Patent Publication Number: US-11046941-B2

Title: Methods of generating nucleic acid fragments

Description:
CROSS-REFERENCE 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/312,510, filed Mar. 10, 2010, which application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Nucleases are enzymes that degrade nucleic acids (e.g., deoxyribonucleic acids, DNA, and ribonucleic acids, RNA) and exist in various biological materials. These enzymes are involved in DNA and RNA metabolism, including degradation, synthesis and genetic recombination of nucleic acids. Nucleases are generally classified into exonucleases and endonucleases according to their mode of action. The former type acts on the terminus of a nucleic acid molecule and hydrolyzes the chain progressively to liberate nucleotides, while the latter type cleaves a phosphodiester bond in a nucleic acid molecule distributively to produce DNA or RNA fragments or oligonucleotides. 
     Deoxyribonucleases (DNases) are phosphodiesterases capable of hydrolyzing polydeoxyribonucleic acid. DNases have been purified from various species to various degrees. Among other uses, DNases find use as reagents in a variety of protocols in molecular biology. DNases have also been used for therapeutic purposes, for example, to reduce the viscosity of pulmonary secretions in such diseases as pneumonia and cystic fibrosis, thereby aiding in the clearing of respiratory airways. 
     Literature 
     Makarova et al. (2002)  Nucleic Acids Res  30:482-496; Makarova et al. (2006)  Biology Direct  1:1-26; Wiedenheft et al. (2009)  Structure  17:904. 
     SUMMARY 
     The present disclosure provides methods of using a Cas1 polypeptide to generate nucleic fragments from a DNA substrate. These methods may be used in vitro or in vivo. Also provided are methods of screening for modulators of Cas1. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts crystal structure of the Cas1 protein from  Pseudomonas aeruginosa.    
         FIGS. 2A-2C  depict two different orientations and two distinct folds in the β-strand domain of Cas1 from  P. aeruginosa . Panel A depicts superimposition of the C-terminal α-helical domains of molecules A and C. Panel B depicts a view down the barrel of α8. Panel C illustrates that superimposing the β-strand domains for molecule A and C highlights two structural differences. 
         FIGS. 3A and 3B  depict dimerization of the Cas1 protein from  P. aeruginosa . Panel A depicts a ribbon diagram of Cas1 homodimer with molecule A shown in grey and molecule C shown in light blue. Panel B shows a view down the dimer interface. 
         FIGS. 4A-4C  provide a schematic of the structural comparison of the Cas1 proteins from  P. aeruginosa  and  A. aeolicus . Panel A shows molecule A from PaCas1 (in grey) superimposed on molecule A from AaCas1 (light green). Panel B depict superposition of the α-helical domains of PaCas1 and AaCas1, as viewed from the perspective of the β-strand domain. Panel C depict the β-strand domains of these two molecules. 
         FIGS. 5A-5D  illustrate that Cas1 is a DNA-specific endonuclease. Panel A shows that Cas1 nuclease activity is restricted to DNA substrates. Panel B depicts the time course of Cas1 nuclease activity on four distinct dsDNA substrates. Panel C depicts the time course of Cas1 nuclease activity on linear dsDNA substrate. Panel D illustrates that Cas1 is an endonuclease. 
         FIGS. 6A-6C  illustrate that Cas1 is a metal-dependent nuclease. Panel A illustrates that manganese supports Cas1 mediated non-specific nuclease activity. Panel B shows that the metal preference of Cas1 is salt dependent. Panel C shows that Cas1 mediated cleavage of ssDNA is supported exclusively by manganese. 
         FIGS. 7A-7D  illustrate that mutants of conserved residues in the metal binding pocket of Cas1 inhibit nuclease activity. Panel A shows a close-up of the metal binding pocket in the α-helical domain of molecule A. Panel B shows SDS-PAGE of the purified Cas1 and Cas1 mutants. Panel C depicts nuclease activity assay for Cas1 and Cas1 mutants. 
         FIGS. 8A and 8B  depict Cas1 protein structure from  A. aeolicus . Panel A shows the superimposition of molecules A (green) and B (pink) of AaCas1. Panel B shows a dimer formed from molecules A (green) and B (pink) of AaCas1. 
         FIGS. 9A to 9F  depict Cas1 amino acid sequences (SEQ ID NOs:1-21). 
         FIG. 10  depicts an amino acid sequence of a  Pseudomonas aeruginosa  Cas1 polypeptide (SEQ ID NO:22). 
     
    
    
     DEFINITIONS 
     The term “biofilm” as used herein refers to an aggregate of microorganisms in which the microorganisms adhere to one another and/or to a surface. Such microorganisms can be embedded within a self-produced matrix of extracellular polymeric substance (EPS). Biofilm EPS, which is also referred to as “slime”, is a mixture of extracellular DNA, proteins, and polysaccharides. Biofilms may form on living or non-living surfaces, and represent a prevalent mode of microbial life in natural, industrial and hospital settings. 
     “Reducing or inhibiting” in reference to a biofilm refers to the prevention of biofilm formation or growth, reduction in the rate of biofilm formation or growth, partial or complete inhibition of biofilm formation or growth. 
     Before the present invention is further described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. 
     It must be noted that and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a method” includes a plurality of such methods and equivalents thereof known to those skilled in the art, and so forth. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. 
     The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. 
     DETAILED DESCRIPTION 
     Provided herein are methods of using Cas1 to generate nucleic fragments from a DNA substrate. These methods may be used in vitro or in vivo. Also provided are methods of screening for modulators of Cas1. 
     Methods Of Generating Nucleic Acid Fragments 
     The present disclosure provides methods for generating nucleic acid fragments of substantially uniform length from a DNA substrate. The methods generally involve contacting a DNA substrate with a Cas1 polypeptide. 
     “Cas1” polypeptide refers to  C RISPR  as sociated (Cas) protein1. CRISPR  C lustered,  r egularly  i nterspaced,  s hort  p alindromic  r epeats is an acronym that describes the architecture of these repetitive elements. Cas1 (COG1518 in the Clusters of Orthologous Group of proteins classification system) is the best marker of the  C RISPR- as sociated  s ystems (CASS). Based on phylogenetic comparisons, seven distinct versions of the CRISPR-associated immune system have been identified (CASS1-7). 
     Cas1 polypeptide used in the methods described herein can be any Cas1 polypeptide present in a prokaryote. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide of an archaeal microorganism. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide of a Euryarchaeota microorganism. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide of a Crenarchaeota microorganism. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide of a bacterium. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide of a gram negative or gram positive bacteria. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide of  Pseudomonas aeruginosa . In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide of  Aquifex aeolicus . In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide that is a member of one of CASS1-7. In certain embodiments, Cas1 polypeptide is a Cas1 polypeptide that is a member of CASS3. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide that is a member of CASS7. In certain embodiments, a Cas1 polypeptide is a Cas1 polypeptide that is a member of CASS3 or CASS7. 
     In some embodiments, a Cas1 polypeptide is encoded by a nucleotide sequence provided in GenBank at, e.g., GeneID number: 2781520, 1006874, 9001811, 947228, 3169280, 2650014, 1175302, 3993120, 4380485, 906625, 3165126, 905808, 1454460, 1445886, 1485099, 4274010, 888506, 3169526, 997745, 897836, or 1193018. 
     In certain embodiments, a Cas1 polypeptide comprises an amino acid sequence having at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 98%, at least about 99%, or 100%, amino acid identity to a contiguous stretch of from about 100 amino acids (aa) to about 150 aa, from about 150 aa to about 200 aa, from about 200 aa to about 250 aa, from about 250 aa to about 275 aa, from about 275 aa to about 300 aa, from about 300 aa to about 325 aa, up to the full length, of an amino acid sequence provided in  FIG. 9 . In certain embodiments, Cas1 polypeptide is a Cas1 polypeptide whose amino acid sequence is provided in  FIG. 9 . 
     In some embodiments, a Cas1 polypeptide comprises an amino acid sequence having at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 98%, at least about 99%, or 100%, amino acid identity to a contiguous stretch of from about 100 amino acids (aa) to about 150 aa, from about 150 aa to about 200 aa, from about 200 aa to about 250 aa, from about 250 aa to about 275 aa, from about 275 aa to about 300 aa, from about 300 aa to about 324 aa, of the amino acid sequence depicted in  FIG. 10 . 
     In certain embodiments, Cas1 protein may be a “functional derivative” of a naturally occurring Cas1 protein. A “functional derivative” of a native sequence polypeptide is a compound having a qualitative biological property in common with a native sequence polypeptide. “Functional derivatives” include, but are not limited to, fragments of a native sequence and derivatives of a native sequence polypeptide and its fragments, provided that they have a biological activity in common with a corresponding native sequence polypeptide. A biological activity contemplated herein is the ability of the functional derivative to hydrolyze a DNA substrate into fragments. The term “derivative” encompasses both amino acid sequence variants of polypeptide, covalent modifications, and fusions thereof. A “fusion” polypeptide is a polypeptide comprising a polypeptide or portion (e.g., one or more domains) thereof fused or bonded to heterologous polypeptide. Examples of fusion polypeptides include immunoadhesins which combine a portion of the Cas1 protein with an immunoglobulin sequence, and epitope tagged polypeptides, which may comprise a Cas1 protein, for example, or portion thereof fused to a “tag polypeptide”. The tag polypeptide has enough residues to provide an epitope against which an antibody can be made, yet is short enough such that it does not interfere with nuclease activity of Cas1. Suitable tag polypeptides generally have at least six amino acid residues and usually between about 6-60 amino acid residues. 
     “Cas1 polypeptide” encompasses a full-length Cas1 polypeptide, an enzymatically active fragment of a Cas1 polypeptide, and enzymatically active derivatives of a Cas1 polypeptide or fragment thereof. Suitable derivatives of a Cas1 polypeptide or a fragment thereof include but are not limited to mutants, fusions, covalent modifications of Cas1 protein or a fragment thereof. Cas1 protein which includes Cas1 protein or a fragment thereof, as well as derivatives of Cas1 protein or a fragment thereof, may be obtainable from a cell or synthesized chemically or by a combination of these two procedures. The cell may be a cell that naturally produces Cas1 protein, or a cell that naturally produces Cas1 protein and is genetically engineered to produce the endogenous Cas1 protein at a higher expression level or to produce a Cas1 protein from an exogenously introduced nucleic acid, which nucleic acid encodes a Cas1 that is same or different from the endogenous Cas1. In some case, the cell does not naturally produce Cas1 protein and is genetically engineered to produce a Cas1 protein. 
     Mutants of Cas1 protein may be generated by performing conservative substitutions which have substantially no effect on antigen binding or other antibody functions. By conservative substitutions is intended combinations such as those from the following groups: gly, ala; val, ile, leu; asp, glu; asn, gln; ser, thr; lys, arg; and phe, tyr. Amino acids that are not present in the same group are “substantially different” amino acids. In certain cases, the conserved residues may not be substituted and the substitutions limited to the non-conserved residues. 
     In certain embodiments, the Cas1 protein may be purified from an organism. The organism may be producing the Cas1 protein from an endogenous gene or from an exogenous gene. The exogenous gene may be present in the organism transiently or stably. For example, a polynucleotide encoding a Cas1 protein can be introduced into a suitable expression vector. The expression vector is introduced into a suitable cell. Expression vectors generally have convenient restriction sites located near the promoter sequence to provide for the insertion of polynucleotide sequences. Transcription cassettes may be prepared comprising a transcription initiation region, cas1 gene or fragment thereof, and a transcriptional termination region. The transcription cassettes may be introduced into a variety of vectors, e.g. plasmid; retrovirus, e.g. lentivirus; adenovirus; and the like, where the vectors are able to transiently or stably be maintained in the cells, usually for a period of at least about one day, more usually for a period of at least about several days to several weeks. 
     The various manipulations may be carried out in vitro or may be performed in an appropriate host, e.g.  E. coli . After each manipulation, the resulting construct may be cloned, the vector isolated, and the DNA screened or sequenced to ensure the correctness of the construct. The sequence may be screened by restriction analysis, sequencing, or the like. 
     Cas1 protein may be recovered and purified from recombinant cell cultures by well-known methods including ammonium sulfate or ethanol precipitation, acid extraction, anion or cation exchange chromatography, phosphocellulose chromatography, hydrophobic interaction chromatography, high performance liquid chromatography, affinity chromatography, protein G affinity chromatography, for example, hydroxyapatite chromatography and lectin chromatography, etc. 
     Cas1 protein may also be recovered from: products of purified cells, whether directly isolated or cultured; products of chemical synthetic procedures; and products produced by recombinant techniques from a prokaryotic or eukaryotic host, including, for example, bacterial, yeast higher plant, insect, and mammalian cells. 
     As mentioned above, methods for generating nucleic acid fragments of substantially uniform length from a DNA substrate are provided. The methods include contacting the DNA substrate with a Cas1 polypeptide. 
     The duration of the contacting step may be about 0.1 hour-48 hours, for example, from about 0.1 hour to about 0.2 hour, from about 0.2 hour to about 0.3 hour, from about 0.3 hour to about 0.5 hour, from about 0.5 hour to about 1 hour, from about 0.3 hour to about 46 hours, about 0.5 hour-45 hours, about 1 hour-40 hours, about 2 hours-35 hours, about 4 hours-30 hours, about 6 hours-24 hours, about 8 hours -20 hours, about 10 hours-18 hours, or about 12 hours-16 hours, such as, 0.3 hour, 0.5 hour, 1 hour, 3 hours, 10 hours, 13 hours, 16 hours, or 18 hours. 
     The amount of Cas1 that is employed is one that is from about 10 units/ml-50,000 units/ml, for example, from about 20 units/ml-30,000 units/ml, about 30 units/ml-10,000 units/ml, about 50 units/ml-5000 units/ml, about 100 units/ml-3000 units/ml, about 200 units/ml-2000 units/ml, about 300 units/ml-1000 units/ml, such as, about 100 units/ml, 300 units/ml, 1000 units/ml, 2000 units/ml, 5000 units/ml, 10,000 units/ml, 20,000 units/ml, or 50,000 units/ml. 
     The temperature at which the enzymatic reaction is carried out is can be from 4° C.-50° C., for example, about 10° C.-45° C., about 16° C.-40° C., about 20° C.-37° C., about 25° C.-35° C., about 30° C.-33° C., e.g., 10° C., 18° C., 25° C., 30° C., 37° C., or 45° C. 
     The contacting step may be carried out in conditions suitable for Cas1 endonuclease activity. In certain embodiments, the conditions suitable for Cas1 endonuclease activity are conditions in which a divalent metal ion such as magnesium (Mg 2+ ) is present. In these embodiments, the Mg 2+  concentration may range from about 1 mM-25 mM, for example, about 1.5 mM-20 mM, about 2 mM-15 mM, about 2 mM-10 mM, about 3 mM-8 mM, or about 5 mM-6 mM, such as, 2 mM, 2.5 mM, 3 mM, or 5 mM. 
     In certain embodiments, the conditions suitable for Cas1 endonuclease activity are conditions in which a divalent metal ion such as Manganese (Mn 2+ ) is present. In these embodiments, the Mn 2+  concentration may range from about 1 mM-25 mM, for example, about 1.5 mM-20 mM, about 2 mM-15 mM, about 2 mM-10 mM, about 3 mM-8 mM, or about 5 mM-6 mM, such as, 2 mM, 2.5 mM, 3 mM, or 5 mM. 
     Under the conditions suitable for Cas1 endonuclease activity, the pH typically ranges from about pH 4.5-pH 10, for example, pH 5-pH 8.5, pH 7-pH 8.5, or pH 7-pH 8, such as, pH 7, pH 7.5, pH 8, or pH 8.5. 
     The DNA substrate may be in the form of genomic DNA, linear DNA, circular DNA, double or single stranded DNA, or a mixture of two or more of these forms of DNA. The DNA substrate may be from any organism, for example, viruses, prokaryotes, e.g. bacteria, archaea and cyanobacteria; and eukaryotes, e.g., members of the kingdom protista, such as flagellates, amoebas and the like, amoeboid parasites, ciliates and the like; members of the kingdom fungi, such as slime molds, acellular slime molds, cellular slime molds, water molds, true molds, conjugating fungi, sac fungi, club fungi, imperfect fungi and the like; plants, such as algae, mosses, liverworts, hornworts, club mosses, horsetails, ferns, gymnosperms and flowering plants, both monocots and dicots; and animals, including sponges, members of the phylum cnidaria, e.g. jelly fish, corals and the like, combjellies, worms, rotifers, roundworms, annelids, molluscs, arthropods, echinoderms, acorn worms, and vertebrates, including reptiles, fishes, birds, snakes, and mammals, e.g. rodents, primates, including humans, and the like. DNA substrates may be obtained from biological fluids, e.g., blood; tissue samples; or cells (including cell lines, cell cultures, etc.), for example. The DNA substrate may be used directly from its naturally occurring source and/or preprocessed in a number of different ways, as is known in the art. 
     The DNA substrate can be present in a living cell, or can be isolated from a living cell. For example, the DNA substrate can be present in a cell lysate. In some embodiments, the DNA substrate is isolated, and can be purified, e.g., the DNA substrate can be at least about 50% pure, at least about 60% pure, at least about 70% pure, at least about 80% pure, at least about 85% pure, at least about 90% pure, at least about 95% pure, at least about 98% pure, at least about 99%, or greater than 99% pure, e.g., free of macromolecules other than the DNA substrate, and free of other contaminants. 
     The term “substantially uniform length” when used in reference to nucleic acid fragments, is used to refer to a population of nucleic acid fragments wherein a majority of the fragments have the same length within an acceptable variation. For example, the acceptable variation in the length of a given fragment in the population can be at most 0.1%, 1%, 2%, 5%, 8%, 10%, or 20% of the average length of fragments in the population. This can be a variation in length of at most about 1 nucleotide, 2 nucleotides, 3 nucleotides, 4 nucleotides, 5 nucleotides, 8 nucleotides, 10 nucleotides, 13 nucleotides, 16 nucleotides, 18 nucleotides, 20 nucleotides, 25 nucleotides, 30 nucleotides, 35 nucleotides, 40 nucleotides, 45 nucleotides, 50 nucleotides, 55 nucleotides, or 60 nucleotides. The population can be composed of at least 50%, 60%, 70%, 80%, 90%, 95%, 99% or 99.9% fragments having a particular length. 
     The substantially uniform length of the nucleic acid fragments generated by contacting a DNA substrate with Cas1 polypeptide may be about 20 base pairs (bp)-1000 bp long, or about 30 bp-750 bp long, or about 40 bp-500 bp long, or about 45 bp-250 bp long, or about 50 bp-200 bp long, or about 60 bp-150 bp long, or about 70 bp-100 bp long, for example, about 30 bp, or about 50 bp, or about 80 bp, or about 100 bp, or about 150 bp, or about 200 bp. About as used herein refers to the value or range indicated±1 bp, or 2 bp, or 3 bp, or 4 bp, or 5 bp. 
     In practicing the subject methods, the order in which the various reagents are contacted with the DNA substrate may vary. As such, in certain embodiments, the Cas1 endonuclease may be introduced into a reaction mix after the introduction of any other reagents, e.g., Mn 2+ . In some embodiments, the Cas1 endonuclease may be introduced into the reaction mix before the introduction of some other reagents, e.g., adapter oligonucleotides. The manner in which contacting is achieved may vary, e.g., by introducing Cas1 endonuclease into the reaction mix, by introducing an amount of DNA substrate in a Cas1 endonuclease containing reaction mix, etc. 
     Screening Methods 
     Methods for identifying modulators of Cas1 endonuclease activity are provided. The methods may comprise assaying the nuclease activity of Cas1 in the presence of a candidate agent wherein an increase or decrease in Cas1 endonuclease activity identifies the candidate agent as a modulator of Cas1 endonuclease activity. 
     Candidate agents of interest for screening include biologically active agents of numerous chemical classes, primarily organic molecules, although including in some instances, inorganic molecules, organometallic molecules, immunoglobulins, genetic sequences, etc. Also of interest are small organic molecules, which comprise functional groups necessary for structural interaction with proteins, particularly hydrogen bonding, and typically include at least an amine, carbonyl, hydroxyl or carboxyl group, frequently at least two of the functional chemical groups. The candidate agents often comprise cyclical carbon or heterocyclic structures and/or aromatic or polyaromatic structures substituted with one or more of the above functional groups. Candidate agents are also found among biomolecules, including peptides, polynucleotides, saccharides, fatty acids, steroids, purines, pyrimidines, derivatives, structural analogs or combinations thereof. 
     Compounds may be obtained from a wide variety of sources including libraries of synthetic or natural compounds. For example, numerous means are available for random and directed synthesis of a wide variety of organic compounds, including biomolecules, including expression of randomized oligonucleotides and oligopeptides. Alternatively, libraries of natural compounds in the form of bacterial, fungal, plant and animal extracts are available or readily produced. Additionally, natural or synthetically produced libraries and compounds are readily modified through conventional chemical, physical and biochemical means, and may be used to produce combinatorial libraries. Known pharmacological agents may be subjected to directed or random chemical modifications, such as acylation, alkylation, esterification, amidification, etc. to produce structural analogs. 
     A plurality of assays may be run in parallel with different concentrations to obtain a differential response to the various concentrations. As known in the art, determining the effective concentration of an agent typically uses a range of concentrations resulting from 1:10, or other log scale, dilutions. The concentrations may be further refined with a second series of dilutions, if necessary. Typically, one of these concentrations serves as a negative control, i.e. at zero concentration or below the level of detection of the agent or at or below the concentration of agent that does not give a detectable change in nuclease activity. 
     The assaying may comprise contacting the candidate agent to a reaction mix that includes Cas1, a source of divalent metal ion, and a DNA substrate; measuring the activity of Cas1 in the reaction mix, comparing the measured activity to the activity of a control reaction mix that includes Cas1, a source of divalent metal ion, and a DNA substrate but not the candidate agent being tested; and identifying a candidate agent that increases or decreases the Cas 1 endonuclease activity. 
     Any type of nuclease assay may be used. In certain examples, the assay may be plasmid DNA digestion assays, such as, supercoiled DNA digestion assay or linear DNA digestion assay, or a hyperchromicity assay. 
     Plasmid DNA Digestion Assays 
     A supercoiled plasmid DNA digestion assay measures the conversion of supercoiled double-stranded plasmid, e.g., pBR322 DNA to relaxed (nicked), linear, and fragmented forms. The linear DNA digestion assay measures the conversion of linear double-stranded DNA to degraded forms. 
     Cas1 protein with or without a candidate agent may be added to a solution containing supercoiled double-stranded plasmid or linear double-stranded DNA in an appropriate reaction mix including a buffer, bovine serum albumin (BSA), salt, divalent metal ion, etc. and incubated at around 25° C. At various times, aliquots of the reaction mixtures may be removed and quenched by the addition of a metal chelator, such as, 25 mM EDTA (ethylene-diamine-tetra-acetic acid), together with reagents for electrophoretic analysis of DNA, such as, xylene cyanol, bromphenol blue, and glycerol. The integrity of the supercoiled or linear DNA in the quenched samples may be analyzed by electrophoresis of the samples on agarose gels (for example, 0.8% weight/vol.). After electrophoresis, the gels may be stained with a solution of ethidium bromide and the DNA in the gels visualized by ultraviolet light. The relative amounts of supercoiled, relaxed, and linear forms of plasmid DNA may be determined by scanning of the gels with a FluorImager and quantitating the amount of DNA in the bands of the gel that corresponded to those different forms. 
     In the supercoiled DNA digestion assay, the overall activity of the Cas1 may be measured as the initial rate of disappearance of supercoiled DNA (as a result of it being converted to relaxed (nicked), linear, or degraded DNA), normalized relative to the rate observed with Cas1 without candidate agent. The ratio of linearized to relaxed forms of DNA may also be determined relative to that observed with Cas1 without candidate agent. In the linear DNA digestion assay, the activity of Cas1 with candidate agent may be measured as the initial rate of disappearance of linear DNA (as a result of it being converted to degraded forms), normalized relative to the rate observed with Cas1 without candidate agent. 
     Modulators of Cas1 endonuclease activity that increase Cas1 activity may be used in vitro or in vivo to enhance Cas1 activity. For instance, such modulators may be added to compositions of Cas1 or used in cell cultures in a laboratory setting. For example, such modulators may serve to enhance the activity of an endogenous Cas1 expressed by a cell in a cell culture and provide an enhanced protection to infection by phages and other pathogens. 
     Modulators of Cas1 endonuclease activity that decrease Cas1 activity may be used in vitro or in vivo to decrease Cas1 activity. For instance, such modulators may be used to increase the susceptibility of an organism that utilizes Cas1 to defend against viral or other pathogens to such pathogens. Therefore, Cas1 modulators that decrease Cas1 activity may be used to weaken an organism, for example. 
     Utility 
     Nucleic Acid Analysis 
     Cas1 may be used to generate DNA fragments for use in a variety of research and diagnostic methods. For example, the nucleic acid fragments of substantially uniform length generated by using Cas1 may be used for sequencing, genotyping, copy number variation analysis, DNA methylation analysis, and the like. 
     In some embodiments, the nucleic acid fragments of substantially uniform size generated by using Cas1 do not usually require size selection by a size separation method such as gel purification and as such almost all of the nucleic acid fragments are available for subsequent use. This is especially advantageous in analysis of nucleic acid from samples where the amount of material is limited, such as biopsies, laser captured cells, limited archival tissues, embryoid bodies, small model systems, and difficult to cultivate organisms such as  Microsporidia.    
     Nucleic Acid Fragment Libraries 
     The nucleic acid fragments of substantially uniform size range generated by using Cas1 may include fragments with blunt ends and/or 3′ and 5′ overhanging ends. The fragment ends may be repaired using methods or kits known in the art to generate ends that are convenient, for example, for insertion into blunt sites in cloning vectors or for ligation of adapters onto the ends of each fragment. 
     Nucleic acid fragment libraries may be prepared from the nucleic acid fragments. Following end repair, double stranded adaptor polynucleotide sequences may be ligated to both ends of the nucleic acid fragments to form adaptor-fragment-adaptor polynucleotide sequences. 
     Ligation methods are known in the art and utilize standard methods (Sambrook and Russell, Molecular Cloning, A Laboratory Manual, third edition Cold Spring Harbor Laboratory Press (2001)). Such methods utilize ligase enzymes such as DNA ligase to effect or catalyze joining of the ends of the two polynucleotide strands of, in this case, the adaptor duplex construct and the nucleic acid fragment, such that covalent linkages are formed. 
     The adaptor constructs may also contain a region on one, or both, of the strands that does not hybridize with a sequence on the other strand of the adaptor. Such “mismatched” adaptors can serve as priming sites for amplification reactions. Optionally, the adaptor-fragment-adaptor molecules may be purified from any components of the ligation reaction, such as enzymes, buffers, salts and the like. Suitable purification methods are known in the art and utilize standard methods (Sambrook and Russell, Supra). 
     In further embodiments, the adaptor-fragment-adaptor molecules may be amplified. The contents of an amplification reaction are known by one skilled in the art and include appropriate reagents (such as, deoxyribonucleotide triphosphates (dNTPs)), enzymes (e.g. a DNA polymerase) and buffer components required for an amplification reaction. Generally amplification reactions use at least two amplification primers, often denoted ‘forward’ and ‘reverse’ primers (primer oligonucleotides) that are capable of annealing specifically to a part of the polynucleotide sequence to be amplified under conditions encountered in the primer annealing step of each cycle of an amplification reaction. In certain embodiments the forward and reverse primers may be identical. 
     The nucleic acid fragment libraries comprising cloned nucleic acid fragments or nucleic acid fragment to which adapters have been ligated may be used in research or diagnostic methods. 
     Generating Labeled Probes 
     The nucleic acid fragments generated by using Cas1 may be labeled to generate labeled nucleic acid fragments that can be used a probes, e.g., for use in research and/or diagnostic methods. 
     Any label detectable by spectroscopic, photochemical, biochemical, immunochemical, electrical, optical or chemical means may be used to label the nucleic acid fragments. Useful labels include biotin for staining with labeled streptavidin conjugate, magnetic beads (e.g., Dynabeads™), fluorescent dyes (e.g., fluorescein, texas red, rhodamine, green fluorescent protein, cyanins and the like), radiolabels (e.g.,  3 H,  35 S,  14 C, or  32 P, enzymes (e.g., horseradish peroxidase, alkaline phosphatase and others commonly used in enzyme-linked immunosorbent assay), and colorimetric labels such as colloidal gold or colored glass or plastic (e.g., polystyrene, polypropylene, latex, etc.) beads. Patents teaching the use of such labels include U.S. Pat. Nos. 3,817,837; 3,850,752; 3,939,350; 3,996,345; 4,277,437; 4,275,149; and 4,366,241, which are herein incorporated by reference. 
     The labels may be incorporated into the nucleic acid fragments by any of a number of means well known to those of skill in the art. The label may be simultaneously incorporated during the amplification step. Thus, for example, polymerase chain reaction (PCR) with labeled primers or labeled nucleotides will provide a labeled amplification product. In certain embodiment, a label may be added directly to the nucleic acid fragments or to the amplification product after the amplification is completed. Means of attaching labels to nucleic acids are well known to those of skill in the art and include, for example nick translation or end-labeling, by kinasing of the nucleic acid and subsequent attachment of a nucleic acid linker joining the nucleic acid to a label. Standard methods may be used for labeling a polynucleotide fragment, for example, as set out in Maniatis et al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Publication (1982). 
     Sequencing 
     Any suitable method of sequencing may be used to determine a sequence read of the nucleic acid fragments prepared using Cas1. Suitable methods of sequencing include the use of sequencing by addition of nucleotide bases, for example sequencing by synthesis (SBS) using nucleoside triphosphates and DNA polymerases (as described in US 2007/0166705 and US 2006/0240439 respectively), or using oligonucleotide cassettes and ligases (as described in U.S. Pat. No. 6,306,597, US 2008/0003571 or Science, 309:5741, 1728-1732 (2005)). 
     In “sequencing by synthesis” or SBS a new polynucleotide strand base-paired to a template strand is built up in the 5′ to 3′ direction by successive incorporation of individual nucleotides complementary to the template strand. In one embodiment of SBS the different nucleotide triphosphates used in the sequencing reaction are each labeled with different labels permitting determination of the identity of the incorporated nucleotide as successive nucleotides are added. The labeled nucleotide triphosphates also have a 3′ blocking group which prevents further incorporation of complementary bases by the polymerase. The label of the incorporated base can then be determined and the blocking group removed to allow further polymerization to occur. Labeled nucleotides are described in WO07135368. 
     Sequencing multiple nucleic acid fragments produced by Cas1 may be performed in parallel using arrays, wherein multiple polynucleotide fragments (with or without adapters) are immobilized on an array and are sequenced in parallel. For example, nucleotide(s) is(are) incorporated into a strand of nucleic acid complementary to the template nucleic and each nucleotide is fluorescently labeled. The inclusion of a fluorescent label facilitates detection/identification of the base present in the incorporated nucleotide(s). Appropriate fluorophores are well known in the art. Use of the polynucleotide fragments of substantially uniform size in nucleic acid analysis is described in US Application Publication No. 20090191563, which is herein incorporated by reference. 
     Treatment Methods 
     Biofilms 
     Cas1 polypeptide may be used to reduce or inhibit biofilms. 
     Biofilms form on living and non-living surfaces and represent a prevalent mode of microbial life in natural, industrial and hospital settings. Biofilms have been found to be involved in a wide variety of microbial infections in the body, by one estimate 80% of all infections. Infectious processes in which biofilms have been implicated include common problems such as urinary tract infections, catheter infections, middle-ear infections, formation of dental plaque, gingivitis, and less common but more lethal processes such as endocarditis, infections in cystic fibrosis, and infections of permanent indwelling devices such as joint prostheses and heart valves. More recently it has been noted that bacterial biofilms may impair cutaneous wound healing and reduce topical antibacterial agents&#39; efficiency in healing or treating infected skin wounds. Biofilms can also form on the inert surfaces of implanted devices such as catheters, prosthetic cardiac valves and intrauterine devices. 
     Cas1 protein may be employed to prevent microorganisms from adhering to surfaces or growing on surfaces, which surfaces may be porous, soft, hard, semi-soft, semi-hard, regenerating, or non-regenerating. These surfaces include, but are not limited to, polyurethane, metal, alloy, or polymeric surfaces in medical devices, enamel of teeth, and cellular membranes in animals, preferably, mammals, more preferably, humans. The surfaces may be coated or impregnated with the Cas1 protein prior to use. Alternatively, the surfaces may be treated with Cas1 protein to control, reduce, or eradicate the microorganisms adhering to these surfaces. 
     Cas1 may be used to reduce the viscoelasticity of DNA-containing material, including sputum, mucus, or other pulmonary secretions of patients with pulmonary disease. Abnormal viscous or inspissated secretions (e.g., sputum, mucus, or other pulmonary secretions) are common in pulmonary diseases such as acute or chronic bronchial pneumonia, including infectious pneumonia, bronchitis or tracheobronchitis, bronchiectasis, cystic fibrosis, asthma, tuberculosis, and fungal infections. For such therapies, a solution or finely divided dry preparation of Cas1 may be instilled in conventional fashion into the airways (e.g., bronchi) or lungs of a patient, for example by aerosolization. 
     Cas1 polypeptide can also useful for treatment of abscesses or severe closed-space infections in conditions such as emphysema, meningitis, abscess, peritonitis, sinusitis, periodontitis, pericarditis, pancreatitis, cholelithiasis, endocarditis and septic arthritis, as well as in topical treatments in a variety of inflammatory and infected lesions such as infected lesions of the skin and/or mucosal membranes, surgical wounds, ulcerative lesions and burns. Cas1 may improve the efficacy of antibiotics used in the treatment of such infections (e.g., gentamicin activity is markedly reduced by reversible binding to intact DNA). 
     Cas1 protein may contribute to the treatment of cystic fibrosis. In cystic fibrosis,  Pseudomonas aeruginosa  reside on the lungs of cystic fibrosis patients. Cas1 protein may prevent, reduce, or eradicate the biofilm of  Pseudomonas aeruginosa.    
     Cas1 polypeptide can be used as a preprocedural rinse for surgery, as an antiseptic rinse, a topical antiseptic and a catheter lock solution. 
     Cas1 polypeptide may also be used for enhancing efficacy of antibiotic therapy against bacterial infections by administration of a pharmaceutical composition of Cas1 polypeptide in combination with or prior to administration of an antibiotic. 
     Cas1 protein or active fragment or derivative thereof can be incorporated in a liquid disinfecting solution. Such solutions may further comprise antimicrobials or antifungals such as alcohol, providone-iodine solution and antibiotics as well as preservatives. These solutions can be used, for example, as disinfectants of the skin or surrounding area prior to insertion or implantation of a device such as a catheter, as catheter lock and/or flush solutions, and as antiseptic rinses for any medical device including, but not limited to catheter components such as needles, Leur-Lok connectors, needleless connectors and hubs as well as other implantable devices. These solutions can also be used to coat or disinfect surgical instruments including, but not limited to, clamps, forceps, scissors, skin hooks, tubing, needles, retractors, scalers, drills, chisels, rasps and saws. 
     Cas1 protein may be formulated into a variety of formulations for therapeutic administration. More particularly, Cas1 protein as disclosed herein can be formulated into pharmaceutical compositions by combination with appropriate pharmaceutically acceptable carriers or diluents, and may be formulated into preparations in solid, semi-solid, liquid forms, such as, powders, granules, solutions, injections, inhalants, gels, hydrogels, microspheres, etc. Pharmaceutical compositions can include, depending on the formulation desired, pharmaceutically-acceptable, non-toxic carriers of diluents, which are defined as vehicles commonly used to formulate pharmaceutical compositions for animal or human administration. The diluent is selected so as not to affect the biological activity of the combination. Examples of such diluents are distilled water, buffered water, physiological saline, phosphate buffered saline (PBS), Ringer&#39;s solution, dextrose solution, and Hank&#39;s solution. In addition, the pharmaceutical composition or formulation can include other carriers, adjuvants, or non-toxic, nontherapeutic, nonimmunogenic stabilizers, excipients and the like. The compositions can also include additional substances to approximate physiological conditions, such as pH adjusting and buffering agents, toxicity adjusting agents, wetting agents and detergents. 
     A pharmaceutical composition comprising Cas1 polypeptide may be an oral preparation, an injection or an aerosol preparation. Preparations suitable for oral administration may be a liquid obtained by dissolving an effective amount of Cas 1 in diluents such as water, physiological saline, a capsule, a sachet or a tablet containing an effective amount of Cas1, suspension containing an effective amount of Cas1 suspended in an appropriate dispersion medium, and emulsion prepared by suspending a solution containing an effective amount of Cas1 dissolved in an appropriate dispersion medium and emulsifying the suspension. The aerosol preparation may include Cas1 compressed with dichlorodifluoromethane, propane or nitrogen or a non-compressed preparation such as nebulizer and atomizer, and can be administered by inhalation or spraying into airways and the like. 
     A Cas1 pharmaceutical composition may be combined with or administered in concert with one or more other pharmacologic agents, such as antibiotics, bronchodilators, anti-inflammatory agents, mucolytics (e.g. n-acetyl-cysteine), actin binding or actin severing proteins (e.g., gelsolin; Matsudaira et al., Cell 54:139-140 (1988); Stossel, et al., PCT Patent Publication No. WO 94/22465 (published Oct. 13, 1994)), protease inhibitors, gene therapy product (e.g., comprising the cystic fibrosis transmembrane conductance regulator (CFTR) gene, Riordan, et al., Science 245:1066-1073 (1989)), glucocorticoids, or cytotoxic agents. 
     The pharmaceutical composition can also include any of a variety of stabilizing agents, such as an antioxidant for example. The Cas1 polypeptide of a composition can also be complexed with molecules that enhance its in vivo attributes. Such molecules include, for example, carbohydrates, polyamines, amino acids, other peptides, ions (e.g., sodium, potassium, calcium, magnesium, manganese), and lipids. Cas1 may be incorporated into liposomes or microvesicles. 
     Wound dressings including but not limited to sponges or gauzes can be impregnated with a composition comprising Cas1 polypeptide or active fragment or derivative thereof to prevent or inhibit bacterial or fungal attachment and reduce the risk of wound infections. Similarly, catheter shields as well as other materials used to cover a catheter insertion sites can be coated or impregnated with Cas1 polypeptide or active fragment or derivative thereof to inhibit bacterial or fungal biofilm attachment thereto. Adhesive drapes used to prevent wound infection during high risk surgeries can be impregnated with the isolated protein or active fragment or variant thereof as well. Additional medical devices which can be coated with Cas1 polypeptide or active fragment or derivative thereof include, but are not limited, central venous catheters, intravascular catheters, urinary catheters, Hickman catheters, peritoneal dialysis catheters, endotracheal catheters, mechanical heart valves, cardiac pacemakers, arteriovenous shunts, scleral buckles, prosthetic joints, tympanostomy tubes, tracheostomy tubes, voice prosthetics, penile prosthetics, artificial urinary sphincters, synthetic pubovaginal slings, surgical sutures, bone anchors, bone screws, intraocular lenses, contact lenses, intrauterine devices, aortofemoral grafts and vascular grafts. Exemplary solutions for impregnating gauzes or sponges, catheter shields and adhesive drapes or coating catheter shields and other medical devices include, but are not limited to, PBS (pH approximately 7.5) and bicarbonate buffer (pH approximately 9.0). 
     The pharmaceutical compositions can be administered for prophylactic and/or therapeutic treatments. Toxicity and therapeutic efficacy of a Cas1 polypeptide can be determined according to standard pharmaceutical procedures in cell cultures and/or experimental animals, including, for example, determining the LD 50  (the dose lethal to 50% of the population) and the ED 50  (the dose therapeutically effective in 50% of the population). The dose ratio between toxic and therapeutic effects is the therapeutic index and it can be expressed as the ratio LD 50 /ED 50 . 
     The data obtained from cell culture and/or animal studies can be used in formulating a range of dosages for humans. The dosage of an active ingredient typically lines within a range of circulating concentrations that include the ED 50  with low toxicity. The dosage can vary within this range depending upon the dosage form employed and the route of administration utilized. 
     The components used to formulate the pharmaceutical compositions are preferably of high purity and are substantially free of potentially harmful contaminants (e.g., at least National Food (NF) grade, generally at least analytical grade, and more typically at least pharmaceutical grade). Moreover, compositions intended for in vivo use are usually sterile. 
     The pharmaceutical compositions may be administered using any medically appropriate routes, e.g., an epithelial route such as intranasal, pulmonary, sublingual, oral, buccal, or other routes such as intravascular (intravenous, intraarterial, intracapillary), injection into the cerebrospinal fluid, intracavity or direct injection into a tissue. 
     The effective amount of a therapeutic composition to be given to a particular patient will depend on a variety of factors, several of which will be different from patient to patient. Utilizing ordinary skill, the competent clinician will be able to optimize the dosage of a particular therapeutic in the course of routine clinical trials. 
     Kits 
     Also provided herein are kits that include one or more containers of the components of the compositions described herein. 
     A subject kit includes at least an isolated Cas1 polypeptide. In some embodiments, the Cas1 polypeptide is lyophilized. In some embodiments, the containers may include a lyophilized Cas1 polypeptide. In some embodiments, the containers may include Cas1 polypeptide suspended in an aqueous medium, where the aqueous medium may be a buffer, for example, PBS, Tris-buffered saline, Tris-Hydrochloride. The medium may include addition components, such as glycerol, or other agents, for example, BSA, dithiothreitol (DTT), that stabilize proteins. The medium may further comprise salt (e.g., sodium chloride, or potassium chloride), additives to prevent microbial growth, such as EDTA, EGTA (ethylene glycol tetra-acetic acid). The kit may further include a container of reaction buffer which may be used in a reaction mixture comprising Cas1 polypeptide. The reaction buffer may include a divalent metal ion, for example, Mg 2+  or Mn 2+ . In addition the reaction buffer may include one or more of: a buffer, one or more salts, glycerol, DTT, BSA, etc. Other suitable components include, e.g., a nuclease inhibitor, a protease inhibitor, and the like. 
     In some cases, the kit may include a first container comprising a Cas1 polypeptide; and a second container comprising at least a second component, e.g., a solution comprising a divalent metal ion, for example, Mg 2+  or Mn 2+ ; a protease inhibitor; a nuclease inhibitor; etc. In some case, the kit may include a first container comprising a Cas1 polypeptide and a divalent metal ion, for example, Mg 2+  or Mn 2+ . In some case, the kit may include a first container comprising a Cas1 polypeptide and a divalent metal ion, for example, Mg 2+  or Mn 2+ ; and a second container comprising a reaction buffer. 
     The kits may further include a suitable set of instructions, generally written instructions, relating to the use of a Cas1 polypeptide for hydrolyzing a DNA substrate in vitro or in vivo. 
     EXAMPLES 
     The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the present invention, and are not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all or the only experiments performed. Efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, temperature, etc.) but some experimental errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, temperature is in degrees Celsius, and pressure is at or near atmospheric. Standard abbreviations may be used, e.g., bp, base pair(s); kb, kilobase(s); pl, picoliter(s); s or sec, second(s); min, minute(s); h or hr, hour(s); aa, amino acid(s); kb, kilobase(s); bp, base pair(s); nt, nucleotide(s); and the like. 
     Materials and Methods 
     PA14_Cas1 expression and purification. Genomic DNA isolated from strain 14 of  P. aeruginosa  (PA14) was used as the template for PCR amplification of the cas1 gene (PA14_33350; GeneID: 4380485) (Lee et al., 2006). The PCR product generated from PA14Cas1_FWD caccatggacgacatttctcccag (SEQ ID NO:23) and PA14Cas1_REV ttatcatgcggatactgtgctc (SEQ ID NO:24) was cloned into pENTR™/TEV/D-TOPO using the Gateway system (Invitrogen). The cas1 sequence was confirmed by DNA sequencing and then recombined into a Gateway compatible expression vector (pHMGWA) containing an N-terminal His6MBP tag. The His6MBP-Cas1 fusion protein was expressed in BL21(DE3) cells that were induced with 0.5 mM isopropyl-β-D-thiogalactopyranoside (IPTG) at OD 600 =0.5 in overnight cultures grown at 16° C. Cells from the overnight expression cultures were harvested by centrifugation (10,000×g) for 20 minutes. The cell pellet was resuspended in lysis buffer (20 mM imidazole, 0.01% Triton X-100, 100 u/ml DNaseI, 2 mM Tris(2-carboxyethyl) phosphine hydrochloride (TCEP), 0.5 mM phenylmethylsulfonyl fluoride (PMSF), protease inhibitors, 10% glycerol) and the slurry was sonicated on ice for 2 min in 10 second bursts. The lysate was clarified by centrifugation (22,000×g for 20 min) and the His6MBP-Cas1 fusion protein was bound to Ni-NTA affinity resin in batch (Qiagen). His6MBP-Cas1 was eluted from the resin in 50 ml lysis buffer containing 300 mM imidazole. The eluted protein was dialyzed at 4° C. overnight against gel filtration buffer (20 mM HEPES pH 7.5, 500 mM KCl, 1 mM Tris(2-carboxyethyl)phosphine hydrochloride (TCEP), 5% Glycerol) in the presence of tobacco etch virus (TEV) protease to remove the N-terminal His6MBP tag. The protein was concentrated (Amicon) for further purification on tandem Superdex 75 (16/60) sizing columns. A similar strategy was used for the expression and purification of the selenomethionine-containing protein, with the only major exception being the expression media. Briefly,  Escherichia coli  BL21(DE3) transformed with the Cas1 expression construct (PaCas1/pHMGWA) were grown in M9 minimal media supplemented with ampicillin. At an OD 600  of 0.5, the following amino acids were added to inhibit methionine biogenesis and to allow for selenomethionine incorporation (Leu, Ile, Val: 50 mg/L; Phe, Lys, Thr: 100 mg/L; Selenomethionine: 75 mg/L) (Vanduyne et al., 1993). IPTG (0.5 mM) was added 15 minutes later and the culture was maintained at 16° C. overnight. The purified protein was concentrated to 9 mg/ml in 20 mM HEPES pH 7.5, with 100 mM KCl, 1 mM TCEP and 5% Glycerol. 
     Crystallization, data collection and structure determination. Native crystals were grown at 18° C. by vapor diffusion in hanging drops composed of equal volumes of protein solution (16 mg/ml Cas1 in 20 mM HEPES pH 7.6, 100 mM KCl, 1 mM TECP, 5% glycerol) and reservoir solution (250 mM CaCl2, 50 mM HEPES pH 7.6, 10% PEG8000). Optimized SeMet-containing crystals were grown at 18° C. by vapor diffusion in hanging drops composed of equal volumes of protein solution (12 mg/ml Cas1 in 20 mM HEPES pH 7.6, 100 mM KCL, 1 mM TECP, 5% glycerol) and reservoir solution (250 mM calcium acetate, 50 mM HEPES pH7.8, 6% PEG5000 MME). All crystals were cryo-protected by soaking in well solution supplemented with 30% glycerol for 15 seconds and then flash cooled in liquid nitrogen. 
     Diffraction data were collected at the Advanced Light Source (beamline 8.2.2), Lawrence Berkeley National Laboratory. Phases for the Cas1 structure were determined from a highly redundant single wavelength anomalous dispersion (SAD) data set collected at the Au L-III edge (λ=1.036652 Å) using native crystals soaked in 10 mM KAu(CN)2 for 10 minutes. Data with an I/sigma of greater than 2.0 was measured out to 3.0 Å resolution. Data were processed in space group P212121 using XDS (Kabsch, 1988; Kabsch, 1993). SOLVE (Terwilliger and Berendzen, 1999) was used to locate six gold atoms in the crystallographic asymmetric unit and to calculate initial phases. 
     Density modification and initial model building was performed using RESOLVE (Adams et al., 2002). A crude initial model was constructed by manually placing alpha helices using COOT (Emsley and Cowtan, 2004). The model was extended by automated model building using RESOLVE (Adams et al., 2002; Terwilliger, 2000, 2003) and Buccaneer (Cowtan, 2006) and completed by iterative rounds refinement and model building using Phenix refine (Afonine et al., 2005) and COOT (Emsley and Cowtan, 2004), respectively. The final model was refined against an isomorphous 2.17 Å data set measured from selenomethionine-containing crystals, yielding a R cryst  of 20.3% and R free  of 25.8%. 
     To locate the metal binding site in Cas1, SeMet-containing Cas1 crystals (grown from 250 mM calcium chloride, 50 mM HEPES pH 7.8, 12% PEG5000 MME) were soaked in 5 mM MnCl 2  for two hours. Diffraction data was measured at the K absorption edge (1.8842 Å). Manganese ions were included in the refinement using elbow (http followed by ://www. followed by phenix-online.org/ followed by documentation/ followed by elbow. followed by htm). 
     Activity assays. Purified recombinant Cas1 from PA14 (15.3 μM) was incubated at 25° C. with 1 μg of the indicated nucleic acid substrate (0.05 μM dsDNA, 0.05 μM ssDNA, 0.3 μM dsRNA and 0.6 μM ssRNA) in the presence of 20 mM HEPES pH 7.5 and 100 mM KCl at 25° C. for 90 minutes. Each reaction was supplemented with no metal (NM) or with 2.5 mM magnesium (Mg 2+ ), manganese (Mn 2+ ), cobalt (Co 2+ ), calcium (Ca 2+ ), iron (Fe 3+ ), zinc (Zn 2+ ), or EDTA, as indicated. RNA substrates were generated by in vitro transcription of the first 10 repeats and nine spacers of CRISPR2 cloned into the plasmid vector pUC19. In vitro transcripts from both strands of CRISPR2 were generated using T7 RNA polymerase at 37° C. for 2-5 hrs, in a reaction including: 30 mM Tris pH 8.4, 25 mM MgCl 2 , 5 mM of each nucleotide tri-phosphate, 10 mM Trition X-100, 10 mM dithiothreitol (DTT), 2 mM spermidine, 200 nM linearized dsDNA template (CRISPR2-pUC19). Transcripts were purified on denaturing polyacrylamide gels. Double-stranded CRISPR2 RNA substrates were generated by annealing the forward and reverse transcripts at 65° C. for 10 minutes. All nuclease assays were performed at 25° C. 
     ACCESSION NUMBERS. Refined models and experimental structure factors for the Cas1 protein from  P. aeruginosa  (PaCas1) have been deposited in the Protein Data Bank under accession number 3GOD. 
     Example 1 
     Identification of the  P. aeruginosa  PA14 cas1 Gene 
       P. aeruginosa , a gram-negative bacterium, is an opportunistic human pathogen known for its ability to grow in low-oxygen environments including the tissues of immunocompromised or cystic fibrosis patients. To investigate the function of the Cas1 protein, CRISPR elements in the genome of  Pseudomonas aeruginosa , strain 14 (PA14) (Lee et al., 2006) were focused upon. Using a CRISPR-finding algorithm, two repetitive genetic elements with the distinct repeat-spacer-repeat architecture characteristic of CRISPRs have been identified in the PA14 genome (Grissa et al., 2007). These two elements flank a cassette of six open reading frames (ORFs) that are annotated as hypothetical proteins. Blast analysis (basic local alignment search tool) identified each of these ORFs (PA14_33300-33350) as CRISPR-associated (cas) genes (Altschul et al., 1997; Zegans et al., 2008). 
     Based on phylogenetic comparisons, seven distinct versions of the CRISPR-associated immune system have been identified (CASS1-7) (Makarova et al., 2006). The identity and genomic arrangement of the PA14 cas genes are characteristic of CASS3. Blast analysis of the predicted protein sequence for PA14 cas gene 33350 identified homologous sequences most typically annotated as Cas1 (COG1518). Cas1, a ˜36 kD protein, has no obvious homology to proteins of known function. Due to its conservation across CRISPR systems, a molecular structure and function for the Cas1 protein from PA14 was determined. 
     Example 2 
     Crystal Structure of Cas1 Revealed a Novel Fold 
     The cas1 gene from  P. aeruginosa  was cloned and over-expressed in  E. coli . The purified protein was crystallized by vapor diffusion in hanging drops with a PEG-salt precipitant. The Cas1 structure was solved by SAD (single-wavelength anomalous dispersion) using a gold derivative and the final structure was refined against a 2.17 Å data set, yielding an R cryst  of 20.3% and an R free  of 25.8%. 
     The Cas1 protein has a novel three-dimensional fold consisting of two structurally distinct domains ( FIG. 1 ). The N-terminal β-strand domain includes residues 1-106 and is composed of 10 β-strands and two α-helices (yellow). This β-strand domain is connected to a C-terminal α-helical domain by a flexible linker (residues 107-112, green). The α-helical domain, including residues 113-324, comprises 10 α-helices (gray). Conserved residues are colored red. Side chains of the four universally conserved residues (E190, N223, H254, and D265), as well a strongly conserved aspartic acid at position 268 (D or E) are displayed as sticks were oxygen&#39;s are red, nitrogen&#39;s are blue and carbons are gray. Residues E190, H254 and D268 coordinate a manganese ion (green sphere). All ribbon diagrams were prepared using PYMOL (DeLano, 2002). 
     Comparison of the four Cas1 molecules (A-D) in the asymmetric unit of the crystal shows that their α-helical domains are nearly identical with an average root mean square deviation (r.m.s.d.) for equivalently positioned Cα atoms between residues 113-324 of 0.32 Å. There are, however, substantial differences in the fold and orientation of the β-strand domains ( FIG. 2 ). Thus, the linker connecting the α and β domains serves as a hinge that allows the two domains of a single molecule to be positioned in different relative orientations ( FIG. 2  Panels A and B).  FIG. 2 , Panel shows that the C-terminal α-helical domains of molecules A and C superimpose (residues 113-321) with an average r.m.s.d of 0.40 Å. The β-strand domains of these two molecules are in different orientations (two-way arrow) with respect to their α-helical domains. Coloring of molecule A is consistent with that in  FIG. 1 . Molecule C is colored light blue, with conserved residues colored pink. The α-helical and β-strand domains of molecules A and B are in the same relative orientation and the two molecules superimpose with an average r.m.s.d. of 0.20 Å for 318 equivalent Cα positions between residues 4-321. Molecules C and D are similar to one-another (0.57 Å Cα r.m.s.d.), but distinct from A and B.  FIG. 2 , Panel B shows a view down the barrel of α8 after rotation of 90° about the Y-axis and 20° about the X-axis. Pronounced structural differences occur at the N-terminus and in the positions of β-strands 8 and 9 ( FIG. 2 , Panel C). Molecules A and B have a well-ordered N-terminal α-helix (residues 6-16), whereas this region is disordered in molecule C. The N-terminus of molecule D forms an extended coil that is oriented in the opposite direction from that observed in molecules A and B; this coil forms crystal contacts with β-strands 3 and 4 (residues 36-52) in molecule C. In  FIG. 2 , panel C the superimposed β-strand domains are rotated 90° about the X-axis and viewed from the perspective of the α-helical domain. β-strands 8 and 9 form a short anti-parallel β-sheet in molecule A. This feature is not observed in molecule C, instead the sequence that would be part of β-strand 9 forms part of the unordered linker. Glutamate 96 (teal sticks) relates the primary sequence of both molecules to secondary structure elements in this region. 
     Cas1 molecules with different β-strand domain structures form dimers in the asymmetric unit of the crystal, yielding A-C and B-D homodimers. The Cas1 homodimer is shaped like a butterfly, where the α-helical and β-strand domains of each molecule represent the upper and lower lobe of each ‘wing’ ( FIG. 3 ). The wingspan of the Cas1 homodimer is ˜86 Å, and each wing stands ˜60 Å from top to bottom and ˜46 Å thick. The two molecules in each dimer are related by a pseudo-two-fold axis of symmetry centered about the dimer interface. Extensive hydrogen bonding and two salt bridges (C/Glu96-A/His248; C/Asp98-A/Arg259) at the dimer interface result in 1,761 Å2 of buried surface area. Notably, the dimer is maintained in high salt (500 mM) buffers and elutes from a calibrated Superdex S-75 size exclusion column with a retention volume consistent with a protein of ˜84 kDa, suggesting that Cas1 (˜36 kDa) is homodimeric in solution.  FIG. 3 , Panel A shows conserved residues colored red in molecule A and pink in molecule C, side chains of the four universally conserved residues displayed as sticks, two of the four universally conserved residues (Glu190 and His254) and a well conserved aspartic acid at position 268 (Asp or Glu) coordinate a manganese ion (green sphere).  FIG. 3 , Panel B provides a look down the dimer interface after sixty-degree rotation about the Y-axis. 
     Structural comparisons performed using the DALI (Holm and Sander, 1993) and VAST (Gibrat et al., 1996) servers reveal a structural homolog of the Cas1 protein. The Cas1 structure from  P. aeruginosa  (PaCas1) is most similar (Z-score 17.5) to the unpublished structure of a hypothetical protein from  Aquifex aeolicus  (pdb id:2YZS). The amino acid sequences of these two proteins are highly divergent (17.6% identity, 37.0% similarity) and are not recognized as homologs by BlastP (Altschul et al., 1997). However, further examination of the  A. aeolicus  protein (gene ID: 1193018) using PSI-Blast and genomic neighborhood analysis reveals that this is a Cas1 protein flanked by cas genes that are most similar to those of the CASS7 subtype. 
     The Cas1 protein from  A. aeolicus  (AaCas1) shares a similar tertiary and quaternary architecture to the Cas1 protein structure from  P. aeruginosa  ( FIG. 4 , Panel A;  FIG. 8 ).  FIG. 4 , Panel A shows that the α-helical domains of molecule A from PaCas1 (PaCas1_A) and molecule A from AaCas1 (AaCas1_A) share 91 equivalent Cα positions that superimpose with an average r.m.s.d of 1.28 Å. The color scheme of PaCas1 is consistent with that in  FIG. 1 . AaCas1 is colored light green and conserved residues are colored pink. Similar to the PaCas1 protein, the AaCas1 is a dimer composed of two molecules with β-domains in distinct orientations, despite having been crystallized under different conditions and in a different space group.  FIG. 4 , Panel B depicts that superposition of the α-helical domains of PaCas1 and AaCas1, as viewed from the perspective of the β-strand domain, highlights two structural differences. The two loops that connect α-helices 10 to 11 and 11 to 12 in PaCas1 are each replaced by two finger-like projections in the AaCas1 structure (gray boxes). Alpha-helix 8 (α8) of PaCas1 is positioned horizontally along the top and the two molecules are displayed at a 90° rotation about the Y-axis.  FIG. 4 , Panel C illustrates that the β-strand domains of these two molecules share 35 equivalent Cα atoms that superimpose with an average r.m.s.d of 1.21 Å. These two molecules are displayed from the perspective of the α-helical domain. β-strands 8 and 9 (gray box) are flipped out of the β-strand domain in molecule A of PaCas1 and positioned adjacent to α8 ( FIG. 4 , Panel A). Comparison of molecule A from PaCas1 (PaCas1_A) and molecule A from AaCas1 (AaCas1_A) reveals two prominent structural differences in the α-helical domain ( FIG. 4 , Panel B). The two short loops that connect α-helix 10 to 11 and 11 to 12 in PaCas1 are each replaced by finger-like projections consisting of two anti-parallel β-strands (residues 238-255 and 273-286, respectively) in the AaCas1 structure. 
     These two Cas1 structures do not share detectable homology with any other protein structure currently deposited in the protein data bank (PDB). 
     Example 3 
     Cas1 Contains a Conserved Divalent Metal Ion Binding Site 
     To investigate whether Cas1 includes a divalent metal ion binding site(s), crystals of the selenomethionine-substituted PaCas1 protein were soaked in solutions containing manganese chloride and diffraction data at the K absorption edge was measured. Anomalous difference electron density maps contoured at five sigma revealed eight unique peaks, three of which correspond to manganese ions (Mn) in molecules A, B and C, while signal from the other five peaks are from selenomethionines. Each of these Mn ions, as well as an additional Mn ion in molecule D (visible at four sigma), are located in equivalent positions in the α-helical domains of each molecule and are coordinated by three conserved residues (Glu190, His254 and Asp286) ( FIG. 1 ). Although the three-dimensional fold of Cas1 is unique, the residues coordinating the Mn ion are typical among nucleases that employ one or more metals in their active site. In fact, the chemical environment of the Cas1 metal binding site is remarkably similar to the active site of the manganese specific endonuclease domain from the cap-snatching subunit of the influenza polymerase (Dias et al., 2009). 
     Although no metal ions were included in the AaCas1 structure, residues E143, H206 and E221 are located in equivalent positions to the metal binding residues in the PaCas1 structure ( FIGS. 8 and 9 ). The conservation of these residues in Cas1 sequences from diverse CASS subfamilies, as well as their conserved three-dimensional arrangement in the AaCas1 and PaCas1 structures, suggests a common role for these residues in coordinating a metal ion. 
     Example 4 
     Cas1 is a Metal-Dependent DNA-Specific Endonuclease 
     Cas1 nuclease activity was tested by adding PaCas1 to a variety of nucleic acid substrates including: linear and circular double-stranded DNA (dsDNA: CRISPR2 cloned into pUC19), circular single-stranded DNA (ssDNA: M13 phage), linear double-stranded RNA (dsRNA; in vitro transcript of CRISPR2) and linear single-stranded RNA (ssRNA; in vitro transcript of CRISPR2). Cas1 is a metal-dependent DNA-specific endonuclease ( FIG. 5 ).  FIG. 5 , Panel A depicts that Cas1 nuclease activity is restricted to DNA substrates. Lanes 1 and 2 are 1 kb and 100 bp DNA ladders, respectively. Lanes 3-6 are dsDNA, lanes 7-10 are ssDNA, lanes 11-14 are dsRNA and lanes 15-18 are ssRNA. The first lane of each substrate type is nucleic acid alone, followed by a lane with nucleic acid and Cas1 in a no metal buffer. The last two lanes of each substrate type include nucleic acid, Cas1 and 2.5 mM Mn 2+ . The last lane of each substrate type was phenol extracted prior to electrophoresis. The dsDNA substrate is CRISPR2 from PA14 cloned into pUC19 (pUC19-C2) and linearized with KpnI (4 Kb). The ssDNA substrate is from M13 phage (reference sequence: NC_003287, 6407nt). RNA substrates are from in vitro transcripts of the first 10 repeats and 9 spacers of CRISPR2 (568nt). All reactions were incubated at 25° C. for 90 minutes prior to electrophoresis on a 1.5% agarose gel. Metal-dependent nuclease activity of Cas1 is independent of both sequence and methylation (dam/dcm) pattern ( FIG. 5 , Panel B).  FIG. 5 , Panel B shows time course of Cas1 nuclease activity on four distinct dsDNA substrates. Lanes 1 and 2 are 1 kb and 100 bp DNA ladders, respectively. The 1 kb ladder is the first lane of each of three subsequent panels. The first panel is pUC19-C2 DNA from isolated from methyltransferases (dam + /dcm + ) component  E. coli  and the second panel is pUC19-C2 DNA from methyltransferases delete (dam − /dcm − )  E. coli . The third and fourth panels are B3 and DMS3 phage DNA (respectively); each isolated from  P. aeruginosa  (PA14). The lanes for each substrate types are: nucleic acid alone, followed by 10, 60 and 390 minute incubations with Cas1 in a reaction buffer containing 2.5 mM Mn 2+ . The non-sequence specific nuclease activity of Cas1 on circular and linear DNA substrates isolated from  E. coli  or from  P. aeruginosa  results in reaction products that migrate as a non-specific smear on agarose gels ( FIG. 5 , Panels B, C and D). The average molecular weight of the DNA cleavage products continually decreases over time, resulting in a minimal cleavage product of approximately 80 base pairs in overnight reactions ( FIG. 5 , Panel C).  FIG. 5 , Panel C illustrates the time course of Cas1 nuclease activity on linear dsDNA substrate. Lanes 1 and 2 are 1 kb and 100 bp DNA ladders, respectively. Lane 3 is linearized dsDNA alone. Cas1 mediated nuclease reactions (lanes 4-11) were phenol extracted at 1, 15, 30, 60, 90, 120, 240 minutes and at 21 hours, prior to electrophoresis. The Cas1 time course is followed by the 100 bp and 1 kb DNA ladders, respectively. Cas1 mediated nuclease activity is inhibited by EDTA in the last two lanes (30 and 60 minute time points).  FIG. 5 , Panel D illustrates that Cas1 is an endonuclease. Lanes 1 and 2 are 1 kb and 100 bp DNA ladders, respectively. Lane 3 is circular dsDNA alone (CRISPR2 from PA14 cloned into pUC19). Cas1 mediated nuclease reactions (lanes 4-6) were phenol extracted at 15, 60, 240 minutes prior to electrophoresis. Endonuclease activity of Cas1 is inhibited by EDTA (lanes 7-6). The last two lanes are 100 bp and 1 kb DNA ladders respectively. 
     Metal ion substitution is a common strategy for understanding the role of the metal ions in metallonucleases. A panel of metal ion cofactors including alkaline earth metals and transition metals that are commonly found in association with metal dependent nucleases were tested for their ability to support Cas1 mediated nuclease activity on dsDNA substrates. In  FIG. 6 , Panel A, lanes 1 and 2 are 1 kb and 100 bp DNA ladders, respectively. Lane 3 is linear dsDNA alone (CRISPR2 from PA14 cloned into pUC19 and linearized with KpnI). Nuclease reactions (lanes 4-9) were performed in 100 mM KCl and 20 mM HEPES pH7.5 at 25° C. for 90 minutes. Each reaction was supplemented with no metal (NM) or with 2.5 mM magnesium (Mg 2+ ), manganese (Mn 2+ ), cobalt (Co 2+ ), calcium (Ca 2+ ), iron (Fe 3+ ), zinc (Zn 2+ ), or EDTA, respectively. Lane 12 is linear dsDNA alone. Lanes 13 and 14 are 100 bp and 1 kb DNA ladders, respectively. All reactions were phenol extracted prior to electrophoresis on a 1.5% agarose gel. Only magnesium (Mg 2+ ) and manganese (Mn 2+ ) support Cas1-mediated cleavage of dsDNA, and metal preference is dependent on monovalent salt concentrations ( FIG. 6 , Panels A and B). In  FIG. 6 , Panel B lanes 1 and 2 are 1 kb and 100 bp DNA ladders, respectively. Lane 3 is linear dsDNA alone. Lane 4 is linear dsDNA in 100 mM KCl and 20 mM HEPES pH7.5. Lanes 5-12 all include Cas1 and linear dsDNA in a 100 mM KCl or 10 mM KCl reaction buffer supplemented with no metal (NM) or with 2.5 mM magnesium (Mg 2+ ), manganese (Mn 2+ ) or EDTA, respectively. Lanes 13 and 14 are 100 bp and 1 kb DNA ladders, respectively. Although most nucleases exhibit highest activity in the presence of Mg, Cas1 is more active with Mn 2+  than with Mg 2+  at physiological KCl concentrations ( FIG. 6 , Panel B). Furthermore, Cas1-mediated cleavage of ssDNA, is supported exclusively by Mn 2+ , regardless of KCl concentration ( FIG. 6 , Panel C). In  FIG. 6 , Panel C lanes 1 and 2 are 1 kb and 100 bp DNA ladders, respectively. Lane 3 is linear ssDNA alone (M13 circular single-stranded DNA). Lanes 4-9 all include Cas1 and ssDNA in a 10 mM KCl or 100 mM KCl reaction buffer supplemented with no metal (NM) or with 2.5 mM magnesium (Mg 2+ ), manganese (Mn 2+ ) or EDTA, as indicated. Lanes 10 and 11 are 100 bp and 1 kb DNA ladders, respectively. 
     A series of mutants were constructed to investigate the role of conserved residues clustered in or around the Cas1 metal-binding pocket. Residues E190, N223, H254, D265 and D268 were mutated to alanine ( FIG. 7 , Panel A). Although each of these mutants was over-expressed, mutation of either E190 or H254 resulted in reduced stability of the protein and we were unable to purify these two point mutants to homogeneity ( FIG. 7B ). The nuclease activity of the three stable mutants (N223A, D265A and D268A) was tested. 
       FIG. 7 , Panel A shows a close-up of the metal binding pocket in the α-helical domain of molecule A. Anomalous difference electron density maps contoured at 5 sigma reveal a manganese ions (green mesh), coordinated by E190, H254 and D268. Asparagine 223 is one of only four universally conserved residues and is the only strictly conserved residue located outside the metal binding pocket ( FIG. 7 , Panel A). Asparagine 223 is located at the N-terminal end of α-helix 8, 15.5 Å away from the metal ion. An alanine substitution at this position (N223A) results in a modest reduction in non-specific nuclease activity ( FIG. 7 , Panel C). This is in contrast to the potent inhibition of nuclease activity observed in mutations made within the metal binding pocket. Mutation of acidic residues in the metal binding pocket at position 265 (D265A) or at metal coordinating residue 268 (D268A), inhibits non-specific nuclease activity. In  FIG. 7 , Panel C, the first two lanes are 1 kb and 100 bp DNA ladders, respectively. Lane 3 is dsDNA alone, lanes 4-7 include dsDNA, 2.5 mM Mn 2+  and one of the following Cas1 proteins in order: wild type Cas1, N223A, D265A and D268A. All reactions were performed at 25° C. for 90 minutes prior to phenol extraction. Samples were resolved by electrophoresis on a 1.5% agarose gel and stained with ethidium bromide. (*) denotes metal binding mutants that inhibit non-specific nuclease activity. In  FIG. 7 , Panel D the two subunits of the Cas1 (A-C) homodimer from  P. aeruginosa  (PA14) are displayed as a charge smoothed surface potential (molecule C) and a ribbon diagram of (molecule A). Basic residues (blue) cluster around the acidic metal binding pocket (red) creating a positive surface potential that may serve to position nucleic acid substrates in proximity to the metal binding site. The Cas1 homodimer is rotated 180° about the Y-axis with respect to the orientation in  FIG. 3 . 
     Thus three independent methods, metal chelation, metal ion substitution or mutation of metal coordinating residues, all suggest that the metal ion is critical for the non-specific degradation of DNA ( FIGS. 5, 6 and 7 , respectively). 
     The metal ion is located on one exposed face of the α-helical domain. An extensive cluster of basic residues including R192, K195, R196, K199, R212, K214, R215, K256 R258, R259 and K271 form a positively charged surface that spans this face of the α-helical domain and may serve to position nucleic acid substrate near this metal ion ( FIG. 7 , Panel D). 
     Example 5 
     Identification of a Cas1 Protein Structure from  Aquifex aeolicus    
       FIG. 8 , Panel A shows that the Cas1 protein structure from  A. aeolicus  (AaCas1) consists of two domains, an N-terminal β-strand domain (residues 2-77) and a C-terminal α-helical domain (88-316). The N and C-terminal domains are connected by a linker (77-83) that allows the two domains to sample different relative orientation. Superimposing residues 88-316 from the α-helical domains of molecules A (green) and B (pink) of AaCas1 (0.59 Å Cα r.m.s.d), reveals differences in domain positioning between these two molecules.  FIG. 8 , Panel B shows that two molecules with α-helical and β-strand domains in different orientations form a dimer. The dimer interface is mediated by hydrogen bonding between β-strand domains and results in 1,439 Å 2  of buried surface area. Conserved residues are yellow and the side chains of universally conserved residues are displayed as sticks with the atoms in each of these side chains colored according to red=oxygen, nitrogen=blue and carbon=gray. The coordinates for this structure were deposited by Ebihara, A., Yokoyama, S., and Kuramitsu, S. on Jul. 6, 2007 (PDB: 2YZS). 
     Example 6 
     Cas1 Sequences Are Diverse 
       FIG. 9  depicts an alignment of Cas1 sequences from each of the 7 major CASS subclasses. Theses sequences were aligned by Mcoffee (http:// followed by www. followed by tcoffee. followed by org/). Twenty-one sequences, 3 from each of the 7 major CASS subfamilies, are labeled by a two letter abbreviation of the genus and species, followed by ‘Cas1’, the NCBI gene identification number and the CASS subfamily number 1-7, previously assigned by Makarova et al (2006) (e.g.  Pseudomonas aeruginosa , Cas1, NCBI gene identification number: 4380485, from the CASS subfamily 3 is abbreviated as, “PaCas1_4380485_CASS3”). Universally conserved residues are in red columns and well-conserved residues are in yellow columns. 
     While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto. 
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