZP 02451021.1 
                 hypothetical protein Bpseu9 29690 [ Burkholderia pseudomallei   
               
               
                 403 
                 ZP 02493334.1 
                 hypothetical protein BpseN 28103 [ Burkholderia pseudomallei   
               
               
                 404 
                 NP 497182.2 
                 hypothetical protein Y50D7A.2 [ Caenorhabditis elegans ] 
               
               
                 405 
                 XP 002640826.1 
                 Hypothetical protein CBG15713 [ Caenorhabditis briggsae ] 
               
               
                 406 
                 ZP 06114232.1 
                 putative helicase [ Clostridium hathewavi  DSM 13479] 
               
               
                 407 
                 YP 002351431.1 
                 helicase c2 domain protein [ Listeria monocytogenes  HCC23] 
               
               
                 408 
                 YP 003898973.1 
                 hypothetical protein HELO 3904 [ Halomonas elongata  DSM 
               
               
                 409 
                 ZP 03450263.1 
                 putative ATP-dependent helicase [ Burkholderia pseudomallei   
               
               
                 410 
                 ZP 02474695.1 
                 hypothetical protein BpseB 28268 [ Burkholderia pseudomallei   
               
               
                 411 
                 BAF62336.1 
                 DNA-repair protein complementing XP-D cells [ Sus scrofa ] 
               
               
                 412 
                 ZP 04588908.1 
                 DEAD 2 protein [ Pseudomonas syringae  py.  oryzae  str. 1 6] 
               
               
                 413 
                 EGH68459.1 
                 hypothetical protein PSYAC 26881 [ Pseudomonas syringae  pv. 
               
               
                 414 
                 ZP 03569477.1 
                 helicase c2 [ Burkholderia multivorans  CGD2M] 
               
               
                 415 
                 ZP 08477214.1 
                 Rad3-related DNA helicase [ Lactobacillus coryniformis  subsp. 
               
               
                 416 
                 ZP 03543327.1 
                 DEAD 2 domain protein [ Comamonas testosteroni  KF-1] 
               
               
                 417 
                 ZP 08976894.1 
                 DEAD 2 domain protein [ Desulfitobacterium metallireducens   
               
               
                 418 
                 ZP 05232273.1 
                 conserved hypothetical protein [ Listeria monocytogenes  FSL 
               
               
                 419 
                 ZP 04744708.1 
                 putative ATP-dependent helicase [ Roseburia intestinalis  L1-82] 
               
               
                 420 
                 YP 001584206.1 
                 helicase c2 [ Burkholderia multivorans  ATCC 17616] 
               
               
                 421 
                 EHF07121.1 
                 hypothetical protein HMPREF1020 01003 [ Clostridium  sp. 
               
               
                 422 
                 YP 001948666.1 
                 putative ATP-dependent DNA helicase [ Burkholderia   
               
               
                 423 
                 CAF32100.1 
                 DNA repair helicase, putative [ Aspergillus fumigatus ] 
               
               
                 424 
                 EHB09904.1 
                 TFIIH basal transcription factor complex helicase subunit, 
               
               
                 425 
                 ZP 09000595.1 
                 helicase c2 [ Paenibacillus lactis  154] &gt;gb|EHB65769.1|helicase 
               
               
                 426 
                 ZP 02881609.1 
                 DEAD 2 domain protein [ Burkholderia graminis  C4D1M] 
               
               
                 427 
                 XP 003341121.1 
                 PREDICTED: LOW QUALITY PROTEIN: TFIIH basal 
               
               
                 428 
                 ZP 08574707.1 
                 DNA helicase (putative) [ Lactobacillus coryniformis  subsp. 
               
               
                 429 
                 ZP 03583039.1 
                 helicase c2 [ Burkholderia multivorans  CGD1] &gt;gb|EEE03212.1| 
               
               
                 430 
                 YP 002234127.1 
                 hypothetical protein BCAM1516A [ Burkholderia cenocepacia   
               
               
                 431 
                 NP 784122.1 
                 DNA helicase (putative) [ Lactobacillus plantarum  WCFS1] 
               
               
                 432 
                 YP 372388.1 
                 Rad3-related DNA helicases-like [ Burkholderia  sp. 383] 
               
               
                 433 
                 XP 002025881.1 
                 GL10160 [ Drosophila persimilis ] &gt;ref|XP 002138963.1| 
               
               
                 434 
                 YP 003607521.1 
                 helicase c2 [ Burkholderia  sp. CCGE1002] &gt;gb|ADG18010.1| 
               
               
                 435 
                 ZP 08981155.1 
                 DEAD 2 domain protein [ Desulfosporosinus meridiei  DSM 
               
               
                 436 
                 XP 001910036.1 
                 hypothetical protein [ Podospora anserina  S mat+] 
               
               
                 437 
                 CBY20154.1 
                 unnamed protein product [ Oikopleura dioica ] 
               
               
                 438 
                 YP 001557350.1 
                 DEAD 2 domain-containing protein [ Clostridium   
               
               
                 439 
                 EHB06437.1 
                 Fanconi anemia group J protein [ Heterocephalus glaber ] 
               
               
                 440 
                 AAS21351.1 
                 helicase-like protein NHL-like protein [ Oikopleura dioica ] 
               
               
                 441 
                 YP 001117339.1 
                 helicase c2 [ Burkholderia vietnamiensis  G4] &gt;gb|ABO57874.1| 
               
               
                 442 
                 ZP 06248030.1 
                 DEAD 2 domain protein [ Clostridium thermocellum  JW20] 
               
               
                 443 
                 ZP 03626657.1 
                 DEAD 2 domain protein [bacterium Ellin514] 
               
               
                 444 
                 EGC82354.1 
                 DEAD2 domain protein [ Anaerococcus prevotii  ACS-065-V- 
               
               
                 445 
                 YP 001669171.1 
                 DEAD 2 domain-containing protein [ Pseudomonas putida  GB- 
               
               
                 446 
                 YP 001779577.1 
                 helicase c2 [ Burkholderia cenocepacia  MC0-3] 
               
               
                 447 
                 EGX97283.1 
                 superfamily II DNA/RNA helicase [ Lactobacillus ruminis   
               
               
                 448 
                 YP 001036721.1 
                 DEAD 2 [ Clostridium thermocellum  ATCC 27405] 
               
               
                 449 
                 EDM05558.1 
                 BRCA1 interacting protein C-terminal helicase 1 (predicted) 
               
               
                 450 
                 ZP 08564414.1 
                 superfamily II DNA/RNA helicase [ Lactobacillus ruminis   
               
               
                 451 
                 YP 338478.1 
                 DNA repair helicase, truncation [ Burkholderia mallei  ATCC 
               
               
                 452 
                 ZP 05429973.1 
                 DEAD 2 domain protein [ Clostridium thermocellum  DSM 
               
               
                 453 
                 ZP 02908774.1 
                 helicase c2 [ Burkholderia ambifaria  MEX-5] &gt;gb|EDT40109.1| 
               
               
                 454 
                 YP 003074789.1 
                 hypothetical protein TERTU 3456 [ Teredinibacter turnerae   
               
               
                 455 
                 CBL10637.1 
                 DEAD 2 [ Roseburia intestinalis  M50/1] 
               
               
                 456 
                 ZP 08080348.1 
                 superfamily II DNA/RNA helicase [ Lactobacillus ruminis   
               
               
                 457 
                 ZP 05714130.1 
                 helicase, putative [ Enterococcus faecium  DO] 
               
               
                 458 
                 YP 002505020.1 
                 DEAD 2 domain-containing protein [ Clostridium   
               
               
                 459 
                 XP 002762284.1 
                 PREDICTED: TFIIH basal transcription factor complex 
               
               
                 460 
                 ZP 07869402.1 
                 helicase c2 domain-containing protein [ Listeria marthii  FSL S4- 
               
               
                 461 
                 YP 004394741.1 
                 hypothetical protein CbC4 0061 [ Clostridium botulinum   
               
               
                 462 
                 YP 004681432.1 
                 Rad3-like DNA helicase [ Cupriavidus necator  N-1] 
               
               
                 463 
                 ZP 00442058.1 
                 putative ATP-dependent helicase [ Burkholderia mallei  GB8 
               
               
                 464 
                 EHA18240.1 
                 DNA repair helicase, subunit of TFIIH [ Aspergillus niger   
               
               
                 465 
                 EGT47910.1 
                 hypothetical protein CAEBREN 01520 [ Caenorhabditis   
               
               
                 466 
                 ZP 03929588.1 
                 DNA-directed DNA polymerase III epsilon subunit 
               
               
                 467 
                 YP 001889149.1 
                 DEAD 2 domain-containing protein [ Burkholderia   
               
               
                 468 
                 ZP 04156392.1 
                 DnaO family exonuclease/DinG family helicase [ Bacillus   
               
               
                 469 
                 YP 003050181.1 
                 helicase c2 [ Methylovorus glucosetrophus  SIP3-4] 
               
               
                 470 
                 XP 002063431.1 
                 GK21904 [ Drosophila willistoni ] &gt;gb|EDW74417.1|GK21904 
               
               
                 471 
                 YP 235981.1 
                 DEAD 2 [ Pseudomonas syringae  pv.  syringae  B728a] 
               
               
                 472 
                 ZP 07004902.1 
                 ATP-dependent helicase [ Pseudomonas savastanoi  pv. 
               
               
                 473 
                 ZP 02371055.1 
                 DNA repair helicase [ Burkholderia thailandensis  TXDOH] 
               
               
                 474 
                 XP 002006040.1 
                 GI20811 [ Drosophila mojavensis ] &gt;gb|EDW09975.1|GI20811 
               
               
                 475 
                 ADI95426.1 
                 DEAD-2 domain-containing protein [ Pseudomonas putida ] 
               
               
                 476 
                 NP 744983.1 
                 DEAD 2 domain-containing protein [ Pseudomonas putida   
               
               
                 477 
                 ADR60507.1 
                 DEAD 2 domain protein [ Pseudomonas putida  BIRD-1] 
               
               
                 478 
                 ZP 06493673.1 
                 DEAD 2 [ Pseudomonas syringae  pv.  syringae  FF5] 
               
               
                 479 
                 YP 001311945.1 
                 DEAD 2 domain-containing protein [ Clostridium beijerinckii   
               
               
                 480 
                 ADP99485.1 
                 Rad3-related DNA helicase [ Marinobacter adhaerens  HP15] 
               
               
                 481 
                 YP 001268166.1 
                 DEAD 2 domain-containing protein [ Pseudomonas putida  F1] 
               
               
                 482 
                 YP 003923571.1 
                 DNA helicase ( ) [ Lactobacillus plantarum  subsp.  plantarum  ST- 
               
               
                 483 
                 XP 340870.3 
                 PREDICTED: BRCA1 interacting protein C-terminal helicase 1 
               
               
                 484 
                 YP 004619190.1 
                 ATP-dependent helicase-like protein [ Ramlibacter tataouinensis   
               
               
                 485 
                 ZP 04947353.1 
                 Rad3-related DNA helicase [ Burkholderia dolosa  AUO158] 
               
               
                 486 
                 YP 623840.1 
                 helicase c2 [ Burkholderia cenocepacia  AU 1054] 
               
               
                 487 
                 YP 877033.1 
                 DNA repair helicase. truncation [ Clostridium novvi  NT] 
               
               
                 488 
                 ZP 02083792.1 
                 hypothetical protein CLOBOL 01315 [ Clostridium bolteae   
               
               
                 489 
                 ZP 03212705.1 
                 Rad3-related DNA helicase [ Lactobacillus rhamnosus  HN001] 
               
               
                 490 
                 ZP 04941727.1 
                 Rad3-related DNA helicase [ Burkholderia cenocepacia  PC184] 
               
               
                 491 
                 ZP 02889716.1 
                 helicase c2 [ Burkholderia ambifaria  IOP40-10] 
               
               
                 492 
                 YP 775689.1 
                 helicase c2 [ Burkholderia ambifaria  AMMD] &gt;gb|ABI89355.1| 
               
               
                 493 
                 ZP 04150622.1 
                 DnaQ family exonuclease/DinG family helicase [ Bacillus   
               
               
                 494 
                 XP 001653621.1 
                 regulator of telomere elongation helicase 1 rtel1 [ Aedes aegypti ] 
               
               
                 495 
                 YP 001230873.1 
                 DEAD 2 domain-containing protein [ Geobacter uraniireducens   
               
               
                 496 
                 YP 840785.1 
                 Rad3-related DNA helicase [ Ralstonia eutropha  H16] 
               
               
                 497 
                 ZP 07324798.1 
                 DEAD 2 domain protein [ Acetivibrio cellulolvticus  CD2] 
               
               
                 498 
                 ZP 05132613.1 
                 DEAD 2 domain-containing protein [ Clostridium  sp. 
               
               
                 499 
                 ZP 05347278.1 
                 putative helicase [ Bryantella formatexigens  DSM 14469] 
               
               
                 500 
                 ZP 08980710.1 
                 Exonuclease RNase T and DNA polymerase III 
               
               
                   
               
             
          
         
       
     
     The XPD helicase is more preferably one of the helicases shown in Table 5 below or a variant thereof. The XPD helicase more preferably comprises the sequence of one of the helicases shown in Table 5, i.e. one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62, or a variant thereof. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 More preferred XPD helicases and most preferred XPD motifs V and VI 
               
             
          
           
               
                   
                   
                   
                   
                 % 
                   
                   
               
               
                 SEQ 
                   
                   
                   
                 Identity 
                 Motif V 
                 Motif VI 
               
               
                 ID 
                   
                   
                   
                 to XPD 
                 (SEQ ID 
                 (SEQ ID 
               
               
                 NO 
                 Name 
                 Strain 
                 Accession No. 
                 Mbu 
                 NO) 
                 NO) 
               
               
                   
               
               
                 10 
                 XPD Mbu 
                 
                   Methanococcoides 
                 
                 YP_566221.1 
                 — 
                 YLWGT 
                 QAMGR 
               
               
                   
                   
                 
                   burtonii 
                 
                 GI:91773529 
                   
                 LSEG 
                 VVRSP 
               
               
                   
                   
                   
                   
                   
                 (11) 
                 TDYGA 
               
               
                   
                   
                   
                   
                   
                   
                 RILLDG 
               
               
                   
                   
                   
                   
                   
                   
                 R (12) 
               
               
                   
               
               
                 13 
                 XPD Hla 
                 
                   Halorubrum 
                 
                 YP_002567268.1 
                 42 
                 SLWGT 
                 QAIGR 
               
               
                   
                   
                 
                   lacusprofundi 
                 
                 GI:222481031 
                   
                 LAEG 
                 VVRGP 
               
               
                   
                   
                   
                   
                   
                 (14) 
                 DDFGV 
               
               
                   
                   
                   
                   
                   
                   
                 RILADR 
               
               
                   
                   
                   
                   
                   
                   
                 R (15) 
               
               
                   
               
               
                 16 
                 XPD Mac 
                 
                   Methanosarcina 
                 
                 NP_615308.1 
                 55 
                 YLWGT 
                 QAMGR 
               
               
                   
                   
                   acetivorans  C2A 
                 GI:20089233 
                   
                 LSEG 
                 VVRSP 
               
               
                   
                   
                   
                   
                   
                 (11) 
                 GDFGV 
               
               
                   
                   
                   
                   
                   
                   
                 RILLDA 
               
               
                   
                   
                   
                   
                   
                   
                 R (17) 
               
               
                   
               
               
                 18 
                 XPD Mzh 
                 
                   Methanosalsum 
                 
                 YP_004617026.1 
                 71 
                 YLWGT 
                 QAMGR 
               
               
                   
                   
                 
                   zhilinae 
                 
                 GI:336477885 
                   
                 LSEG 
                 VVRSPS 
               
               
                   
                   
                   
                   
                   
                 (11) 
                 DYGAR 
               
               
                   
                   
                   
                   
                   
                   
                 ILLDGR 
               
               
                   
                   
                   
                   
                   
                   
                 (19) 
               
               
                   
               
               
                 20 
                 XPD Nph 
                 
                   Natronomonas 
                 
                 YP_326312.1 
                 43 
                 SLWGT 
                 QALGR 
               
               
                   
                   
                 
                   pharaonis 
                 
                 GI:76801304 
                   
                 LAEG 
                 VVRSP 
               
               
                   
                   
                   
                   
                   
                 (14) 
                 TDFGV 
               
               
                   
                   
                   
                   
                   
                   
                 RVLVD 
               
               
                   
                   
                   
                   
                   
                   
                 ER (21) 
               
               
                   
               
               
                 22 
                 XPD Hoc 
                 
                   Haliangium 
                 
                 YP_003265400.1 
                 15 
                 VTGGV 
                 QAAGR 
               
               
                   
                   
                   ochraceum  DSM 
                 GI:262194191 
                   
                 FAEG 
                 VLRTPE 
               
               
                   
                   
                   
                   
                   
                 (23) 
                 DRGVI 
               
               
                   
                   
                   
                   
                   
                   
                 ALLGR 
               
               
                   
                   
                   
                   
                   
                   
                 R (24) 
               
               
                   
               
               
                 25 
                 XPD Sbe 
                 
                   Shewanella 
                 
                 ZP_02159638.1 
                 21 
                 LGTGA 
                 QGVGR 
               
               
                   
                   
                 
                   benthica 
                 
                 GI:163752447 
                   
                 FWEG 
                 LIRDER 
               
               
                   
                   
                   
                   
                   
                 (26) 
                 DRGVLI 
               
               
                   
                   
                   
                   
                   
                   
                 LCDNR 
               
               
                   
                   
                   
                   
                   
                   
                 (27) 
               
               
                   
               
               
                 28 
                 XPD Mev 
                 
                   Methanohalobium 
                 
                 YP_003727831.1 
                 71 
                 YIWGT 
                 QAMGR 
               
               
                   
                   
                 
                   evestigatum 
                 
                 GI:298676081 
                   
                 LSEG 
                 VVRSP 
               
               
                   
                   
                   
                   
                   
                 (29) 
                 TDYGA 
               
               
                   
                   
                   
                   
                   
                   
                 RILIDG 
               
               
                   
                   
                   
                   
                   
                   
                 R (30) 
               
               
                   
               
               
                 31 
                 XPD Mba 
                 
                   Methanosarcina 
                 
                 YP_304917.1 
                 56 
                 YLWGT 
                 QAMGR 
               
               
                   
                   
                 
                   barkeri fusaro 
                 
                 GI:73668902 
                   
                 LSEG 
                 IVRSPD 
               
               
                   
                   
                   
                   
                   
                 (11) 
                 DYGVR 
               
               
                   
                   
                   
                   
                   
                   
                 ILLDSR 
               
               
                   
                   
                   
                   
                   
                   
                 (32) 
               
               
                   
               
               
                 33 
                 XPD Hvo 
                 
                   Haloferax volcanii 
                 
                 YP_003534113.1 
                 43 
                 SLWGT 
                 QALGR 
               
               
                   
                   
                   
                 GI:292654216 
                   
                 LAEG 
                 VIRAPD 
               
               
                   
                   
                   
                   
                   
                 (14) 
                 DFGVR 
               
               
                   
                   
                   
                   
                   
                   
                 VLADK 
               
               
                   
                   
                   
                   
                   
                   
                 R (34) 
               
               
                   
               
               
                 35 
                 XPD Tac 
                 
                   Thermoplasma 
                 
                 NP_393536.1 
                 24 
                 VSGGR 
                 QEIGRL 
               
               
                   
                   
                 
                   acidophilum 
                 
                 GI:16082477 
                   
                 LSEG 
                 IRSAED 
               
               
                   
                   
                   
                   
                   
                 (36) 
                 TGACVI 
               
               
                   
                   
                   
                   
                   
                   
                 LDKR 
               
               
                   
                   
                   
                   
                   
                   
                 (37) 
               
               
                   
               
               
                 38 
                 XPD Tsi 
                 
                   Thermococcus 
                 
                 YP_002994616.1 
                 24 
                 VMGGR 
                 QAAGR 
               
               
                   
                   
                 
                   sibiricus 
                 
                 GI:242399192 
                   
                 NSEG 
                 VHRSE 
               
               
                   
                   
                   
                   
                   
                 (39) 
                 EEKGA 
               
               
                   
                   
                   
                   
                   
                   
                 VVVLD 
               
               
                   
                   
                   
                   
                   
                   
                 YR (40) 
               
               
                   
               
               
                  41 
                 XPD Pab 
                 
                   Pyrococcus abyssi 
                 
                 NP_127020.1 
                 24 
                 VMGGR 
                 QAAGR 
               
               
                   
                   
                   
                 GI:14521544 
                   
                 NSEG 
                 VHRSE 
               
               
                   
                   
                   
                   
                   
                 (39) 
                 EEKGSI 
               
               
                   
                   
                   
                   
                   
                   
                 VILDYR 
               
               
                   
                   
                   
                   
                   
                   
                 (42) 
               
               
                   
               
               
                 43 
                 XPD Pfu 
                 
                   Pyrococcus 
                 
                 NP_578662.1 
                 25 
                 VMGGR 
                 QAAGR 
               
               
                   
                   
                 
                   furiosus 
                 
                 GI:18977305 
                   
                 NSEG 
                 VHRSE 
               
               
                   
                   
                   
                   
                   
                 (39) 
                 EEKGSI 
               
               
                   
                   
                   
                   
                   
                   
                 VILDYR 
               
               
                   
                   
                   
                   
                   
                   
                 (42) 
               
               
                   
               
               
                 44 
                 XPD Hsp 
                   Halobacterium  sp. 
                 NP_281042.1 
                 41 
                 SLWGT 
                 QAMGR 
               
               
                   
                   
                 NRC-1 
                 GI:15791218 
                   
                 LAEG 
                 VIRSPE 
               
               
                   
                   
                   
                   
                   
                 14) 
                 DFGVR 
               
               
                   
                   
                   
                   
                   
                   
                 MLVDR 
               
               
                   
                   
                   
                   
                   
                   
                 R (45) 
               
               
                   
               
               
                 46 
                 XPD Mja 
                 
                   Methanocaldococcus 
                 
                 NP_247937.1 
                 19 
                 LATGR 
                 QMIGR 
               
               
                   
                   
                 
                   jannaschii 
                 
                 GI:15669132 
                   
                 FAEG 
                 LIRTEN 
               
               
                   
                   
                   
                   
                   
                 (47) 
                 DYGVV 
               
               
                   
                   
                   
                   
                   
                   
                 VIQDK 
               
               
                   
                   
                   
                   
                   
                   
                 R (48) 
               
               
                   
               
               
                 49 
                 XPD Sso 
                 
                   Sulfolobus 
                 
                 NP_341859.1 
                 19 
                 IARGKL 
                 QSIGRA 
               
               
                   
                   
                 
                   solfataricus 
                 
                 GI:15897254 
                   
                 AEG 
                 IRGPTD 
               
               
                   
                   
                   
                   
                   
                 (50) 
                 NATIW 
               
               
                   
                   
                   
                   
                   
                   
                 LLDKR 
               
               
                   
                   
                   
                   
                   
                   
                 (51) 
               
               
                   
               
               
                 52 
                 XPD Sac 
                 
                   Sulfolobus 
                 
                 YP_254904.1 
                 20 
                 VGKGK 
                 QAIGR 
               
               
                   
                   
                 
                   acidocaldarius 
                 
                 GI:70606034 
                   
                 LAEG 
                 AIRDV 
               
               
                   
                   
                   
                   
                   
                 (53) 
                 NDKCN 
               
               
                   
                   
                   
                   
                   
                   
                 VWLLD 
               
               
                   
                   
                   
                   
                   
                   
                 KR (54) 
               
               
                   
               
               
                 55 
                 XPD Tba 
                 
                   Thermococcus 
                 
                 YP_004071335.1 
                 25 
                 VMGGR 
                 QAAGR 
               
               
                   
                   
                   barophilus  MP 
                 GI:315230899 
                   
                 NSEG 
                 VHRSA 
               
               
                   
                   
                   
                   
                   
                 (39) 
                 EEKGAI 
               
               
                   
                   
                   
                   
                   
                   
                 IILDYR 
               
               
                   
                   
                   
                   
                   
                   
                 (56) 
               
               
                   
               
               
                 57 
                 XPD Nma 
                 
                   Natrialba magadii 
                 
                 YP_003479905.1 
                 42 
                 SLWGT 
                 QALGR 
               
               
                   
                   
                   
                 GI:289581439 
                   
                 LAEG 
                 VIRSPE 
               
               
                   
                   
                   
                   
                   
                 (14) 
                 DVGVR 
               
               
                   
                   
                   
                   
                   
                   
                 ALLDR 
               
               
                   
                   
                   
                   
                   
                   
                 R (58) 
               
               
                   
               
               
                 59 
                 XPD Hpa 
                 
                   Haladaptatus 
                 
                 ZP_08042768.1 
                 40 
                 SLWGT 
                 QALGR 
               
               
                   
                   
                 
                   paucihalophilus 
                 
                 GI: 322368199 
                   
                 LAEG 
                 VIRSPE 
               
               
                   
                   
                   
                   
                   
                 (14) 
                 DFGVRI 
               
               
                   
                   
                   
                   
                   
                   
                 LLDKR 
               
               
                   
                   
                   
                   
                   
                   
                 (60) 
               
               
                   
               
               
                 61 
                 XPD Htu 
                 
                   Haloterrigena 
                 
                 YP_003401909.1 
                 38 
                 SLWGT 
                 QALGR 
               
               
                   
                   
                 
                   turkmenica 
                 
                 GI:284163630 
                   
                 LAEG 
                 VIRSPE 
               
               
                   
                   
                   
                   
                   
                 (14) 
                 DVGVR 
               
               
                   
                   
                   
                   
                   
                   
                 ALLDR 
               
               
                   
                   
                   
                   
                   
                   
                 R (58) 
               
               
                   
               
               
                 62 
                 XPD 
                 
                   Methanosarcina 
                 
                 NP_633611.1 
                 55 
                 YLWGT 
                 QAMGR 
               
               
                   
                 Mmaz 
                 
                   mazei 
                 
                 GI:21227689 
                   
                 LSEG 
                 VVRSP 
               
               
                   
                   
                   
                   
                   
                 (11) 
                 GDFGV 
               
               
                   
                   
                   
                   
                   
                   
                 RILLDA 
               
               
                   
                   
                   
                   
                   
                   
                 R (17) 
               
               
                   
               
             
          
         
       
     
     The XPD helicase most preferably comprises the sequence shown in SEQ ID NO: 10 or a variant thereof. 
     A variant of a XPD helicase is an enzyme that has an amino acid sequence which varies from that of the wild-type helicase and which retains polynucleotide binding activity. In particular, a variant of any one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 is an enzyme that has an amino acid sequence which varies from that of any one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 and which retains polynucleotide binding activity. A variant of SEQ ID NO: 10 is an enzyme that has an amino acid sequence which varies from that of SEQ ID NO: 10 and which retains polynucleotide binding activity. The variant retains helicase activity. Methods for measuring helicase activity are known in the art. Helicase activity can also be measured as described in the Examples. The variant must work in at least one of the two modes discussed below. Preferably, the variant works in both modes. The variant may include modifications that facilitate handling of the polynucleotide encoding the helicase and/or facilitate its activity at high salt concentrations and/or room temperature. Variants typically differ from the wild-type helicase in regions outside of XPD motifs V and VI discussed above. However, variants may include modifications within one or both of these motifs. 
     Over the entire length of the amino acid sequence of any one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62, such as SEQ ID NO: 10, a variant will preferably be at least 10%, preferably 30% homologous to that sequence based on amino acid identity. More preferably, the variant polypeptide may be at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% and more preferably at least 95%, 97% or 99% homologous based on amino acid identity to the amino acid sequence of any one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62, such as SEQ ID NO: 10, over the entire sequence. There may be at least 70%, for example at least 80%, at least 85%, at least 90% or at least 95%, amino acid identity over a stretch of 150 or more, for example 200, 300, 400, 500, 600, 700, 800, 900 or 1000 or more, contiguous amino acids (“hard homology”). Homology is determined as described above. The variant may differ from the wild-type sequence in any of the ways discussed above with reference to SEQ ID NOs: 2 and 4. 
     In particular, variants may include fragments of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62. Such fragments retain polynucleotide binding activity. Fragments may be at least about 200, at least about 300, at least about 400, at least about 500, at least about 600 or at least about 700 amino acids in length. The length of the fragment will typically depend on the length of the wild-type sequence. As discussed in more detail below, fragments preferably comprise the XPD motif V and/or the XPD motif VI of the relevant wild-type sequence. 
     Amino acid substitutions may be made to the amino acid sequence of SEQ ID NO: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 or 62, for example up to 1, 2, 3, 4, 5, 10, 20 or 30 substitutions. The substitutions are preferably conservative substitutions as discussed above. 
     A variant, such as a fragment, of any one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 preferably comprises the XPD motif V and/or the XPD motif VI of the relevant wild-type sequence. A variant, such as a fragment, of any one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 more preferably comprises the XPD motif V and the XPD motif VI of the relevant wild-type sequence. For instance, a variant of SEQ ID NO: 10 preferably comprises XPD motif V of SEQ ID NO: 10 (YLWGTLSEG; SEQ ID NO: 11) and/or XPD motif VI of SEQ ID NO: 10 (QAMGRVVRSPTDYGARILLDGR; SEQ ID NO: 12). A variant of SEQ ID NO: 10 more preferably comprises both XPD motifs V and VI of SEQ ID NO: 10. The XPD motifs V and VI of each of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 are shown in Table 5. However, a variant of any one SEQ ID NOs 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 may comprise XPD motifs V and/or VI from a different wild-type sequence. For instance, a variant of SEQ ID NO: 10 may comprise XPD motif V of SEQ ID NO: 13 (SLWGTLAEG; SEQ ID NO: 14) and/or XPD motif VI of SEQ ID NO: 13 (QAIGRVVRGPDDFGVRILADRR; SEQ ID NO: 15). A variant of any one SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 may comprise any one of the preferred motifs shown in Table 5. Variants of any one of SEQ ID NOs: 10, 13, 16, 18, 20, 22, 25, 28, 31, 33, 35, 38, 41, 43, 44, 46, 49, 52, 55, 57, 59, 61 and 62 may also include modifications within XPD motif V and/or XPD motif VI of the relevant wild-type sequence. Suitable modifications to these motifs are discussed above when defining the two motifs. 
     The helicase may be covalently attached to the pore. The helicase is preferably not covalently attached to the pore. The application of a voltage to the pore and helicase typically results in the formation of a sensor that is capable of sequencing target polynucleotides. This is discussed in more detail below. 
     Any of the proteins described herein, i.e. the transmembrane protein pores or XPD helicases, may be modified to assist their identification or purification, for example by the addition of histidine residues (a his tag), aspartic acid residues (an asp tag), a streptavidin tag, a flag tag, a SUMO tag, a GST tag or a MBP tag, or by the addition of a signal sequence to promote their secretion from a cell where the polypeptide does not naturally contain such a sequence. An alternative to introducing a genetic tag is to chemically react a tag onto a native or engineered position on the pore or helicase. An example of this would be to react a gel-shift reagent to a cysteine engineered on the outside of the pore. This has been demonstrated as a method for separating hemolysin hetero-oligomers (Chem. Biol. 1997 July; 4(7):497-505). 
     The pore and/or helicase may be labelled with a revealing label. The revealing label may be any suitable label which allows the pore to be detected. Suitable labels include, but are not limited to fluorescent molecules, radioisotopes, e.g.  125 I,  35 S, enzymes, antibodies, antigens, polynucleotides and ligands such as biotin. 
     Proteins may be made synthetically or by recombinant means. For example, the pore and/or helicase may be synthesized by in vitro translation and transcription (IVTT). The amino acid sequence of the pore and/or helicase may be modified to include non-naturally occurring amino acids or to increase the stability of the protein. When a protein is produced by synthetic means, such amino acids may be introduced during production. The pore and/or helicase may also be altered following either synthetic or recombinant production. 
     The pore and/or helicase may also be produced using D-amino acids. For instance, the pore or helicase may comprise a mixture of L-amino acids and D-amino acids. This is conventional in the art for producing such proteins or peptides. 
     The pore and/or helicase may also contain other non-specific modifications as long as they do not interfere with pore formation or helicase function. A number of non-specific side chain modifications are known in the art and may be made to the side chains of the protein(s). Such modifications include, for example, reductive alkylation of amino acids by reaction with an aldehyde followed by reduction with NaBH 4 , amidination with methylacetimidate or acylation with acetic anhydride. 
     The pore and helicase can be produced using standard methods known in the art. Polynucleotide sequences encoding a pore or helicase may be derived and replicated using standard methods in the art. Polynucleotide sequences encoding a pore or helicase may be expressed in a bacterial host cell using standard techniques in the art. The pore and/or helicase may be produced in a cell by in situ expression of the polypeptide from a recombinant expression vector. The expression vector optionally carries an inducible promoter to control the expression of the polypeptide. These methods are described in described in Sambrook, J. and Russell, D. (2001). Molecular Cloning: A Laboratory Manual, 3rd Edition. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. 
     The pore and/or helicase may be produced in large scale following purification by any protein liquid chromatography system from protein producing organisms or after recombinant expression. Typical protein liquid chromatography systems include FPLC, AKTA systems, the Bio-Cad system, the Bio-Rad BioLogic system and the Gilson HPLC system. 
     The method of the invention involves measuring one or more characteristics of the target polynucleotide. The method may involve measuring two, three, four or five or more characteristics of the target polynucleotide. The one or more characteristics are preferably selected from (i) the length of the target polynucleotide, (ii) the identity of the target polynucleotide, (iii) the sequence of the target polynucleotide, (iv) the secondary structure of the target polynucleotide and (v) whether or not the target polynucleotide is modified. Any combination of (i) to (v) may be measured in accordance with the invention. 
     For (i), the length of the polynucleotide may be measured using the number of interactions between the target polynucleotide and the pore. 
     For (ii), the identity of the polynucleotide may be measured in a number of ways. The identity of the polynucleotide may be measured in conjunction with measurement of the sequence of the target polynucleotide or without measurement of the sequence of the target polynucleotide. The former is straightforward; the polynucleotide is sequenced and thereby identified. The latter may be done in several ways. For instance, the presence of a particular motif in the polynucleotide may be measured (without measuring the remaining sequence of the polynucleotide). Alternatively, the measurement of a particular electrical and/or optical signal in the method may identify the target polynucleotide as coming from a particular source. 
     For (iii), the sequence of the polynucleotide can be determined as described previously. Suitable sequencing methods, particularly those using electrical measurements, are described in Stoddart D et al., Proc Natl Acad Sci, 12; 106(19):7702-7, Lieberman K R et al, J Am Chem. Soc. 2010; 132(50):17961-72, and International Application WO 2000/28312. 
     For (iv), the secondary structure may be measured in a variety of ways. For instance, if the method involves an electrical measurement, the secondary structure may be measured using a change in dwell time or a change in current flowing through the pore. This allows regions of single-stranded and double-stranded polynucleotide to be distinguished. 
     For (v), the presence or absence of any modification may be measured. The method preferably comprises determining whether or not the target polynucleotide is modified by methylation, by oxidation, by damage, with one or more proteins or with one or more labels, tags or spacers. Specific modifications will result in specific interactions with the pore which can be measured using the methods described below. For instance, methylcyotsine may be distinguished from cytosine on the basis of the current flowing through the pore during its interaction with each nucleotide. 
     A variety of different types of measurements may be made. This includes without limitation: electrical measurements and optical measurements. Possible electrical measurements include: current measurements, impedance measurements, tunneling measurements (Ivanov A P et al., Nano Lett. 2011 Jan. 12; 11(1):279-85), and FET measurements (International Application WO 2005/124888). Optical measurements may be combined 10 with electrical measurements (Soni G V et al., Rev Sci Instrum. 2010 January; 81(1):014301). The measurement may be a transmembrane current measurement such as measurement of ionic current flowing through the pore. 
     Electrical measurements may be made using standard single channel recording equipment as describe in Stoddart D et al., Proc Natl Acad Sci, 12; 106(19):7702-7, Lieberman K R et al, J Am Chem. Soc. 2010; 132(50):17961-72, and International Application WO-2000/28312. Alternatively, electrical measurements may be made using a multi-channel system, for example as described in International Application WO-2009/077734 and International Application WO-2011/067559. 
     In a preferred embodiment, the method comprises: 
     (a) contacting the target polynucleotide with a transmembrane pore and a XPD helicase such that the target polynucleotide moves through the pore and the XPD helicase controls the movement of the target polynucleotide through the pore; and 
     (b) measuring the current passing through the pore as the polynucleotide moves with respect to the pore wherein the current is indicative of one or more characteristics of the target polynucleotide and thereby characterising the target polynucleotide. 
     The methods may be carried out using any apparatus that is suitable for investigating a membrane/pore system in which a pore is inserted into a membrane. The method may be carried out using any apparatus that is suitable for transmembrane pore sensing. For example, the apparatus comprises a chamber comprising an aqueous solution and a barrier that separates the chamber into two sections. The barrier has an aperture in which the membrane containing the pore is formed. 
     The methods may be carried out using the apparatus described in International Application No. PCT/GB08/000,562 (WO 2008/102120). 
     The methods may involve measuring the current passing through the pore as the polynucleotide moves with respect to the pore. Therefore the apparatus may also comprise an electrical circuit capable of applying a potential and measuring an electrical signal across the membrane and pore. The methods may be carried out using a patch clamp or a voltage clamp. The methods preferably involve the use of a voltage clamp. 
     The methods of the invention may involve the measuring of a current passing through the pore as the polynucleotide moves with respect to the pore. Suitable conditions for measuring ionic currents through transmembrane protein pores are known in the art and disclosed in the Example. The method is typically carried out with a voltage applied across the membrane and pore. The voltage used is typically from +2 V to −2 V, typically −400 mV to +400 mV. The voltage used is preferably in a range having a lower limit selected from −400 mV, −300 mV, −200 mV, −150 mV, −100 mV, −50 mV, −20 mV and 0 mV and an upper limit independently selected from +10 mV, +20 mV, +50 mV, +100 mV, +150 mV, +200 mV, +300 mV and +400 mV. The voltage used is more preferably in the range 100 mV to 240 mV and most preferably in the range of 120 mV to 220 mV. It is possible to increase discrimination between different nucleotides by a pore by using an increased applied potential. 
     The methods are typically carried out in the presence of any charge carriers, such as metal salts, for example alkali metal salt, halide salts, for example chloride salts, such as alkali metal chloride salt. Charge carriers may include ionic liquids or organic salts, for example tetramethyl ammonium chloride, trimethylphenyl ammonium chloride, phenyltrimethyl ammonium chloride, or 1-ethyl-3-methyl imidazolium chloride. In the exemplary apparatus discussed above, the salt is present in the aqueous solution in the chamber. Potassium chloride (KCl), sodium chloride (NaCl) or caesium chloride (CsCl) is typically used. KCl is preferred. The salt concentration may be at saturation. The salt concentration may be 3M or lower and is typically from 0.1 to 2.5 M, from 0.3 to 1.9 M, from 0.5 to 1.8 M, from 0.7 to 1.7 M, from 0.9 to 1.6 M or from 1 M to 1.4 M. The salt concentration is preferably from 150 mM to 1 M. As discussed above, XPD helicases surprisingly work under high salt concentrations. The method is preferably carried out using a salt concentration of at least 0.3 M, such as at least 0.4 M, at least 0.5 M, at least 0.6 M, at least 0.8 M, at least 1.0 M, at least 1.5 M, at least 2.0 M, at least 2.5 M or at least 3.0 M. High salt concentrations provide a high signal to noise ratio and allow for currents indicative of the presence of a nucleotide to be identified against the background of normal current fluctuations. 
     The methods are typically carried out in the presence of a buffer. In the exemplary apparatus discussed above, the buffer is present in the aqueous solution in the chamber. Any buffer may be used in the method of the invention. Typically, the buffer is HEPES. Another suitable buffer is Tris-HCl buffer. The methods are typically carried out at a pH of from 4.0 to 12.0, from 4.5 to 10.0, from 5.0 to 9.0, from 5.5 to 8.8, from 6.0 to 8.7 or from 7.0 to 8.8 or 7.5 to 8.5. The pH used is preferably about 7.5. 
     The methods may be carried out at from 0° C. to 100° C., from 15° C. to 95° C., from 16° C. to 90° C., from 17° C. to 85° C., from 18° C. to 80° C., 19° C. to 70° C., or from 20° C. to 60° C. The methods are typically carried out at room temperature. The methods are optionally carried out at a temperature that supports enzyme function, such as about 37° C. 
     The method is typically carried out in the presence of free nucleotides or free nucleotide analogues and an enzyme cofactor that facilitate the action of the helicase. The free nucleotides may be one or more of any of the individual nucleotides discussed above. The free nucleotides include, but are not limited to, adenosine monophosphate (AMP), adenosine diphosphate (ADP), adenosine triphosphate (ATP), guanosine monophosphate (GMP), guanosine diphosphate (GDP), guanosine triphosphate (GTP), thymidine monophosphate (TMP), thymidine diphosphate (TDP), thymidine triphosphate (TTP), uridine monophosphate (UMP), uridine diphosphate (UDP), uridine triphosphate (UTP), cytidine monophosphate (CMP), cytidine diphosphate (CDP), cytidine triphosphate (CTP), cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), deoxyadenosine monophosphate (dAMP), deoxyadenosine diphosphate (dADP), deoxyadenosine triphosphate (dATP), deoxyguanosine monophosphate (dGMP), deoxyguanosine diphosphate (dGDP), deoxyguanosine triphosphate (dGTP), deoxythymidine monophosphate (dTMP), deoxythymidine diphosphate (dTDP), deoxythymidine triphosphate (dTTP), deoxyuridine monophosphate (dUMP), deoxyuridine diphosphate (dUDP), deoxyuridine triphosphate (dUTP), deoxycytidine monophosphate (dCMP), deoxycytidine diphosphate (dCDP) and deoxycytidine triphosphate (dCTP). The free nucleotides are preferably selected from AMP, TMP, GMP, CMP, UMP, dAMP, dTMP, dGMP or dCMP. The free nucleotides are preferably adenosine triphosphate (ATP). The enzyme cofactor is a factor that allows the helicase to function. The enzyme cofactor is preferably one or more divalent metal cations. Suitable divalent metal cations include, but are not limited to, Mg 2+ , Mn 2+ , Ca 2+ , Co 2+  and Fe 2+ . The enzyme cofactor is preferably Fe 2+  or Mg 2+ . The enzyme cofactor is most preferably Fe 2+  and Mg 2+   
     The target polynucleotide may be contacted with the XPD helicase and the pore in any order. In is preferred that, when the target polynucleotide is contacted with the XPD helicase and the pore, the target polynucleotide firstly forms a complex with the helicase. When the voltage is applied across the pore, the target polynucleotide/helicase complex then forms a complex with the pore and controls the movement of the polynucleotide through the pore. 
     As discussed above, XPD helicases may work in two modes with respect to the pore. First, the method is preferably carried out using the XPD helicase such that it moves the target sequence through the pore with the field resulting from the applied voltage. In this mode the 5′ end of the DNA is first captured in the pore, and the enzyme moves the DNA into the pore such that the target sequence is passed through the pore with the field until it finally translocates through to the trans side of the bilayer. Alternatively, the method is preferably carried out such that the enzyme moves the target sequence through the pore against the field resulting from the applied voltage. In this mode the 3′ end of the DNA is first captured in the pore, and the enzyme moves the DNA through the pore such that the target sequence is pulled out of the pore against the applied field until finally ejected back to the cis side of the bilayer. 
     The method of the invention most preferably involves a pore derived from MspA and a helicase comprising the sequence shown in SEQ ID NO: 8 or a variant thereof. Any of the embodiments discussed above with reference to MspA and SEQ ID NO: 8 may be used in combination. 
     Other Methods 
     The invention also provides a method of forming a sensor for characterising a target polynucleotide. The method comprises forming a complex between a pore and a XPD helicase. The complex may be formed by contacting the pore and the helicase in the presence of the target polynucleotide and then applying a potential across the pore. The applied potential may be a chemical potential or a voltage potential as described above. Alternatively, the complex may be formed by covalently attaching the pore to the helicase. Methods for covalent attachment are known in the art and disclosed, for example, in International Application Nos. PCT/GB09/001,679 (published as WO 2010/004265) and PCT/GB10/000,133 (published as WO 2010/086603). The complex is a sensor for characterising the target polynucleotide. The method preferably comprises forming a complex between a pore derived from Msp and a XPD helicase. Any of the embodiments discussed above with reference to the method of the invention equally apply to this method. 
     Kits 
     The present invention also provides kits for characterising a target polynucleotide. The kits comprise (a) a pore and (b) a XPD helicase. Any of the embodiments discussed above with reference to the method of the invention equally apply to the kits. 
     The kit may further comprise the components of a membrane, such as the phospholipids needed to form an amphiphilic layer, such as a lipid bilayer. 
     The kits of the invention may additionally comprise one or more other reagents or instruments which enable any of the embodiments mentioned above to be carried out. Such reagents or instruments include one or more of the following: suitable buffer(s) (aqueous solutions), means to obtain a sample from a subject (such as a vessel or an instrument comprising a needle), means to amplify and/or express polynucleotides, a membrane as defined above or voltage or patch clamp apparatus. Reagents may be present in the kit in a dry state such that a fluid sample resuspends the reagents. The kit may also, optionally, comprise instructions to enable the kit to be used in the method of the invention or details regarding which patients the method may be used for. The kit may, optionally, comprise nucleotides. 
     Apparatus 
     The invention also provides an apparatus for characterising a target polynucleotide. The apparatus comprises a plurality of pores and a plurality of a XPD helicase. The apparatus preferably further comprises instructions for carrying out the method of the invention. The apparatus may be any conventional apparatus for polynucleotide analysis, such as an array or a chip. Any of the embodiments discussed above with reference to the methods of the invention are equally applicable to the apparatus of the invention. 
     The apparatus is preferably set up to carry out the method of the invention. 
     The apparatus preferably comprises: 
     a sensor device that is capable of supporting the membrane and plurality of pores and being operable to perform polynucleotide characterising using the pores and helicases; 
     at least one reservoir for holding material for performing the characterising; 
     a fluidics system configured to controllably supply material from the at least one reservoir to the sensor device; and 
     a plurality of containers for receiving respective samples, the fluidics system being configured to supply the samples selectively from the containers to the sensor device. The apparatus may be any of those described in International Application No. PCT/GB08/004,127 (published as WO 2009/077734), PCT/GB10/000,789 (published as WO 2010/122293), International Application No. PCT/GB10/002,206 (not yet published) or International Application No. PCT/US99/25679 (published as WO 00/28312). 
     Characterisation without a Pore 
     In some embodiments, the target polynucleotide is characterised, such as partially or completely sequenced, using a XPD helicase, but without using a pore. In particular, the invention also provides a method of characterising a target polynucleotide which comprises contacting the target polynucleotide with a XPD helicase such that the XPD helicase controls the movement of the target polynucleotide. In this method, the target polynucleotide is preferably not contacted with a pore, such as a transmembrane pore. The method involves taking one or more measurements as the XPD helicase controls the movement of the polynucleotide and thereby characterising the target polynucleotide. The measurements are indicative of one or more characteristics of the target polynucleotide. Any such measurements may be taken in accordance with the invention. They include without limitation: electrical measurements and optical measurements. These are discussed in detail above. Any of the embodiments discussed above with reference to the pore-based method of the invention may be used in the method lacking a pore. For instance, any of the XPD helicases discussed above may be used. 
     The invention also provides an analysis apparatus comprising a XPD helicase. The invention also provides a kit a for characterising a target polynucleotide comprising (a) an analysis apparatus for characterising target polynucleotides and (b) a XPD helicase. These apparatus and kits preferably do not comprise a pore, such as a transmembrane pore. Suitable apparatus are discussed above. 
     The following Examples illustrate the invention. 
     Example 1 
     This example illustrates the use of a XPD helicase (XPD MBu) to control the movement of intact DNA strands through a nanopore. The general method and substrate employed throughout this example is shown in  FIG. 1  and described in the figure caption. 
     Materials and Methods 
     Primers were designed to amplify a ˜400 bp fragment of PhiX174. Each of the 5′-ends of these primers included a 50 nucleotide non-complimentary region, either a homopolymeric stretch or repeating units of 10 nucleotide homopolymeric sections. These serve as identifiers for controlled translocation of the strand through a nanopore, as well as determining the directionality of translocation. In addition, the 5′-end of the forward primer was “capped” to include four 2′-O-Methyl-Uracil (mU) nucleotides and the 5′-end of the reverse primer was chemically phosphorylated. These primer modifications then allow for the controlled digestion of predominantly only the antisense strand, using lambda exonuclease. The mU capping protects the sense strand from nuclease digestion whilst the PO4 at the 5′ of the antisense strand promotes it. Therefore after incubation with lambda exonuclease only the sense strand of the duplex remains intact, now as single stranded DNA (ssDNA). The generated ssDNA was then PAGE purified as previously described. 
     The DNA substrate design used in all the experiments described here is shown in  FIG. 1B . The DNA substrate consists of a 400 base section of ssDNA from PhiX, with a 50T 5′-leader to aid capture by the nanopore (SEQ ID NO: 63). Annealed to this strand just after the 50T leader is a primer (SEQ ID NO: 64) containing a 3′ cholesterol tag to enrich the DNA on the surface of the bilayer, and thus improve capture efficiency. An additional primer (SEQ ID NO: 65) is used towards the 3′ end of the strand to aid the capture of the strand by the 3′ end. 
     Buffered solution: 400 mM NaCl, 10 mM Hepes pH 8.0, 1 mM ATP, 1 mM MgCl 2 , 1 mM DTT 
     Nanopore:  E. coli  MS (B2)8 MspA ONLP3476 MS-(L88N/D90N/D91N/D93N/D118R/D134R/E139K)8 
     Enzyme: XPD Mbu (ONLP3696, ˜6.2 μM) 16.1 μl→100 nM final. 
     Electrical measurements were acquired from single MspA nanopores inserted in 1,2-diphytanoyl-glycero-3-phosphocholine lipid (Avanti Polar Lipids) bilayers. Bilayers were formed across ˜100 μm diameter apertures in 20 μm thick PTFE films (in custom Delrin chambers) via the Montal-Mueller technique, separating two 1 mL buffered solutions. All experiments were carried out in the stated buffered solution. Single-channel currents were measured on Axopatch 200B amplifiers (Molecular Devices) equipped with 1440A digitizers. Ag/AgCl electrodes were connected to the buffered solutions so that the cis compartment (to which both nanopore and enzyme/DNA are added) is connected to the ground of the Axopatch headstage, and the trans compartment is connected to the active electrode of the headstage. After achieving a single pore in the bilayer, DNA polynucleotide and helicase were added to 100 μL of buffer and pre-incubated for 5 mins (DNA=6 nM, Enzyme=1 μM) This pre-incubation mix was added to 900 μL of buffer in the cis compartment of the electrophysiology chamber to initiate capture of the helicase-DNA complexes in the MspA nanopore (to give final concentrations of DNA=0.6 nM, Enzyme=0.1 μM). Helicase ATPase activity was initiated as required by the addition of divalent metal (1 mM MgCl 2 ) and NTP (1 mM ATP) to the cis compartment. Experiments were carried out at a constant potential of +140 mV. 
     Results and Discussion 
     The addition of Helicase-DNA substrate to MspA nanopores as shown in  FIG. 1  produces characteristic current blocks as shown in  FIGS. 2 and 3 . DNA which is not complexed with a helicase interacts transiently with the nanopore producing short-lived blocks in current (&lt;&lt;1 second). DNA with helicase bound and active (ie. moving along the DNA strand under ATPase action) produces long characteristic block levels with stepwise changes in current as shown in  FIGS. 2 and 3 . Different DNA motifs in the nanopore give rise to unique current block levels. For a given substrate, we observe a characteristic pattern of current transitions that reflects the DNA sequence. 
     In the implementation shown in  FIG. 1 , the DNA strand is sequenced from a random starting point as the DNA is captured with a helicase at a random position along the strand. 
     Salt Tolerance 
     Nanopore strand sequencing experiments of this type generally require ionic salts. The ionic salts are necessary to create a conductive solution for applying a voltage offset to capture and translocate DNA, and to measure the resulting sequence dependent current changes as the DNA passes through the nanopore. Since the measurement signal is dependent in the concentration of the ions, it is advantageous to use high concentration ionic salts to increase the magnitude of the acquired signal. For nanopore sequencing salt concentrations in excess of 100 mM KCl are ideal, and salt concentrations in excess of 400 mM are preferred. 
     However, many enzymes (including some helicases and DNA motor proteins) do not tolerate high salt conditions. Under high salt conditions the enzymes either unfold or lose structural integrity, or fail to function properly. The current literature for known and studied helicases shows that almost all helicases fail to function above salt concentrations of approximately 100 mM KCl/NaCl, and there are no reported helicases that show correct activity in conditions of 400 mM KCl and above. While potentially halophilic variants of similar enzymes from halotolerant species exist, they are extremely difficult to express and purify in standard expression systems (e.g.  E. coli ). 
     We surprisingly show in this Example that XPD from Mbu displays salt tolerance up to very high levels of salt. We find that the enzyme retains functionality in salt concentrations of 400 mM KCl through to 1 M KCl, either in fluorescence experiments or in nanopore experiments. 
     Forward and Reverse Modes of Operation 
     Most helicases move along single-stranded polynucleotide substrates in uni-directional manner, moving a specific number of bases for each NTPase turned over. Although  FIG. 1  illustrates the use of this movement to feed threaded DNA through the nanopore into the trans chamber in the same direction as the applied potential, helicase movement could be exploited in other manners to feed DNA through the nanopore in a controlled fashion.  FIG. 4  illustrates two basic ‘forward’ and ‘reverse’ modes of operation. In the forward mode, the DNA is fed into the pore by the helicase in the same direction as the DNA would move under the force of the applied field. The direction of movement of the DNA is shown by the trans arrows. For XPD Mbu, which is a 5′-3′ helicase, this requires capturing the 5′ end of the DNA in the nanopore until a helicase contacts the top of the nanopore, and the DNA is then fed into the nanopore under the control of the helicase with the field from the applied potential, moving from cis to trans. The reverse mode requires capturing the 3′ end of the DNA, after which the helicase proceeds to pull the threaded DNA back out of the nanopore against the field from the applied potential, moving the DNA from cis to trans as indicated by the arrows.  FIG. 4  shows these two modes of operation using XPD Mbu. 
     Example 2 
     This example illustrates the salt tolerance of a XPD helicase (XPD Mbu) using a fluorescence assay for testing enzyme activity. 
     A custom fluorescent substrate was used to assay the ability of the helicase to displace hybridised dsDNA ( FIG. 5A ). As shown in 1) of  FIG. 5A , the fluorescent substrate strand (50 nM final) has a 5′ ssDNA overhang, and a 40 base section of hybridised dsDNA. The major upper strand has a carboxyfluorescein base at the 3′ end, and the hybrised complement has a black-hole quencher (BHQ-1) base at the 5′ end. When hybrised the fluorescence from the fluorescein is quenched by the local BHQ-1, and the substrate is essentially non-fluorescent. 1 μM of a capture strand that is complementary to the shorter strand of the fluorescent substrate is included in the assay. As shown in 2), in the presence of ATP (1 mM) and MgCl 2  (10 mM), helicase (150 nM) added to the substrate binds to the 5′ tail of the fluorescent substrate, moves along the major strand, and displaces the complementary strand as shown. As shown in 3), once the complementary strand with BHQ-1 is fully displaced the fluorescein on the major strand fluoresces. As shown in 4), an excess of capture strand preferentially anneals to the complementary DNA to prevent re-annealing of initial substrate and loss of fluorescence. 
     Substrate DNA: SEQ ID NO 66 with a carboxyfluorescein near the 3′ end and SEQ ID NO: 67 with a Black Hole Quencher-1 at the 5′ end. 
     Capture DNA: SEQ ID NO: 68 with a carboxyfluorescein near the 3′ end. 
     The graph in  FIG. 5B  shows the initial rate of activity in buffer solutions (100 mM Hepes pH 8.0, 1 mM ATP, 10 mM MgCl 2 , 50 nM fluorescent substrate DNA, 1 μM capture DNA) containing different concentrations of KCl from 100 mM to 1 M. The helicase works at 1 M.