PHARMACOLOGICAL TARGETING OF mRNA CAP FORMATION

Abstract: This invention provides methods for the discovery of molecules that target an essential aspect of eukaryotic gene expression - the formation of the mRNA 5' cap m7GpppN. An underlying principle of this invention is the use of different strains of a test organism that differ only in the composition or source of the essential cap-forming enzymes. The invention provides isogenic yeast strains that derive all their capping activities from fungal sources versus mammalian sources. These strains form the basis of a differential growth inhibition assay to identify molecules that specifically target the fungal capping apparatus. This invention also provides a method to screen in vitro for molecules that inhibit fungal RNA triphosphatase, an essential enzyme that catalyzes the first of three steps in cap synthesis.

Description: PHARMACOLOGICAL TARGETING OF mRNA CAP FORMATION BACKGROUND OF THE INVENTION Cross-Re.ference to Related Application This application is a continuation-in-part of US SN 09/188,579, filed November 9, 1998. Federal Funding T .egend This invention was created using federal funds under Grant No. GM-52470 from the National Institutes of Health. Accordingly, the United States Government has certain rights in the invention. Field of the Invention The present invention relates generally to the fields of ge ne biochemical pharmacology and drug discovery. More specifically, the present invention relates to method of screening for a compound that inhibits formation of an organism' s 5' mRNA cap structure. Description of thp. Related Art Processing of eukaryotic mRNA in vivo is coordinated temporally and physically with transcription. The earliest event is the modification of the 5' terminus of the nascent transcript to form the cap structure m7GpppN. The cap is formed by three enzymatic reactions: (i) the 5' triphosphate end of the nascent RNA is hydrolyzed to a diphosphate by RNA 5' triphosphatase; (ii) the diphosphate end is capped with GMP by GTP:RNA guanylyltran sferase; and (iii) the GpppN cap is methylated by AdoMet:RNA (guanine-N7) methyltransferase [1] . RNA capping is essential for cell growth. Mutations of the triphosphatase, guanylyltransferase, or methyltransferase components of the yeast capping apparatus that abrogate catalytic activity are lethal in vivo [2-12] . Genetic and biochemical experiments highlight roles for the cap in protecting mRNA from untimely degradation by cellular 5' exonucleases [13] and in recruiting the mRNA to the ribosome during translation initiation [14] . The physical and functional organizations of the capping apparatus differ in significant respects in fungi, metazoans, protozoa, and viruses. Hence, the cap-forming enzymes are potential targets for antifungal, antiviral, and antiprotozoal drugs that would interfere with capping of pathogen mRNAs, but spare the mammalian host capping enzymes. A plausible strategy for drug discovery is to identify compounds that block cell growth contingent on pathogen-encoded capping activities without affecting the growth of otherwise identical cells bearing the capping enzymes of the host organism. For this approach to be feasible, the capping systems of interest must be interchangeable in vivo. The architecture of the capping apparatus differs between metazoans, fungi, protozoa, and DNA viruses. Metazoan species encode a two-component capping system consisting of a bifunctional triphosphatase-guanylyltransferase polypeptide (named Mcelp in the mouse and Hcelp in humans) and a separate methyltransferase polypeptide (Hcmlp in humans) [6, 9, 15-22]. The budding yeast Sac char omyces c e re v i s ia e encodes a three component system consisting of separate triphosphatase (Cetlp), guanylyltransferase (Ceglp), and methyltransferase (Abdlp) gene products [7, 10, 11 , 23]. In yeast, the triphosphatase (Cetlp) and guanylyltransferase (Ceglp) polypeptides interact to form a heteromeric complex [11], whereas in mammals, autonomous triphosphatase and guanylyltransferase domains are linked in cis within a single polypeptide (Mcelp) [18] . Vaccinia virus capping enzyme is a multifunctional protein that catalyzes all three reactions . The triphosphatase, guanylyltransferase, and methyltransferase active sites are arranged in a modular fashion within a single polypeptide - the Vaccinia DI protein [24-30]. Other DNA viruses encode a subset of the capping activities; e. g. , baculoviruses encode a bifunctional triphosphatase- guanylyltransferase (LEF-4) and Chlorella virus PBCN- 1 encodes a monofunctional guanylyltransferase [31-33] . The guanylyltransferase and methyltransferase domains are conserved between DΝA viruses, fungi, and metazoans. In contrast, the triphosphatase components are structurally and mechanistically divergent. RΝA Guanylyltransferas - Transfer of GMP from GTP to the 5' diphosphate terminus of RΝA occurs in a two-step reaction involving a covalent enzyme-GMP intermediate [34] . Both steps require a divalent cation cofactor, either magenesium or manganese. (i) E + pppG < > E-pG + PPi (ii) E-pG + ppRΝA < >GpppRΝA + E The GMP is covalently linked to the enzyme through a phosphoamide (P-N) bond to the epsilon-amino group of a lysine residue within a conserved KxDG element (motif I) found in all known cellular and DNA virus-encoded capping enzymes (Figure 1). Five other sequence motifs (III, Ilia, IN, V, and NI) are conserved in the same order and with similar spacing in the capping enzymes from fungi, metazoans, DΝA viruses, and trypanosomes (Figure 1) [35]. Hakansson et al. [36] have determined the crystal structure of the Chlorella virus capping enzyme in the GTP-bound state and with GMP bound covalently. The protein consist of a larger Ν-terminal domain (domain 1, containing motifs I, III, Ilia, and IN) and a smaller C-terminal domain (domain 2, containing motif NI) with a deep cleft between them. Motif N bridges the two domains. Motifs I, III, Ilia, IN, and V form the nucleotide binding pocket. The crystal structure reveals a large conformational change in the GTP-bound enzyme, from an "open" to a "closed" state, that brings motif VI into contact with the beta and gamma phosphates of GTP and reorients the phosphates for in-line attack by the motif I lysine. When the crystal is soaked in manganese, guanylyltransferase reaction chemistry occurs in crystallo and the covalent enzyme-GMP intermediate is formed. However, only the enzyme in the closed conformation is reactive. Identification of essential enzymic functional groups has been accomplished by site-directed mutagenesis of Ceglp, the RΝA guanylyltransferase of Sa c cha romy c e s c e re v is ia e . The guanylyltransferase activity of Ceglp is essential for cell viability. Hence, mutational effects on Ceglp function in vivo can be evaluated by simple exchange of mutant CEG1 alleles for the wild type gene. The effects of alanine substitutions for individual amino acids in motifs I, III, Ilia, IN, N, and VI [2, 5, 6] have been examined. Sixteen residues were defined as essential (denoted by asterisks in Figure 1 ) and structure-activity relationships at these positions were subsequently determined by conservative replacements. Nine of the essential Ceglp side chains correspond to moieties which, in the Ch l o re l la virus capping enzyme crystal structure, make direct contact with GTP (arrowheads in Figure 2). These include: the motif I lysine nucleophile which contacts the alpha-phosphate of GTP; the motif I arginine and motif III glutamate, which contact the 3' and 2' ribose hydroxyls, respectively; the motif III phenylalanine, which stacks on the guanine base; the two motif V lysines, which contact the alpha-phosphate; the motif V asp artate, which interacts with the beta-phosphate; the motif VI arginine that interacts with the beta-phosphate; and the motif VI lysine, which contacts the gamma-phosphate of GTP [6, 36]. On the basis of sequence conservation outside motifs I, III, Ilia, IV, V, and VI, the capping enzymes of fungi {S. cerevisiae, S. p o m b e, C. albicans,), metazoans (C. e legans and mammals) and Chlorella virus can be grouped into a discrete subfamily [6, 37]. The sequence alignment in Figure 2 highlights two motifs that are present in these capping enzymes, but not in the poxvirus enzymes, which can be designated motif P and motif Vc. In the Chlorella virus capping enzyme, motif P forms one wall of the guanosine binding pocket of domain 1 [36]. Motif Vc - (K/R)I(I/V)EC - is situated between motifs V and VI in domain 2. The glutamate residue of motif Vc is essential for the activity of the fungal guanylyltransferase Ceglp [37]. RNA Triphosph ata se - There are at least two mechanistically and structurally distinct classes of RNA 5' triphosphatases: (i) the divalent cation-dependent RNA triphosphatase/NTPase family (exemplified by yeast Cetlp, baculovirus LEF-4, and vaccinia DI ), which require three conserved collinear motifs (A, B, and C) for activity [12, 28, 31 , 32] , and (ii) the divalent cation-independent RNA triphosphatases, e . g. , the metazoan cellular enzymes and the baculovirus enzyme BVP [ 15, 17, 38-40] , which require the HCxAGxGR(S/T)G phosphate-binding motif. The existence of additional classes of RNA 5'-triphosphatases is likely, given that the candidate capping enzymes of several RNA viruses and trypanosomatid protozoa lack the defining motifs of the two known RNA triphosphatase families [41 , 42] . Hence, the triphosphatase components of the capping apparatus provide attractive targets for the identification of specific antifungal, antiviral, and antiprotozoal drugs that will block capping of pathogen mRNAs, but spare the mammalian host enzyme. Mammali an RNA Tri phosph ata se - Metazoan capping enzymes consist of an N-terminal RNA triphosphatase domain and a C- terminal guanylyltransferase domain. In the 496-amino acid mouse enzyme Mce lp, the two catalytic domains are autonomous and nonoverlapping [18] . The metazoan RNA triphosphatase domains contain a (I/V)HCxAGxGR(S/T)G signature motif initially described for the protein tyrosine phosphatase/dual-specificity protein phosphatase enzyme family. These enzymes catalyze phosphoryl transfer from a protein phosphomonoester substrate to the thiolate of the cysteine of the signature motif to form a covalent phosphocysteine intermediate, which is then hydrolyzed to liberate phosphate (Figure 3). The metazoan capping enzymes hydrolyze the phosphoanhydride bond between the beta and gamma phosphates of triphosphate-terminated RNA; they are not active on nucleoside triphosphates. The conserved cysteine of the signature motif is essential for RNA triphosphatase function [11, 15, 38, 39]. A characteristic of the cysteine-phosphatases is their lack of a requirement for a divalent cation cofactor. The N-terminal portion of Mcelp from residues 1-210 is an autonomous RNA triphosphatase domain [18]. Recombinant Mce l ( l - 210)p has been expressed in bacteria and purified to near- homogeneity. Mcel(l -210)p sediments in a glycerol gradient as a discrete peak of 2.5 S, indicating that the domain is monomeric in solution. The RNA triphosphatase activity of Mcel (l-210)p can be assayed by the release of 3 2 Pi from γ 3 2 P-labeled poly(A). A kinetic analysis showed that the initial rate of Pi release was proportional to enzyme concentration. Mcel(l-210)p hydrolyzed 1.2 to 2 molecules of Pi per enzyme per second at steady state. RNA triphosphatase activity was optimal in 50 mM Tris buffer at pH 7.0 to 7.5. Activity was optimal in the absence of a divalent cation and was unaffected by EDTA. Inclusion of divalent cations elicited a concentration dependent inhibition of RNA triphosphatase activity. 75% inhibition was observed at 0.5 mM MgCl2 or MnCl2 - Mcel (l-210) did not catalyze release of 3 2 Pi from [γ^PJATP. Metal-dependent R N A Triphosphata ses - The RNA triphosphatases of S. cerevisiae and DNA viruses are structurally and mechanistically unrelated to the metazoan RNA triphosphatases. The vaccinia virus RNA triphosphatase depends absolutely on a divalent cation cofactor. Vaccinia triphosphatase displays broad specificity in its ability to hydrolyze the gamma phosphate of ribonucleoside triphosphates, deoxynucleoside triphosphates, and triphosphate- terminated RNAs [43, 44]. The NTPase and RNA triphosphatase reactions occur at a single active site within an 545-amino acid N- terminal domain of vaccinia capping enzyme that is distinct from the guanylyltransferase active site [27-30] . The vaccinia RNA triphosphatase is optimal with magnesium, is 12% as active in manganese, and is inactive with cobalt [43]. In contrast, the vaccinia NTPase is fully active with cobalt, manganese, or magnesium [43, 44]. Baculovirus LEF-4 hydrolyzes the g phosphate of RNA and NTPs; the LEF-4 NTPase is activated by manganese or cobalt, but not by magnesium [31] . The yeast RNA 5'-triphosphatase Cetlp also hydrolyzes the gamma phosphate of nucleoside triphosphates [12]. The NTPase of Cetlp is activated by manganese and cobalt. This is a property shared with the triphosphatase components of the vaccinia D I and baculovirus LEF-4 capping enzymes. Recent studies illuminate a common structural basis for metal-dependent catalysis by these enzymes. The metal-dependent RNA triphosphatases share 3 collinear sequence motifs, designated A, B, and C (Figure 4). These are present in yeast Cetlp, in the Cetlp homolog from yeast Candida albicans, in the triphosphatase-guanylyltransferase domains of the vaccinia virus, Shope fibroma virus, molluscum contagiosum virus, and African swine fever virus capping enzymes, and in baculovirus LEF-4. Mutational analysis identified several residues within these motifs that are essential for the RNA triphosphatase and ATPase activities of vaccinia virus capping enzyme; the essential residues include two glutamates in motif A, an arginine in motif B, and two glutamates in motif C [28]. Alanine substitutions at any of these positions in the vaccinia capping enzyme reduced phosphohydrolase specific activity by 2 to 3 orders of magnitude. These 5 residues may comprise part of the triphosphatase active site. All five residues essential for vaccinia triphosphatase activity are conserved in LEF-4 and Cetlp. Mutations of the equivalent residues of Cetlp result in loss of triphosphatase activity [12]. Physi cal A ssoci ati on of th e Tri ph osph ata se a n d Guanylyltransferase Components of the Capping Apparatus - Yeast and mammals use different strategies to assemble a bifunctional enzyme with triphosphatase and guanylyltransferase activities. In yeast, separate triphosphatase (Cetlp) and guanylyltransferase (Ceg lp) enzymes interact to form a heteromeric complex, whereas in mammals, autonomous triphosphatase and guanylyltransferase domains are linked in cis within a single polypeptide (Mcelp). Cetl p and Cegl p Form a Heterodimeric Capping Enzyme Complex In Vitro. - The native size of purified recombinant yeast RNA triphosphatase Cetlp was gauged by glycerol gradient sedimentation. Cetlp sedimented as a single component of 4.3 S . The yeast guanylyltransferase Ceglp sediments similarly in a glycerol gradient. Yet, when equal amounts of recombinant Cetlp and Ceglp were mixed in buffer containing 0.1 M NaCl and the mixture was analyzed by glycerol gradient sedimentation, the two proteins, as well as the triphosphatase and guanylyltransferase activities, cosedimented as a single discrete peak of 7.5 S. Thus, Ceglp and Cetlp interact in vitro to form a heteromeric complex [11]. Cetlp does not form a complex with the structurally homologous RNA guanylyltransferase domain of the mouse capping enzyme. Recombinant mouse guanylyltransferase, Mcel(211-597)p, was purified to homogeneity, mixed with Cetlp or with a buffer control, and then subjected to sedimentation analysis in parallel with the Cetl -Cegl mixtures. The 45 kDa Mcel (21 1 -597) protein alone sedimented as a single monomeric peak [ 18] . Sedimentation of the Mcel(21 1-597)p plus Cetlp mixture revealed no shift in the distribution of the mouse guanylyltransferase or the yeast triphosphatase to a more rapidly sedimenting form [11]. Hence, yeast RNA triphosphatase forms a heteromeric complex in vitro with yeast guanylyltransferase, but not with the mammalian enzyme. Subsequent studies of the Candida albicans RNA triphosphatase (named CaCetlp) showed that it could interact with S. cerevisiae guanylyltransferase Ceglp in vivo as gauged by a two-hybrid reporter assay [45]. Cetl p-Cegl Heterodimeri zation i s Essenti al In Vivo - Truncated proteins Cetl (201 -549)p and Cetl (246-549)p were expressed in bacteria and purified from soluble bacterial lysates by Ni- agarose and phosphocellulose column chromatography. Purified Cetl (201-549)p and Cetl (246-549)p catalyzed the release of 32 Pi from γ^P-labeled triphosphate-terminated poly(A) or [γ^PJATP with the same specific activity as full-length Cetlp [1 1]. The CETl (201 -259) gene in single copy was functional in vivo in supporting yeast cell growth [1 1] . The finding that the CETI (246- 549) gene on a C E N plasmid could not support cell growth, even though the gene product has full RNA triphosphatase activity in vitro, suggests that the catalytic activity of Cetlp, though essential for cell growth (see below), may not suffice for Cetlp function in vivo. Glycerol gradient analysis showed that Cetl (201-549)p by itself sedimented as a discrete species of -4.1 S. When Cetl(201-549)p was mixed with Ceglp and the mixture was analyzed by glycerol gradient sedimentation, the two proteins cosedimented as a 6.8 S heteromeric complex [1 1] . The more extensively truncated Cetl (246-549)p did not sediment as a discrete species like full-sized Cetlp and Cetl(201-549)p. Rather, most of the Cetl (246-549)p sedimented as a high molecular weight oligomer (~ 13 S) that retained RNA triphosphatase activity. When a mixture of Cetl(246-549)p and Ceglp was sedimented most of the Cetl(246- 549)p remained aggregated; only a minor fraction of the input Ceglp was shifted to the size expected for the heteromeric complex. Deletion from residues 201-245 (which results in loss of function in vivo despite retention of triphosphatase activity in vitro) affects the interaction of Cetlp with Ceglp. The implication of these data is that the interaction of Cetlp with Ceglp is essential for yeast cell growth. Pharmacological interference with Ceg lp-Cetlp heterodimerization is a potential mechanism for blocking gene expression in fungi without impacting on mammalian cells. Cap Methyltransferase - The enzyme RNA (guanine-N7-) methyltransferase (referred to hereafter as cap methyltransferase) catalyzes the transfer of a methyl group from AdoMet to the GpppN terminus of RNA to produce m7GpppN-terminated RNA and AdoHcy [1]. The Saccharomyces cerevisiae cap methyltransferase is the product of the A B D 1 gene [7] . ABD1 encodes a 436-amino acid polypeptide. A catalytic domain of Abdlp from residues 110 to 426 suffices for yeast cell growth (8, 9); this segment of Abdlp is homologous to the methyltransferase catalytic domain of the vaccinia virus capping enzyme [7]. A key distinction between the yeast and vaccinia virus cap methyltransferases is their physical linkage, or lack thereof, to the other cap-forming enzymes. The vaccinia virus methyltransferase active site is encoded within the same polypeptide as the triphosphatase and guanylyltransferase, whereas the yeast methyltransferase is a monomeric protein that is not associated with the other capping activities during fractionation of yeast extracts [7] . Mutational analyses of the yeast and vaccinia cap methyltransferases have identified conserved residues that are critical for cap methylation [8, 9, 46] . In the case of Abdlp, mutations that abolished methyltransferase activity in vitro were lethal in vivo. A putative cap methyltransferase from C. elegans was identified on phylogenetic grounds [9]. An alignment of the sequence of the predicted 402-amino acid C. elegans protein (Genbank accession Z81038) with the yeast cap methyltransferase Abdlp is shown in Figure 5. Although it remains to be demonstrated that the nematode protein has cap methyltransferase activity, the extensive sequence conservation suggested that other metazoans might also encode homologues of Abdlp. A human cDNA that encodes a bona fide cap methyltransferase has been identified and a physical and biochemical characterization of the recombinant human cap methyltransferase (Hcmlp) produced in bacteria was conducted. A functional C-terminal catalytic domain of Hcmlp was defined by deletion analysis [22]. Interaction of the, Cellular Capping Apparatus with the Phosphorylated CTD of RNA Polymerase II. Cap formation in eukaryotic cells in vivo is targeted to the nascent chains synthesized by RNA polymerase II (pol II). A solution to the problem of how pol II transcripts are specifically singled out for capping has been described whereby the cellular capping enzymes are targeted to pre-mRNA by binding to the phosphorylated carboxyl-terminal domain (CTD) of the largest subunit of pol II [16-18, 47, 48]. The CTD, which is unique to pol II, consists of a tandem array of a heptapeptide repeat with the consensus sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser. The mammalian pol II large subunit has 52 tandem repeats whereas the S. cerevisiae subunit has 27 copies. The pol II largest subunit exists in two forms, a nonphosphorylated IIA form and a phosphorylated IIO form, which are interconvertible and functionally distinct. In vivo, the pol IIO enzyme contains as many as 50 phosphorylated amino acids (primarily phosphoserine) within the CTD. During transcription initiation, pol IIA is recruited to the DNA template by the general transcription factors. The pol IIA CTD undergoes extensive phosphorylation and conversion to IIO during the transition from preinitiation complex to stable elongation complex. Several CTD kinase activities have been implicated in CTD hyperphosphorylation, each of which contains a cyclin and cyclin-dependent kinase subunit pair. The cdk7 and cyclin H subunits of the general transcription factor TFIIH catalyze phosphorylation of Ser-5 of the CTD heptapeptide. Other CTD kinases include the cdk8/cyclin C pair found in the pol II holoenzyme, CTDK-I, a heterotrimeric kinase with cdk-like and cyclin-like subunits, and P- TEFb, a regulator of polymerase elongation with a cdc2-like subunit. The recombinant S. c e r e v i s i a e and Sc. pombe guanylyltransferases Ceg lp and Pce lp bind specifically to the phosphorylated form of the CTD [16] . Moreover, recombinant yeast cap methyltransferase Abdlp also binds specifically to CTD-P04 [16] . Phosphorylation at Ser-5 of the heptad repeat was sufficient to confer guanylyltransferase and methyltransferase binding capacity to the CTD [16]. This analysis has been extended to mammalian capping enzyme where the key finding is that the guanylyltransferase domain Mce l (21 1 -597)ρ by itself binds to CTD-P04, whereas the triphosphatase domain Mcel ( l -210)p does not [18] . These findings suggest that the mammalian RNA triphosphatase is targeted to the nascent pre-mRNA by virtue of its connection in c i s to the guanylyltransferase. The phosphorylation-dependent interaction between guanylyltransferase and the CTD is conserved from yeast to mammals. It is not clear if the structural elements on the yeast and mammalian enzymes that interact with CTD-P04 are conserved or divergent. Nonetheless, pharmacological interference with the binding of guanylyltransferase or cap methyltransferase to the CTD is a potential mechanism for blocking gene expression in fungi or mammalian cells. Such interference can occur either by direct blocking of capping enzyme/CTD-P04 binding or indirectly by affecting the phosphorylation state of the CTD. The prior art is deficient in the lack of methods of screening for a compound that inhibits formation of an organism' s 5' mRNA cap structure. The present invention fulfills this longstanding need in the art. SUMMARY OF THE INVENTION The present invention facilitates the discovery of drugs that target an essential aspect of eukaryotic gene expression - the formation of the mRNA 5' cap m7GpppN. The underlying principle of the invention is the use of a different strains of a test organism that differ only in the composition or source of the essential cap-forming enzymes. For example, the construction of isogenic yeast strains that derive all their capping activities from fungal sources versus mammalian sources provides the basis to identify molecules that specifically target the fungal capping apparatus. The methods disclosed herein test a battery of candidate molecules for their ability to selectively impair the growth of a strain containing capping enzymes differing in composition or source. In a simple embodiment of the invention, this would entail the local application of an array of candidate molecules to agar culture plates that had been inoculated with an engineered yeast strain containing capping enzymes differing in composition or source. The plates are incubated to permit growth of the yeast cells to form a confluent lawn. Growth inhibition by the applied compound is detected as a "halo" of no-growth or slow-growth around the site of application. The methods will necessarily detect many compounds that inhibit the growth of all test strains; these are regarded as non-specific to capping and would likely not be pursued. The positive candidates are those that inhibit growth of one strain, but not of an otherwise identical strain containing a capping apparatus that differs in composition or source. For example, molecules that inhibit the growth of yeast cells containing an all-fungal capping apparatus, but have little or no effect on the growth of yeast cells containing the mammalian capping apparatus, would be regarded as promising leads for antifungal drugs. Conversely, molecules that inhibit the growth of cells containing an all-mammalian capping apparatus, but have little or no effect on the growth of cells containing the fungal capping apparatus merit further consideration as specific inhibitors of capping in mammalian cells, with the potential for development as an antineoplastic agent. The methods of the present invention are also applicable to the identification of potential antiviral agents and antiparasitic agents that specifically target virus-encoded or parasite-encoded capping enzymes. In this embodiment of the invention, the growth inhibition screen would include testing the array of candidate molecules against a strain that contains one or more virus-encoded or parasite-encoded capping activities in lieu of the endogenous enzyme(s). Molecules that selectively inhibit growth of the strain bearing the viral or parasite capping enzyme component, but not both of the strain bearing the mammalian capping apparatus, would be regarded as promising leads for further evaluation of antiviral or antiparasitic activity. The invention is not restricted to the use of fungi as the test organisms. Advances in gene targeting in mammalian cells make it feasible to construct mammalian cell lines in which one or more of the genes encoding cellular capping activities is deleted and replaced by a gene encoding the analogous enzyme from another source. Thus, candidate cap-targeting compounds could be identified by screening in parallel for selective growth inhibition of one of several cell lines differing only in the composition or source of the capping apparatus. Eukaryotic viruses that depend on virus-encoded capping activities can also be developed as the targets for testing of growth inhibition. In this embodiment, a viral gene encoding an essential capping activity would be deleted and replaced by a gene encoding the analogous enzyme from another source. Virus plaque formation on permissive host cells provides an easy visual screen (by plaque number and plaque size) for inhibition of virus replication by candidate agents added to the medium. Agents that selectively inhibit the replication of virus containing capping enzymes from one source, but not from another source, are presumed to do so by selective targeting of cap formation. The invention is also not restricted to identifying exogenous molecules that target cap formation. Another embodiment facilitates the DNA-based identification of natural or synthetic gene products that inhibit cell growth via intracellular effects on the capping enzymes. In this application of the invention, candidate genes or gene libraries (either natural or synthetic) would be transformed into a test yeast strain, e. g. , a strain bearing an all-fungal capping apparatus. The genes in the library are under the control of a regulated promoter {e.g. a GAL promoter) so that their expression can be repressed (in glucose medium) or induced (in galactose medium) by the experimenter. The initial screen selects for library-transformed cells that grow on glucose, but are inhibited on galactose. Plasmids recovered from such cells would be clonally amplified in bacteria and then re-transformed in parallel into yeast strains containing the fungal capping apparatus and strains containing one or both mammalian capping enzymes. Plasmids that elicit galactose-dependent growth inhibition of the strain with the fungal capping system, but do not inhibit the strains with mammalian capping components, are good candidates to encode specific antagonists of fungal cap formation. Sequencing the plasmid encoded gene product will reveal the identity of the presumptive inhibitor. Structure-activity relationships for the gene product can then be delineated by DNA-based mutagenesis . The growth-inhibiting molecules or genes identified using the methods described in this invention could conceivably target cap formation via a number of distinct mechanisms, including: (i) direct inhibition of the catalytic activity of one of the cap-forming enzymes by the identified molecule or a metabolite thereof; (ii) interference with protein-protein interactions required for in vivo function of one of the cap-forming enzymes; (iii) alterations in the level of available substrates for cap formation {e. g. , GTP and AdoMet) or the level of endogenous inhibitors of cap formation {e. g. , AdoHcy); and (iv) the synthesis of abnormal cap structures in the presence of the growth- inhibiting molecule (or metabolites thereof) that effectively "poison" cellular transactions dependent on the RNA cap. An advantage of the present invention is that it is geared to detect specific targeting of capping enzymes in vivo based on differences in their composition or origin, without bias with respect to the mechanism of inhibition. Once candidate molecules are identified, further testing of growth inhibition of isogenic strains differing in one component of the capping system can reveal which enzyme is targeted. Further mechanistic studies can ensue using purified capping enzymes from the relevant sources. The invention also encompasses an in vitro screen to identify candidate inhibitors of the catalytic activity of the fungal RNA 5' triphosphatase. The method exploits the fact that the yeast RNA triphosphatase Cetlp has a vigorous ATPase activity that depends on either manganese or cobalt as the divalent cation cof actor [12]. The method is simple, quantitative, and adaptable to a colorimetric detection assay that is suited to high-throughput screening for inhibitors. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the guanylyltransferase signature motifs. Six collinear sequence elements, designated motifs I, III, Ilia IV, V, and VI, are present in cellular and viral capping enzymes. The amino acid sequences are aligned for the enzymes of S. cerevisiae (See), S. pomb e (Spo), C. albicans (Cal), Chlorella virus PBCV-1 (ChV), mouse (Mus), African swine fever virus (ASF), Trypanosoma brucei gambiense (Tbr), Crithidia fasciculata (Cfa), AcNPN baculovirus (Lef4), vaccinia virus (Vac), Shope fibroma virus (SFV), and molluscum contagiosum virus (MCV). The numbers of amino acid residues separating the motifs are indicated. The amino acids of the yeast enzyme motifs that are essential for Ceglp function in vivo are denoted by asterisks. Figure 2 shows the conserved sequence elements in fungal, metazoan, and Chlorella virus mRΝA capping enzymes. The amino acid sequence of the S. cerevisiae (see) guanylyltransferase Ceglp from residues 43 to 399 is aligned with the homologous regions of the guanylyltransferases encoded by Candida alb icans (cal), Schizosaccharomyces pombe (spo), mouse (mus), Caeno rhabditis elegans (eel), and Chlorella virus PBCV-1. Eight conserved collinear sequence elements, designated motifs P, I, III, Ilia IV, V, Vc, and VI, are shown in shaded boxes. Gaps in the sequence are indicated by dashes (-). Ceglp residues defined by mutational analysis as essential for enzyme function in vivo are denoted by asterisks. Residues in proximity to GTP in the Chlorella virus capping enzyme-GTP co-crystal are indicated by arrowheads below the aligned sequences. Figure 3 shows the RNA triphosphatase domains of metazoan capping enzymes and baculovirus phosphatase BVP. The amino acid sequence of the 168-amino acid baculovirus phosphatase BVP (Baculo) is aligned with the N-terminal RNA triphosphatase domains of mouse capping enzyme (Mus CE) and C. elegans capping enzyme (Cel CE). Gaps in the sequences are indicated by dashes (-). Amino acids conserved in all three proteins are denoted by asterisks. The protein phosphatase signature motif is highlighted in the shaded box. The active site cysteine is in bold face. The presumptive reaction pathway involving formation of a cysteinyl phosphate intermediate is shown. Figure 4 shows the conserved sequence elements of the metal-dependent RNA triphosphatases. Three conserved motifs, designated A, B, and C, in the RNA triphosphatases of S. cerevisiae (Cetl), vaccinia virus (vvDl), Shope fibroma virus (SFV), molluscum contagiosum virus (MCV), African swine fever virus (ASF), baculovirus (Lef4), and Candida albicans (Cal) are aligned in the figure. Also included in the alignment is the predicted translation product of the S. cerevisiae YMR180c open reading frame (180c). Cetlp residues conserved in at least two other family members are shaded. The numbers of amino acids separating the motifs are indicated. The five amino acids in the vaccinia virus capping enzyme that are essential for triphosphatase activity are underlined. Cetlp residues Glu-305, Glu- 307, Arg-454, Glu-492, Glu-494, and Glu-496 that are essential for CETl function in vivo and for Cetlp RNA triphosphate activity in vitro are denoted by arrowheads. The locations of motifs A, B, and C within the yeast 456-amino acid RNA triphosphatase Cetlp is diagrammed above the aligned sequences. The margins of the C-terminal catalytic domain sufficient for g-phosphate hydrolysis are indicated by brackets. Figure 5 shows the amino acid sequence conservation in human and yeast cap methyltransferases. The complete amino acid sequence of the 476-amino acid Hcmlp protein (hem) is aligned with the complete sequence of yeast Abdlp (abd) and with the predicted 402-amino acid C. elegans C25Al.f gene product (eel) from residue 1 to 373. Gaps in the sequences are indicated by dashes (-). The C- termini of Hcmlp and Abdlp are indicated by asterisks. Residues in Hcmlp that are identical or similar in Abdlp or the C. elegans protein are shown in shaded boxes. Amino acids essential for Abdlp function are denoted by dots. Figure 6 shows the mutational effects on the RNA triphosphatase activity of Cetl (201-549)p. (A) Protein purification. Aliquots (3 μg) of the Ni-agarose preparations of recombinant wild type (WT) Cetl (201-549)p and the indicated Cetl (201-549)p-Ala mutants were electrophoresed through a 12% polyacrylamide gel containing 0.1 % SDS. Polypeptides were visualized by staining with Coomassie blue dye. The positions and sizes (in kDa) of marker proteins are indicated on the left. (B) RNA triphosphatase activity. Reaction mixtures (10 μl) containing 50 mM Tris HC1 (pH 7.5), 5 mM DTT, 1 mM MgCl2 , 20 pmol (of triphosphate termini) of [γ 32 P]-poly(A), and either WT or mutant proteins as specified were incubated for 15 min at 30°C. Aliquots of the mixtures were applied to a polyethyleneimine-cellulose TLC plate, which was developed with 0.75 M potassium phosphate (pH 4.3). 32 Pi release is plotted as a function of input protein. Figure 7 shows the hydrolysis of ATP by Cetlp. Reaction mixtures (10 μl) containing 50 mM Tris HCl (pH 7.5), 5 mM DTT, 1 mM [γ^PJATP, 250 ng of recombinant Cetlp (phosphocellulose fraction), and 1 mM divalent cation as specified were incubated for 15 min at 30°C. An aliquot (4 μ l) of the mixture was applied to a polyethyleneimine-cellulose TLC plate, which was developed with 0.5 M LiCl2, 1 M formic acid. An autoradiograph of the TLC plate is shown. The positions of Pi and ATP are indicated on the left. Figure 8 shows a kinetic analysis of ATP hydrolysis by Cetlp. (A) Protein titration. Reaction mixtures (10 μl) containing 50 mM Tris HCl (pH 7.0), 5 mM DTT, 2 mM MnCl2, 1 mM tf jATP, and either Cetlp, Cetl (201-549)p, or Cetl (246-549)ρ were incubated for 15 min at 30°C. Pi release is plotted as a function of input protein. (B) Kinetics. Reaction mixtures (100 μl) containing 50 mM Tris HCl (pH 7.0), 5 mM DTT, 2 mM MnCl2, 1 mM [V 2 P]ATP, and either 8, 16, or 32 nM Cetlp were incubated at 30°C. Aliquots (10 μl) were withdrawn at the times indicated and quenched immediately by the addition of 2.5 ml of 5 M formic acid. Pi release is plotted as a function of time. (C) Hydrolysis of [α 32 P]ATP. Reaction mixtures (100 μl) containing 50 mM Tris HCl (pH 7.0), 5 mM DTT, 2 mM MnCl2 , 1 mM [cc 3 2 P]ATP, and 100 ng of Cetlp were incubated at 30°C. Aliquots (10 μl) were withdrawn at the times indicated and quenched with formic acid. The products were analyzed by TLC. The levels of [α 32 P]ADP (o) and [α 32 P]AMP (q) are plotted as a function of time. An otherwise identical reaction containing 1 mM [γ^P ATP was analyzed in parallel; Pi release (•). (D) ATP-dependence. Reaction mixtures (20 μl) containing 50 mM Tris HCl (pH 7.0), 5 mM DTT, 2 mM MnCl2 , 40 pg of Cetlp, and [γ^PJATP as indicated were incubated for 15 min at 30°C. Pi release is plotted as a function of ATP concentration. Inset: a double-reciprocal plot of the data is shown. Figure 9 shows cap methyltransferase activity of recombinant Hcm lp. The complete reaction mixture (lane 1 ) contained (in 10 μl) 50 mM Tris HCl (pH 7.5), 5 mM DTT, 16 fmol of cap-labeled poly(A), 50 μM AdoMet, and -50 fmol of Hcmlp (Ni- agarose 0.2 M imidazole eluate fraction). Reaction components were varied as follows: omit AdoMet (lane 2); omit AdoMet, include 50 μM AdoHcy (lane 3); include 50 μM AdoMet plus 500 μM AdoHcy (lane 4); omit Hcmlp (lane 5); omit Hcmlp, include purified recombinant vaccinia capping enzyme (lane 6). After incubation at 37°C for 10 min, the reaction mixtures were heated at 95 °C for 5 min, then adjusted to 50 mM sodium acetate (pH 5.5). The samples were incubated with 5 μg of nuclease PI for 30-60 min at 37°C. The digests were then spotted on polyethyleneimine cellulose TLC plates that were developed with 0.2 M (NH4)2S04. An autoradiograph of the chromatogram is shown. The chromatographic origin and the positions of cap dinucleotides m7GpppA and GpppA are indicated on the right. Figure 10 shows glycerol gradient sedimentation of human cap methyltransferase. The Ni-agarose fractions of Hcmlp (panel A), and N-terminal deletion mutants Δ 120 (panel B) and Δ 151 (panel C) were sedimented in glycerol gradients as described under Methods. Fractions were collected from the bottom of the tubes (fraction 1 ). Aliquots (20 μl) of alternate fractions were analyzed by SDS-PAGE along with an aliquot of the material that had been applied to the gradient (lane Ni). The gels were fixed and stained with Coomassie blue dye. The positions and sizes (kDa) of coelectrophoresed marker polypeptides are indicated at the left of each gel. Methyltransferase reaction mixtures contained 16 fmol of cap-labeled poly(A), 50 μM AdoMet, and 1 ml of a 1/500 dilution of the indicated fractions from the Hcmlp and Δ 120 glycerol gradients or 1 ml of a 1/50 dilution of fractions from the Δ 151 glycerol gradient. The reaction products were digested with nuclease PI and analyzed by TLC. The extent of cap methylation [m7GpppA/(m7GpppA + GpppA)] was determined by scanning the chromatogram using a FUJIX BAS 1000 phosphorimager. The peaks of marker proteins catalase, BSA and cytochrome C, which were centrifuged in a parallel gradient, are indicated by arrowheads. Figure 11 shows Cap methylation by N-terminal deletion mutants of Hcmlp. (A) Polypeptide composition. Aliquots ( 10 μg) of the peak glycerol gradient fractions of wild type Hcmlp and deletion mutants Δ 120 and Δ151 were analyzed by SDS-PAGE. Polypeptides were visualized by staining with Coomassie Blue dye. The positions and sizes (kDa) of marker polypeptides are indicated at the left. (B) Methyltransferase activity. Reaction mixtures contained 20 fmol of cap-labeled poly(A), 50 μM AdoMet, and WT, Δ 120, or Δ 151 enzyme as specified. Figure 12 shows Human cap methyltransferase substitutes for yeast Abdlp in vivo. Yeast strain YBS 10 {MATa ura3 trpl lys2 his3 leu2 abdl r. hisG [pCEN URA3 ABDl])was transformed with 2m TR P I plasmids (panel A) or CEN TRPI plasmids (panel B) containing the wild type HCM1 gene or N-terminal deletion mutants as specified. Control transformations were performed using the TRP I vectors alone and a CEN TRPI plasmid containing A B D 1. Trp-f- isolates were streaked on agar plates containing 0.75 mg/ml 5-FOA. The plates were photographed after incubation for 3 days at 30° C. Figure 13 shows replacement of the yeast capping apparatus with mammalian capping enzymes. (A) Yeast strain YBS50 {Δcetl Acegl) was transformed with CEN TRPI plasmids containing the indicated genes. Trp-f- isolates were streaked on agar containing 0.75 mg/ml 5-FOA. The plate was photographed after incubation for 3 days at 30°C. (B) Isogenic strains deleted at one or more yeast capping enzyme loci were transformed with CEN TRPI plasmids as specified below and then selected on 5-FOA. FOA-resistant isolates were streaked on a YPD agar plate. The plate was photographed after incubation for 3 days at 30°C. HCM1Δ120 MCEl is YBS52 transformed with HCMl ( 1 2 1 - 4 76 ) and M C E 1 ; ABD1 CETl CEGl is YBS52 transformed with ABD1 CETl and CE G l on a single plasmid; ABD 1 MCEl is YBS50 transformed with MCEl ; HCM1Δ120 CETl CEGl is YBS40 transformed with HCMl (121 -476). Figure 14 shows the cloning of the C. albicans cap methyltransferase gene by genetic complementation in S. cerevisiae. (Figure 14A) The C. albicans genomic DNA inserts of the 2μ library plasmid pCCM l -6.6 and the subclone pCCM l -3.9 are illustrated. Restriction sites used in subcloning and gene mapping are indicated. The CCM1 open reading frame is depicted as a black bar, with the orientation of the coding strand indicated by an arrow. (Figure 14B) Complementation of a bdl - 5 by CCM1. Yeast strain a b d l - 5 was transformed with 2μ URA 3 plasmids pCCMl-6.6, pCCMl-3.9, YEp24 (vector), and YEp24-ABDl . Transformants were streaked on agar medium lacking uracil and incubated at either 25°C or 37°C. (Figure 14 C ) The sequence of the 474-amino acid Ccmlp protein is shown. The Ccmlp residues corresponding to the 8 amino acids found by alanine scanning to be essential for Hcmlp function are denoted by dots . Figure 15 shows complementation of a S. cerevisiae Δabdl strain by the cap methyltransferase of Candida albicans and the effects of N-terminal deletions on C CMl function in vivo. Yeast strain YBS40 was transformed with CEN TRPI plasmids containing the indicated alleles of CCMl. Control transformations were performed using the TRPI vector alone and p358-ABDl . Trp + isolates were streaked on an agar plate containing 0.75 mg/ml of 5-FOA. The plate was photographed after incubation for 3 days at 30° C. Figure 16 shows an in vivo deletion analysis of Candida albicans RNA triphosphatase. YBS20 was transformed with CEN TRPI plasmids containing either CETl or the indicated alleles of CaCETl. A control transformation was performed using the TRP I vector. Trp+ isolates were streaked on agar plates containing 0.75 mg/ml of 5-FOA. The plates were photographed after incubation for 7 days at 25°C. The amino acid sequence of S. cerevisiae (Sc) Cetlp from residues 225-265 is aligned with the homologous segment of C. albicans (Ca) Cetlp. Identical amino acids are denoted by a colon (:); positions of side-chain similarity are denoted by a dot (.). The margins of the viable N- terminal deletion alleles of C E T l and CaCETl are denoted by arrowheads above and below the aligned sequences. The N-terminus of the lethal Ca CETl (21 7 -520) mutant is marked by a small arrowhead with a cross. Figure 17 shows the effect of N-terminal deletions on 5. cerevisiae CETl function in vivo. Yeast strain YBS20 was transformed with CEN TRPI plasmids containing the indicated N-terminal deletion alleles of CETl . A control transformation was performed using the TRP I vector. Trp + isolates were streaked on agar plates containing 0.75 mg/ml of 5-FOA. The plates were photographed after incubation for 3 days at 30°C. Figure 18 shows the replacement of the triphosphatase and guanylyltransferase components of the Saccharomyces capping apparatus by Candida enzymes. Yeast strain YBS50 {Δcetl Δcegl) was transformed with CEN TRPI plasmids containing the indicated genes. Trp + isolates were streaked on agar containing 0.75 mg/ml 5-FOA. The plate was photographed after incubation for 3 days at 30°C. Figure 19 shows the complete replacement of the Saccharomyces capping apparatus by Candida enzymes. Yeast strain YBS52 {Δcetl Δcegl Δabdl ) was transformed with TRPI plasmids as specified below and then selected on 5-FOA. FOA-resistant isolates were streaked on a YPD agar plate. The plate was photographed after incubation for 3 days at 30°C. ABD1 CETl CEGl was transformed with the three S. cerevisiae genes on a single CEN plasmid. CaCETl (179-520) CGT1 CCMl (137-474) and CaCET(179-520) CGT1 CCMl w e r e transformed with the indicated sets of three C. albicans genes on CEN plasmids pCAN-CAP-2 and ρCAN-CAP-2, respectively. M C E l HCMl (121 -476) was cotransformed with CEN MCEl and 2μHCMl (121 - 476) . Figure 20 shows the purification of recombinant CaCetlp, CaCetl (203-520)p (Δ 202), and CaCetl (217-520)p (Δ 216). The phosphocellulose column elution profiles were analyzed by SDS-PAGE. Aliquots (20 μl) of the Ni-agarose fractions that were applied to the phosphocellulose columns (lanes Ni); the phosphocellulose flow- through fraction (lanes F), the phosphocellulose 50 mM NaCl wash fractions (lanes 50), the 100 mM NaCl eluate fractions (lanes 100), the 200 mM NaCl eluate fractions (lanes 200), the 500 mM NaCl eluate fractions (lanes 500) and the 1 M NaCl eluate fractions (lanes 1000) were electrophoresed through 12.5% polyacrylamide gels containing 0.1 % SDS . The polypeptides were visualized by staining with Coomassie blue dye. The positions and sizes (kDa) of co- electrophoresed marker polypeptides are indicated at the right of each gel. The recombinant Candida proteins are indicated on the left. Figure 21 shows the manganese-dependent ATPase activity of CaCetlp and N-terminal deletion mutants of CaCetlp. (Figure 21A) Polypeptide composition. Aliquots (4 μg) of the phosphocellulose preparations of recombinant CaCetlp, CaCetl (203- 520)p (Δ202), and CaCetl (217-520)ρ (Δ 216) were electrophoresed through a 12.5 % polyacrylamide gel containing 0.1 % SDS . Polypeptides were visualized by staining with Coomassie blue dye. The positions and sizes (in kDa) of marker proteins are indicated on the left. (Figure 2 IB) ATPase activity. Reaction mixtures ( 10 μl) containing 50 mM Tris-HCl (pH 7.5), 5 mM DTT, 2 mM MnCl2 , 1 mM [γ^PJATP, and CaCetlp, Δ202, or Δ216 as specified were incubated for 15 min at 30°C. The reactions were quenched by adding 2.5 μl of 5 M formic acid. An aliquot (2.5 μl) of each mixture was applied to a polyethyleneimine-cellulose TLC plate, which was developed with 0.5 M LiCl 2 , 1 M formic acid. The release of 3 2 Pi from [γ^PJATP was quantitated by scanning the TLC plate with a FUJIX BAS2000 Bio- Imaging Analyzer. Figure 22 shows that ATP hydrolysis by the Candida albicans RNA triphosphatase is specifically activated by manganese and cobalt. Figure 22A: Divalent cation specificity. Reaction mixtures (10 μl) containing 50 mM Tris-HCl (pH 7.5), 1 mM [γ^PJATP, 50 ng of CaCetl (203-520)p (Δ202), and 2 mM divalent cation as specified were incubated for 15 min at 30°C. Mg, Mn, Ca, Co, and Zn were added as chloride salts; Cu was added as copper sulfate. The reaction products were analyzed by TLC. Figure 22B : Titration of manganese and cobalt. Reaction mixtures (10 μl) containing 50 mM Tris-HCl (pH 7.5), 50 ng of Δ202, 1 mM [γ^PJATP, and either MnCl 2 or CoCl 2 as specified were incubated for 15 min at 30°C. Pi release is plotted as a function of manganese or cobalt concentration. Figure 23 shows the RNA triphosphatase activity of recombinant CaCetl (203-540)p. Reaction mixtures (10 μl) containing 50 mM Tris-HCl (pH 7.5), 5 mM DTT, 1 mM MgCl 2 , 20 pmol (of triphosphate termini) of [γ P]-poly(A), and Δ202 as specified were incubated for 15 min at 30°C. Reactions were quenched by adding 2.5 μl of 5 M formic acid. Aliquots (5 μl) of the mixtures were applied to a polyethyleneimine-cellulose TLC plate, which was developed with 0.75 M potassium phosphate (pH 4.3). 3 2 Pi release is plotted as a function of input Δ202 protein. DETAILED DESCRIPTION OF THE INVENTION The present invention describes isogenic yeast strains containing fungal versus mammalian capping enzymes. These strains provide an attractive means of drug discovery aimed at blocking cap formation in, for example, pathogenic fungi. Any compound that is selectively cytotoxic to the "fungal capping" strain, but not to the "mammalian capping" strain, is a strong candidate for a specific inhibitor of the fungus. Secondary screens for cytotoxicity by comparing strains in which only a subset of the genes encoding the host organism' s capping enzymes are replaced by a corresponding subset of genes encoding fungal or mammalian capping enzyme' s can pinpoint which of the capping enzyme(s) is targeted by such a compound. The invention described herein further elaborates on an i n vivo strategy to identify antifungal compounds that block cell growth via their effects on the fungal capping apparatus. An underlying principle of this invention is the use of different strains of a test organism, such as Saccharomyces cerevisiae , that differ only in the composition or source of the essential cap-forming enzymes. The invention further provides isogenic yeast strains that derive all their capping activities from a pathogenic fungal source or a mammalian source. The present invention describes the construction of a yeast strain in which the entire 3-component Saccharomyces cerevisiae capping apparatus is replaced by the 3-component capping apparatus of the pathogenic fungus Candida albicans. Construction of this strain was contingent upon the cloning and characterization of the Candida albicans gene encoding cap methyltransferase, which is also described herein. This invention also provides a method to screen in vitro for molecules that inhibit fungal RNA triphosphatase, an essential enzyme that catalyzes the first of three steps in cap synthesis. This screening method exploits the intrinsic capacity of a fungal RNA triphosphatase, exemplified by the S. c e re vis ia e enzyme, Cetlp, to hydrolyze nucleoside triphosphates in the presence of a divalent cation cofactor such as manganese or cobalt. The present invention additionally includes demonstration of an intrinsic manganese- or cobalt-dependent nucleoside triphosphatase activity of the RNA triphosphatase encoded by the pathogenic fungus Candida albicans. The present invention is directed to a method of screening for a compound that inhibits formation of an organism' s 5' mRNA cap structure, comprising the steps of: a) replacing a host organism's genes encoding 5' mRNA capping functions with genes encoding 5' mRNA capping functions from a first organism, thereby producing a host organism expressing the first organism' s capping apparatus; b) replacing a host organism's genes encoding 5' mRNA capping functions with genes encoding the 5' mRNA capping functions from a second organism, thereby producing a host organism expressing the second organism' s capping apparatus, wherein said first organism and said second organism are not the same; c) treating the host organism expressing the first organism' s capping apparatus and the host organism expressing the second organism' s capping apparatus with a test compound; and d) comparing growth inhibition of the host organism expressing the first organism' s capping apparatus with growth inhibition of the host organism expressing the second organism' s capping apparatus. Growth inhibition of only the host organism expressing the first organism' s capping apparatus relative to the host organism expressing the second organism' s capping apparatus indicates that the test compound inhibits the 5' mRNA capping functions of the first organism, whereas growth inhibition of only the host organism expressing the second organism' s capping apparatus relative to the host organism expressing the first organism' s capping apparatus indicates that the test compound inhibits the 5' mRNA capping functions of the second organism. Representative host organisms are selected from the group consisting of viruses, fungal cells, insect cells, plant cells, and mammalian cells. When the host organism is a virus, comparison of growth inhibition of the virus expressing the first organism' s capping apparatus with the virus expressing the second organism' s capping apparatus is determined by the number of viral plaques formed on viral-permissive host cells and the viral plaque size formed on viral-permissive host cells. A preferred fungal host organism is Saccharomyces cerevisiae. Preferably, the first organism and second organism are selected from the group consisting of viruses, fungi, protozoa, plants, insects and mammals. Generally, the test compound will be either a chemical, a drug or protein. Preferably, the protein is encoded by DNA that can be expressed in the host organism, wherein the DNA is operably linked to an inducible promoter. Preferably, the 5' mRNA capping function inhibited by this method is RNA triphosphatase, RNA guanylyltransferase, or RNA (guanine-N7)-methyltransf erase. The present invention is directed to a method of screening for a compound that inhibits formation of an organism's 5' mRNA cap structure, comprising the steps of: a) replacing Saccharomyces cerevisiae ' s genes encoding 5 ' mRNA capping functions with genes encoding 5' mRNA capping functions from a first organism, thereby producing S. cerevisiae cells expressing the first organism' s capping apparatus; b) replacing S. cerevisiae 's genes encoding 5' mRNA capping functions with genes encoding the 5' mRNA capping functions from a second organism, thereby producing S. cerevisiae cells expressing the second organism's capping apparatus, wherein said first organism and said second organism are not the same; c) treating S. cerevisiae cells expressing the first organism' s capping apparatus and S. cerevisiae cells expressing the second organism' s capping apparatus with a test compound; and d) comparing growth inhibition of the S. cerevisiae cells expressing the first organism' s capping apparatus with growth inhibition of the S. cerevisiae cells expressing the second organism' s capping apparatus. Growth inhibition of only S. cerevisiae cells expressing the first organism' s capping apparatus relative to S. cerevisiae cells expressing the second organism's capping apparatus indicates that the test compound inhibits the 5' mRNA capping functions of the first organism, whereas growth inhibition of only S. cerevisiae cells expressing the second organism' s capping apparatus relative to S. cerevisiae cells expressing the first organism' s capping apparatus indicates that the test compound inhibits the 5 ' mRNA capping functions of the second organism. Preferably, the first organism and second organism are selected from the group consisting of viruses, fungi, protozoa, plants, insects and mammals. Generally, the test compound will be either a chemical, a drug or protein. Preferably, the protein is encoded by a D N A expressed in the S. cerevisiae cells, wherein the DNA is operably linked to an inducible promoter. Preferably, the 5' mRNA capping function inhibited by this method is RNA triphosphatase, RNA guanylyltransferase, or RNA (guanine-N7)- me thy ltransf erase. The present invention is directed to a method of screening for a compound that inhibits formation of an organism's 5' mRNA cap structure, comprising the steps of: a) replacing Saccharomyces cerevisiae ' s genes encoding 5' mRNA capping functions with genes encoding 5' mRNA capping functions from a fungal organism, thereby producing S. cerevisiae cells expressing the fungal organism's capping apparatus; b) replacing S. cerevisiae ' s genes encoding 5' mRNA capping functions with genes encoding the 5' mRNA capping functions from a mammalian organism, thereby producing S. cerevisiae cells expressing the mammalian organism' s capping apparatus; c) treating S. cerevisiae cells expressing the fungal organism' s capping apparatus and S. c e re vis ia e cells expressing the mammalian organism' s capping apparatus with a test compound; and d) comparing growth inhibition of the S. cerevisiae cells expressing the fungal organism' s capping apparatus with growth inhibition of the S. cerevisiae cells expressing the mammalian organism' s capping apparatus. Growth inhibition of only S. cerevisiae cells expressing the fungal organism' s capping apparatus relative to S. cerevisiae cells expressing the mammalian organism' s capping apparatus indicates that the test compound inhibits the 5 ' mRNA capping functions of the fungal organism, whereas growth inhibition of only S. cerevisiae cells expressing the mammalian organism' s capping apparatus relative to S. cerevisiae cells expressing the fungal organism's capping apparatus indicates that the test compound inhibits the 5 ' mRNA capping functions of the mammalian organism. Generally, the test compound will be either a chemical, a drug or protein. Preferably, the protein is encoded by a DNA expressed in the S. cerevisiae cells, wherein the DNA is operably linked to an inducible promoter. Preferably, the 5' mRNA capping function inhibited by this method is RNA triphosphatase, RNA guanylyltransferase, or RNA (guanine-N7)-methyltransferase. A representative fungal organism from which the replacement genes are derived is Candida albicans. The present invention is directed to a method of screening for a compound that inhibits the catalytic activity of fungal RNA 5' triphosphatase, comprising the steps of: a) contacting said fungal RNA 5' triphosphatase with a nucleoside triphosphate and a divalent cation cofactor in the presence and absence of a test compound; and detecting hydrolysis of said nucleoside triphosphate. A lack of hydrolysis of said nucleoside triphosphate or a reduction in the hydrolysis of said nucleoside triphosphate indicates inhibition of said fungal RNA 5' triphosphatase by said test compound. Preferably, the divalent cation cofactor is selected from the group consisting of manganese and cobalt. Although detection of hydrolysis may be by any method readily known to those having ordinary skill in this art, preferable methods include radioisotope assay and a colorimetric assay. Representative fungal RNA triphosphatases are RNA triphosphatases from Saccharomyces cerevisiae and Candida albicans. The present invention is also directed to a transformed host organism, wherein said host organism' s genes encoding 5 ' mRNA capping functions are replaced with replacement genes encoding 5 ' mRNA capping functions. Representative host organisms which can be transformed as described below include viruses, fungal cells, insect cells, plant cells, and mammalian cells. Preferably, replacement genes are from an organism selected from the group consisting of viruses, fungi, protozoa, plants, insects and mammals. A preferred host organism is Saccharomyces c e re v i s i a e , and a preferred strain of Saccharomyces cerevisiae is YBS52 {Δcetl Δcegl Δabdl ). Even more preferably, the replacement genes are from an organism selected from the group consisting of a fungus and a mammal, wherein replacement genes from the fungus are A B D 1, CETl and CEGl , and the resultant genotype is MA Ta. Ieu2 αde2 trpl his3 urα3 cαnl ceglr. hisG cetl::LEU2 αbdl : :KAN p360-CETl/CEGl/ABDl, or wherein the replacement genes from said mammal are HCM1 and MCEl . A preferable organism from which the replacement genes are derived is Candida albicans; those replacement genes comprise CaCETl, CGT1 and CCMl. For example, the ABD1 gene from S. cerevisiae may be replaced with the C CM l gene from Candida albicans, or the CETl and CEGl genes from S. cerevisiae may be replaced with the CaCETl and CGT1 genes from Candida albicans. The present invention is also directed to a transformed host S. cerevisiae, wherein said host organism's ABD1 gene is replaced with the Candida albicans CCMl gene. The present invention is also directed to a transformed host S. c e revis iae , wherein said host organism' s CETl and CEGl genes are replaced the Candida albicans CaCETl and CGT1 genes. Also provided in the present invention is an isolated C CM l gene enco din g an mRNA c ap methyl ating enzyme AdoMet:RNA(guanine-N7)-methyltransferase from Candida albicans. In accordance with the present invention, there may be employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e. g. , Sambrook, Fritsch & Maniatis, "Molecular Cloning: A Laboratory Manual (1982); "DNA Cloning: A Practical Approach," Volumes I and II (D.N. Glover ed. 1985); "Oligonucleotide Synthesis" (M.J. Gait ed. 1984); "Nucleic Acid Hybridization" [B.D. Hames & S.J. Higgins eds. (1985)] ; "Transcription and Translation" [B.D. Hames & S.J. Higgins eds. (1984)] ; "Animal Cell Culture" [R.I. Freshney, ed. (1986)] ; "Immobilized Cells And Enzymes" [IRL Press, (1986)] ; B. Perbal, "A Practical Guide To Molecular Cloning" (1984). Therefore, if appearing herein, the following terms shall have the definitions set out below. A "DNA molecule" refers to the polymeric form of deoxyribonucleotides (adenine, guanine, thymine, or cytosine) in its either single stranded form, or a double-stranded helix. This term refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear DNA molecules {e. g. , restriction fragments), viruses, plasmids, and chromosomes. In discussing the structure herein according to the normal convention of giving only the sequence in the 5' to 3' direction along the nontranscribed strand of DNA {i. e. , the strand having a sequence homologous to the mRNA). A "vector" is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. A "replicon" is any genetic element {e. g. , plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo; i. e., capable of replication under its own control. An "origin of replication" refers to those DNA sequences that participate in DNA synthesis. An "expression control sequence" is a DNA sequence that controls and regulates the transcription and translation of another DNA sequence. A coding sequence is "operably linked" and "under the control" of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then translated into the protein encoded by the coding sequence. In general, expression vectors containing promoter sequences which facilitate the efficient transcription and translation of the inserted DNA fragment are used in connection with the host. The expression vector typically contains an origin of replication, promoter(s), terminator(s), as well as specific genes which are capable of providing phenotypic selection in transformed cells. The transformed hosts can be fermented and cultured according to means known in the art to achieve optimal cell growth. A DNA "coding sequence" is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5' (amino) terminus and a translation stop codon at the 3' (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences, cDNA from eukaryotic mRNA, genomic DNA sequences from eukaryotic {e.g., mammalian) DNA, and even synthetic DNA sequences. A polyadenylation signal and transcription termination sequence will usually be located 3' to the coding sequence. A "cDNA" is defined as copy-DNA or complementary-DNA, and is a product of a reverse transcription reaction from an mRNA transcript. An "exon" is an expressed sequence transcribed from the gene locus, whereas an "intron" is a non-expressed sequence that is from the gene locus . Transcriptional and translational control sequences are DNA regulatory sequences, such as promoters, enhancers , polyadenylation signals, terminators, and the like, that provide for the expression of a coding sequence in a host cell. A "cis-element" is a nucleotide sequence, also termed a "consensus sequence" or "motif", that interacts with other proteins which can upregulate or downregulate expression of a specicif gene locus. A "signal sequence" can also be included with the coding sequence. This sequence encodes a signal peptide, N-terminal to the polypeptide, that communicates to the host cell and directs the polypeptide to the appropriate cellular location. Signal sequences can be found associated with a variety of proteins native to prokaryotes and eukaryotes. A "promoter sequence" is a DNA regulatory region capable of binding RNA polymerase in a cell and initiating transcription of a downstream (3' direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3' terminus by the transcription initiation site and extends upstream (5' direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter sequence will be found a transcription initiation site, as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Eukaryotic promoters often, but not always, contain "TATA" boxes and "CAT" boxes. Prokaryotic promoters contain Shine-Dalgarno sequences in addition to the -10 and -35 consensus sequences. A cell has been "transformed" or "transfected" with exogenous or heterologous DNA when such DNA has been introduced inside the cell. The transforming DNA may or may not be integrated (covalently linked) into the genome of the cell. In prokaryotes, yeast, and mammalian cells for example, the transforming DNA may be maintained on an episomal element such as a vector or plasmid. With respect to eukaryotic cells, a stably transformed cell is one in which the transforming DNA has become integrated into a chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by the ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cells containing the transforming DNA. A "clone" is a population of cells derived from a single cell or ancestor by mitosis. A "cell line" is a clone of a primary cell that is capable of stable growth in vitro for many generations. An organism, such as a plant or animal, that has been transformed with exogenous DNA is termed "transgenic". As used herein, the term "host organism" is meant to include not only prokaryotes but also eukaryotes such as yeast, plant and animal cells. A recombinant DNA molecule or gene can be used to transform a host using any of the techniques commonly known to those of ordinary skill in the art. Prokaryotic hosts may include E. coli, S. tymph im u rium, Serratia marcescens and Bacillus subtilis. Eukaryotic hosts include yeasts such as Pichia pastoris, mammalian cells, insect cells, and plant cells, such as Arabidopsis thaliana and To ba c c u m nicotiana. The following examples are given for the purpose of illustrating various embodiments of the invention and are not meant to limit the present invention in any fashion: EXAMPLE 1 Expression and purification of yeast RNA tri ph osph ata se Induced expression of the His-tagged Cetlp, Cetl (201- 549)p, and Cetl (246-549)ρ in Escherichia coli BL21(DE3) cells was performed as described by Ho et al. [11] . The recombinant proteins were purified from soluble bacterial lysate by Ni-agarose and phosphocellulose column chromatography [ 11] . EXAMPLE 2 Mntational analysis of yeast RNA tri phosph a ta se Alanine substitution mutations were introduced into the CETl (201 - 549) gene by PCR using the two-stage overlap extension method. Residues targeted for amino acid substitutions were Glu-305, Glu-307, Arg-454, Glu-492, Glu-494, and Glu-496. Plasmid p358- CES5(201-549) was the template for the first stage amplifications. The DNA products of the second stage amplification were digested with Ndel and B a mϋl and inserted into pET-16b. The presence of the desired mutations was confirmed by DNA sequencing; the inserted restriction fragment was sequenced completely in order to exclude acquisition of unwanted mutations during amplification and cloning. The His-tagged mutant proteins and the wild type protein were purified from soluble bacterial lysates by Ni-agarose chromatography as described by Ho et al. [11]. The 0.2 M imidazole eluate fractions containing Cetl(201-549)p were dialyzed against buffer C (50 mM Tris HCl [pH 8.0], 50 mM NaCl, 2 mM DTT, 10% glycerol, 0.05% Triton X- 100). Protein concentration was determined by using the Bio-Rad dye reagent with bovine serum albumin as the standard. EXAMPLE HCMl expression vectors A DNA fragment containing the 476-amino acid HCMl open reading frame was amplified by polymerase chain reaction from the pHG0376 template (Genbank AB007858) using Pfu DNA polymerase and oligonucleotide primers designed to introduce an Ndel restriction site at the translation start codon and a B a HI site 3' of the stop codon. The PCR product was digested with Ndel and BamHl and inserted into the T7 RNA polymerase-based expression plasmid pET16b to generate plasmid pET-His-Hcml . N-terminal deletion mutants of HCMl were constructed by PCR amplification with mutagenic sense- strand primers that introduced an Ndel restriction site and a methionine codon in lieu of the codons for Gly- 120 and Ala- 151 or an Ndel restriction site at the Met- 179 codon. The PCR products were digested with Ndel and B a mHl, then inserted into pET16b to yield plasmids pET-His-Hcml ( 121 -476), pET-His-Hcml (152-476), and pET- His-Hcml(179-476). The inserts of each plasmid were sequenced to verify that no unwanted coding changes were introduced during amplification and cloning. EXAMPLE 4 Hcm l p expression and purificati n pET-His-Hcml was transformed into Escherichia coli BL21 (DE3). A 1-liter culture of E. coli BL21 (DE3)/pET-His-Hcml was grown at 37°C in Luria-Bertani medium containing 0.1 mg/ml ampicillin until the A 60 o reached 0.5. The culture was adjusted to 0.4 mM isopropyl-β-D-thiogalactopyranoside (IPTG) and incubation was continued at 30°C for 4 h. Cells were harvested by centrifugation and the pellet was stored at -80°C. All subsequent procedures were performed at 4°C. Thawed bacteria were resuspended in 50 ml of lysis buffer (50 mM Tris HCl [pH 7.5], 0.15 M NaCl, 10% sucrose). Cell lysis was achieved by addition of lysozyme and Triton X-100 was to a final concentrations of 50 μg/ml and 0.1 %, respectively. The lysate was sonicated to reduce viscosity and insoluble material was removed by centrifugation. The soluble extract was mixed for 1 h with 2-ml of Ni- NTA-agarose resin that had been equilibrated with lysis buffer. The suspension was poured into a column and washed with lysis buffer. The column was eluted step-wise with IMAC buffer (20 mM Tris HCl [pH 7.9], 0.5 M NaCl, 1 mM phenylmethylsulfonyl fluoride, 10% glycerol) containing 25, 50, 200, and 500 mM imidazole. The polypeptide composition of the column fractions was monitored by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The recombinant Hcmlp protein was retained on the column and recovered in the 200 mM imidazole eluate. An aliquot of this fraction (100 μg of protein) was applied to a 4.8-ml 15-30% glycerol gradients containing 0.5 M NaCl in buffer A (50 mM Tris-HCl [pH 8.0], 1 mM EDTA, 2 mM DTT, and 0.1 % Triton X- 100). The gradient were centrifuged at 50,000 rpm for 15 h at 4°C in a Beckman SW50 rotor. Fractions (-0.2 ml) were collected from the bottom of the tube. Protein concentration was determined using the BioRad dye reagent with bovine serum albumin as the standard. EXAMPLE 5 Expression and purification of N-terminal deletion mutants of Hcm l p A modified strategy was employed to optimize the expression of the Hcml (121-476)p and Hcml(152-476)p proteins in soluble form in bacteria. Cultures (1-liter) of E. coli BL21(DE3) bearing the pET-based plasmids were grown at 37°C until the A600 reached 0.5. The cultures were adjusted to 2% ethanol and incubation was continued at 17°C for 24 h. The recombinant Hcml( 121-476)ρ and Hcml (152-476)p proteins were purified from soluble lysates by Ni- agarose chromatography and glycerol gradient sedimentation as described above for wild type Hcmlp. Hcml (179-476)p was expressed in bacteria, but was recovered exclusively in the insoluble pellet fraction of the cell lysate. EXAMPLE 6 Yeast HCM l expression plasmids Ndel-BamHl restriction fragments containing the wild type HCMl gene and the N-terminal deletion mutants were excised from the respective pET16b-based plasmids and inserted into a customized yeast expression vector pYXl-His, a derivative of pYX132 {CEN TRPI) in which six consecutive histidine codons and a unique Ndel site are inserted between the Ncol and BamHl sites of pYX132. (pYX132 was purchased from Novagen). The single copy expression plasmids were named pYX-Hcml , pYX-Hcml(121-476), pYX-Hcml (152-476), and pYX- Hcml ( 179-476). Nc ol-Xhol fragments containing the wild type HCMl gene and the N-terminal deletion mutants were excised from the respective pYX-based C E N plasmids and inserted into the yeast expression vector pYX232 (2m TRPI). In these vectors, expression of the human methyltransferase is under the control of the yeast TPIl promoter. EXAMPLE 7 Expression plasmids for yeast capping enzymes Plasmids p360-CEGl/CETl {CEN URA3 CETl CEGl) and p358-CETl/CEGl {CEN TRPI CETl CEGl) contain the yeast RNA triphosphatase and guanylyltransferase genes under the control of their natural promoters; the CETl and CEGl genes are arrayed head-to- head and transcribed divergently. Plasmids p360-CETl/CEGl/ABDl {CEN URA3 CETl CEGl ABDl) and p358-CETl/CEGl/ABDl {CEN TRPI CETl CEGl ABDl) were constructed by insertion of A BD l (under the control of its natural promoter) into p360-CEGl/CETl and p358- CET1/CEG1, respectively; the ABDl gene was placed next to CEGl in a tail-to-tail orientation. EXAMPLE 8 Yeast strains Yeast strain YBS50 {MA T a leu2 αde2 trpl his3 urα3 cαnl ceglr. hisG cetl::LEU2 p360-CETl/CEGl) is deleted at the chromosomal loci encoding RNA triphosphatase and guanylyltransferase. YBS40 {MA Ta leul αde2 trpl his3 urα3 cαnl αbdl::hisG p360-A D l ) is deleted at the chromosomal locus encoding cap methyltransferase YBS52 {MA Ta leu2 αde2 trpl his3 urα3 cαnl ceglr. hisG cetl::LEU2 αbdl : :KAN p360-CETl/CEGl/ABDl) is deleted at the chromosomal loci encoding all three components of the yeast capping apparatus. YBS50, YBS40, and YBS52 were derived by targeted gene disruptions in the diploid strain W303, followed by tetrad dissection and genotyping of haploid progeny. Gene disruptions were confirmed by Southern blotting. EXAMPLE 9 RNA Triphosphatase Activity of Cetl p is Essential for Yeast Cell Growth The fungal RNA triphosphatase Cetlp is a promising drug target because it executes the first step of RNA cap formation via a reaction mechanism that is completely different from that used by the mammalian RNA triphosphatase. Before attempting to exploit the distinctions in catalytic mechanism to design approaches to antifungal drug discovery, one needs to ascertain that the RNA triphosphatase activity of Cetlp is essential for yeast cell growth. This was shown to be the case by characterizing the effects of single amino acid mutations on Cetlp function in vitro and in vivo. Mutations were targeted to conserved residues within motifs A, B, and C of the metal-dependent RNA triphosphatase/NTPase family (Figure 4). Cetlp residues Glu-305 and Glu-307 (motif A), Arg-454 (motif B), and Glu-492, Glu-494, and Glu-496 (motif C) were replaced individually by alanine. The Ala mutations were introduced into Cetl(201-549)p, which is fully active in vitro and in vivo [11]. The six mutated proteins were expressed as His-tagged fusions and purified from soluble lysates by Ni-agarose column chromatography in parallel with wild type Cetl(201-549)p (Figure 6A). The RNA triphosphatase activities of the wild type and six mutant Cetl(201-549)p proteins were assayed by the release of 32 Pi from 2 μM [γ 3 2 ?] -labeled poly(A). Specific enzyme activity was determined from the slopes of the protein titration curves in the linear range of enzyme-dependence (Figure 6B). The specific activity of the wild type enzyme (16.5 nmol Pi released per microgram of protein in 15 min) corresponds to turnover number of -0.8 s ~ l . Analysis of RNA triphosphate cleavage by wild type Cetl(201-549)p as a function of RNA substrate concentration revealed a Km of 1 μM for poly(A) triphosphate termini and Vmax of 1 s ~ l . The specific activities of the alanine mutants, calculated from the data in Figure 6B and expressed as the percent of the wild type value, were: E305A (0.03%); E307A (0.03%); R454A (0.2%); E492A (1.7%); E494A (0.03%); E496A (0.8%). Mutant alleles of CETl (201 -549) encoding triphosphatase- defective enzymes were tested for their function in vivo using the plasmid shuffle assay described by Ho et al. [11] . The wild type and mutants coding sequences were cloned into a CEN TRPI vector so as to place the CETl (201 - 549) gene under the control of the natural CETl promoter. The plasmids were transformed into the Δcetl strain YBS20, in which the chromosomal CETl locus has been deleted and replaced by LEU2. Growth of YBS20 is contingent of the maintenance of a wild type CETl allele on a CEN URA3 plasmid. Hence, YBS20 is unable to grow on agar medium containing 0.75 mg/ml of 5-fluoroorotic acid (5- FOA), which selects against the URA 3 plasmid, unless it is transformed with a biologically active CETl allele or a functional homologue from another organism. Growth on 5-FOA was complemented by CETl (201 - 549) , but not by mutant alleles E305A, E307A, R454A, E492A, E494A, or E496A [12]. The correlation of in vitro and in vivo mutational effects indicates that the RNA triphosphatase activity of Cetlp is essential for yeast cell growth. EXAMPLE 10 Manganese- and Cobalt-dependent NTPase Activity of Cegl p Figure 7 illustrates that Ceglp also catalyzes the near- quantitative release of 3 2 Pi from 1 mM [γ^PJATP in the presence of 1 mM manganese as the divalent cation cofactor. There was no detectable ATP hydrolysis in the absence of a divalent cation and 1 mM magnesium was extremely feeble in supporting catalysis. The failure of previous investigators to appreciate the NTPase activity of the capping enzyme isolated from yeast extracts or recombinant Cetlp is most likely attributable to the reliance on magnesium as the divalent cation cofactor. Divalent cation specificity was tested in reaction mixtures containing 1 mM ATP and 2 mM divalent cation. Cobalt was at least as effective as manganese in activating the ATPase. Calcium, copper, and zinc did not activate the ATPase. Cofactor titration experiments showed that hydrolysis of 1 mM ATP was optimal at 1 to 3 mM MnCl2 - Cobalt-dependent hydrolysis of 1 mM ATP was optimal at 1 to 5 mM C0CI2. ATP hydrolysis was measured in 50 mM Tris buffer from pH 6.0 to pH 9.5. Activity was optimal from pH 6.5 to pH 7.0 and declined with increased alkalinity. Activity at pH 9.5 was 25% that at pH 7.0. EXAMPLE 11 Kinetics of ATP Hydrolysis hy Cetl p The extent of 3 2 Pi release from [γ 32 P]ATP during a 15 min reaction was proportional to the amount of input Cetlp protein (Figure 8 A). 32 Pi accumulated with time over 30 min; the rate of reaction varied linearly with Cetlp concentration (Figure 8B). From a plot of initial rate versus enzyme concentration, a turnover number of 25 s " was calculated. Cetlp catalyzed the quantitative conversion of [α 3 2 P]ATP to [α 3 2 P]ADP. The rate of [α 32 P]ADP formation was identical to the rate of 3 2 Pi release from [γ 32 P]ATP assayed in a parallel reaction mixture (Figure 8C). No formation of [α 3 2 P]AMP from [α 32 P]ATP was detected, even after 20-45 min of incubation, by which time all of the nucleotide had been converted to ADP. Thus, Cetlp catalyzes the hydrolysis of ATP to ADP plus Pi and is unable to further hydrolyze the ADP reaction product. Kinetic parameters were determined by measuring ATPase activity as a function of input [γ 32 P]ATP concentration (Figure 8D). From a double-reciprocal plot of the data, a Km of 2.8 μM ATP was calculated and a Vmax of 25 s " 1 (Figure 8D). The phosphohydrolase activity of Cetlp was not restricted to ATP. Cetlp also catalyzed manganese-dependent hydrolysis of [α 32 P]GTP to [α 3 2 P]GDP, [α 3 2 P]dCTP to [α 3 2 P]dCDP, and [α 3 2 P]dATP to [α 32 P]dADP. The turnover number of the yeast enzyme in ATP hydrolysis (25 s " 1 ) is similar to the values reported for the baculovirus (30 s " 1 ) and vaccinia virus ( 10 s " 1 ) triphosphatases, although the affinity of the yeast enzyme for ATP (Km = 2.8 μM) is significantly higher than that of either LEF-4 (Km = 43 μM) or vaccinia triphosphatase (Km = 800 μM) [12]. EXAMPLE 12 ATPase, Activity of CetU2 1 -549;, and Cetl r246-549) Two N-terminal truncation mutants, Cetl (201 -549)p and Cetl(246-549)p, that retained full activity in catalyzing the release of 3 2 Pi from γ 32 P-labeled triphosphate-terminated poly (A) were previously purified and characterized [11]. Similarly, the two truncated proteins were as active as full-length Cetlp in hydrolyzing ATP (Figure 3A). Both reactions are catalyzed by the same catalytic site within the carboxyl-terminal domain. The kinetic parameters determined for ATP hydrolysis by Cetl (201 -549)ρ [Km = 3.3 μM ATP, Vmax = 33 s " 1 ] were similar to those of full-length Cetlp. These data suggest that the deleted N-terminal 200-amino acid segment does not contribute to nucleotide binding or reaction chemistry. Hence, purified recombinant Cetl (201 -540)p can be used to screen in vitro for candidate inhibitors of the first step of fungal cap formation based on the effects of exogenous compounds on the ATPase activity of Cetl(201-540)p. EXAMPLE 13 Purifi cati on and Characteri zati on of Recombin ant Human Cap Mp.thyltransfp.rase The 476-amino acid polypeptide encoded by human cDNA KIAA0398 (Genbank accession AB007858), named Hcmlp, displays sequence conservation throughout its length with the 426-amino acid Abdlp protein (Figure 5). Abdlp contains an 11-amino acid C- terminal extension that has no counterpart in Hcmlp; the C-terminal decapeptide of Abdlp is dispensable for cap methyltransferase activity in vitro and in vivo [9] . Hcmlp also displays extensive sequence similarity to the candidate cap methyltransferase from C. elegans (Figure 5). All six enzymic functional groups that have been shown to be essential for cap methylation by Abdlp (denoted by dots above the aligned sequences in Figure 5) are conserved in Hcmlp [8, 9] . The Hcmlp protein was expressed in bacteria fused to short histidine-rich amino-terminal leader segment. The His-tag allowed for rapid enrichment of Hcmlp based on the affinity of the tag for immobilized nickel. The bacterial lysate was applied to Ni-agarose and adsorbed material was step-eluted with increasing concentrations of imidazole. SDS-PAGE analysis revealed a prominent 56 kDa Coomassie blue-stained species in the 200 mM imidazole eluate (see Figure 10A). This polypeptide was not recovered when lysates of IPTG-induced BL21 (DE3) carrying either the pET vector alone or pET plasmids expressing other His-tagged gene products were subjected to the same Ni-agarose chromatography procedure. RNA (guanine-7-) methyltransferase activity of the Ni- agarose Hcmlp preparation was detected by the conversion of 3 2 P cap- labeled poly(A) to methylated cap-labeled poly(A) in the presence of AdoMet [7] . The reaction products were digested to cap dinucleotides with nuclease PI and then analyzed by PEI-cellulose thin layer chromatography, which resolves the GpppA cap from the methylated cap m7GpppA. The radiolabeled product synthesized by Hcmlp (Figure 9, lane 1) co-migrated with m7GpppA generated in a parallel reaction mixture containing purified recombinant vaccinia virus cap methyltransferase (Figure 9, lane 6). Cap methylation by Hcmlp depended on inclusion of S-adenosylmethionine in the reaction mixture (Figure 9, lane 2). S-adenosylhomocysteine did not support cap methylation (Figure 9, lane 3) and was partially inhibitory in the presence of AdoMet (Figure 9, lane 4). Hcmlp was further purified by centrifugation of the Ni- agarose fraction through a 15-30% glycerol gradient. Cap methyltransferase activity sedimented as a single peak coincident with the peak of the Hcmlp polypeptide (Figure 10A). The apparent sedimentation coefficient of 4 S (relative to markers analyzed in parallel) indicated that the recombinant human cap methyltransferase is a monomer. Characterization of the enzyme was performed using the peak glycerol gradient fraction of Hcmlp. Methylation of capped poly(A) varied linearly with input enzyme and was quantitative at saturation (Figure 1 1B). Hcmlp formed 0.24 fmol of methylated capped ends per fmol of protein during a 10 min reaction. MgCl2 strongly inhibited activity in a concentration-dependent fashion; methylation was reduced by an order of magnitude by 1 mM magnesium. The extent of methylation varied with AdoMet concentration. Half-maximal activity was observed at -25 μM AdoMet. The reaction product AdoHcy inhibited cap methylation in a concentration-dependent manner; cap methylation in the presence of 10 μM AdoMet was reduced by 80% in the presence of 100 μM AdoHcy. EXAMPLE 14 Effects of N-terminal Deletions on Cap Methyltransferase Activity. His-tagged versions of N-terminal truncated proteins Hcm l ( 121 -476)p, Hcm l ( 152-476)p, and Hcm l ( 179-476)p were expressed in E. co li . Hcml (121 -476)p (referred to as Δ 120) and Hcml ( 152-476)p (Δ 151) were purified from soluble bacterial lysates by Ni-agarose chromatography and glycerol gradient sedimentation (Figure 10B and 10C). Hcml(179-476)p was insoluble and therefore not amenable to purification. The Δ 120 and Δ 151 proteins sedimented as discrete peaks of 4 S and 3 S respectively. Both preparations displayed cap methyltransferase activity that cosedimented with the recombinant proteins. Thus, both truncated enzymes are likely monomeric. SDS-PAGE analysis of the peak glycerol gradient fractions of wild type Hcmlp, Δ 120, and Δ 151 revealed that the proteins were of comparable purity and that the truncated versions migrated more rapidly than the full-sized Hcmlp, as expected (Figure 1 1 A). The specific activities of Δ 120 and Δ 151 in cap methylation were calculated from the slopes of the titration curves in the linear range of enzyme- dependence (Figure 11B). Δ 120 retained 50% of the activity of full- length Hcmlp. Δ151 was 22% as active as the wild type enzyme. EXAMPLE 15 Human Cap Methyltransferase is Functional In Vivo in Yeast. HC 7 and the truncated alleles HCMl (121 -476), HCMl (152-476) , and HCMl (179-476) were cloned into a yeast 2μ TRPI plasmid such that their expression was under the control of the yeast TPIl promoter. The HCMl plasmids were introduced into yeast strain YBS10 in which the chromosomal ABDl locus was deleted. Growth of YBS 10 is contingent on maintenance of an extrachromosomal ABDl gene on a CEN URA3 plasmid. Trp+ transformants were plated on medium containing 5-FOA to select against the URA3 ABDl plasmid. Control cells transformed with a TRPI ABDl plasmid grew on 5-FOA, whereas cells transformed with the TR P I vector were incapable of growth on 5-FOA (Figure 5A). The instructive finding was that cells bearing the HCMl or HCMl (121 -476) plasmids grew on 5-FOA. Thus, human cap methyltransferase was functional in vivo in lieu of the endogenous yeast enzyme. The more extensively truncated alleles HCMl (152-476) and HC i( 179-476) did not support growth on FOA (Figure 12A). The full-length and truncated HCMl alleles were also cloned into CEN TRPI vectors. Expression of the Δ 120 protein in single-copy complemented the abdl deletion, whereas expression of the full-length Ηcmlp protein did not (Figure 12B). The copy-number dependence of complementation by full-length HCMl may be indicative of lower levels of Ηcmlp expression in yeast compared to the Δ 120 derivative. Based on the alignment in Figure 5, the Δ 120 deletion of Ηcmlp would be roughly comparable to a deletion of 93-amino acids from the N-terminus of yeast Abdlp, whereas the Δ 151 deletion would be analogous to removal of 124-amino acids from Abdlp. Deleting 52 or 109 amino acids from the N-terminus of Abdlp did not affect the ability of the deleted alleles to support yeast cell growth [8] . However, more extensive deletions of 120, 129, or 142 amino acids from the N- terminus were lethal [9] . The N-terminal margins of the minimal functional domains of the yeast and human cap methyltransferases appear to be quite similar. EXAMPLE 16 Complete Replacement of the Yeast Capping Apparatus by Mammalian Enzymes. The strategy for drug discovery underpinning the present invention is to identify compounds that block cell growth contingent on pathogen-encoded capping activities without affecting the growth of otherwise identical cells bearing the capping enzymes of the host organism. For this approach to be feasible, the capping systems of interest must be interchangeable in vivo. Thus, yeast strains were constructed in which the entire fungal capping apparatus was replaced by mammalian enzymes. Expres sion of the mamm ali an triphosphatase- guanylyltransferase in yeast can complement the growth of singly deleted Δcegl or Δcetl strains [11, 17, 18, 20]. The critical next step was to test the ability of the mammalian capping enzyme to complement a new Δcetl Δcegl double-deletion strain (YBS50), growth of which depends on maintenance of a CEN URA3 CETl CEGl plasmid. Control experiments showed that transformation of YBS50 with a CEN TRPI CETl CEGl plasmid permitted the cells to grow on 5-FOA, whereas a CEN TRPI plasmid containing only CETl or only CEGl was unable to rescue growth on 5-FOA (Figure 10A). Expression of MCEl on a CEN TRPI plasmid under control of the yeast TPIl promoter fully complemented the Δcetl Δcegl double-deletion (Figure 13A). A new Δcetl Δcegl Δabdl triple-deletion strain (YBS52), was then constructed, growth of which is sustained by a CEN URA3 CETl CEGl ABDl plasmid. Control plasmid shuffle experiments showed that YBS52 cells transformed with a CEN TRPI CETl CEGl ABDl plasmid grew on 5-FOA, whereas plasmids containing only CETl , only CEGl , or only ABDl did not complement growth on 5-FOA. Cotransformation with MCEl plus HCMl ( 121 -476) complemented the triple-deletion; neither M CEl alone nor HCMl ( 121 -476) alone permitted growth of YBS52 on 5-FOA. FOA-resistant isolates were then streaked on YPD plates at 30°C. Using colony size as a rough estimate of growth, cells containing either MCEl in place of CETl plus CEGl , or HCMl (121 -476) in place of ABDl grew about as well as the strain containing an all- yeast capping apparatus (Figure 13B). However, colony size was smaller when all three yeast genes were replaced by M C E l plus HCMl ( 121 -476) on CEN plasmids (Figure 13B). Colony size was increased when the yeast genes were replaced with CEN MCEl plus 2μ HCMl ( 121 - 476) , implying that the human methyltransferase was limiting for growth in single copy in this background. To better gauge the growth of isogenic yeast cells containing yeast versus mammalian capping enzyme components, their doubling times were measured in YPD medium in suspension cultures at 30°C. The growth rates of cells containing expressing either Mcelp or Hcml(121-476)p in lieu of the yeast enzymes (generation times 1.6 to 1.8 h) were similar to that of cells with an all-yeast capping system ( 1.4 h). Cells with an all-mammalian capping system expressed from single copy plasmids grew more slowly (doubling time 3.3 h), but the defect was suppressed by high-copy expression of the human methyltransferase (doubling time 2.2 h). Thus, the entire three- component fungal capping system can be replaced by the two- component mammalian system. The two yeast strains provide the tools to implement the method of drug discovery outlined in this application. The method consists of a series of primary and secondary screening steps that are outlined below. The procedures described in the following section are applicable to the embodiment of the invention that uses yeast as the target organism in a screen for molecules that target the fungal capping apparatus. Other embodiments can be adapted as discussed above. EXAMPLE 17 Screening for Differential Growth Inhibition Based on the Composition or Source of the Capping Apparatus. The two yeast strains used for screening differential growth inhibition are the "fungal capping strain" (FCS) and "mammalian capping strain" (MCS), respectively. The primary screen entails plating a suspension of fungal capping strain cells on one agar plate and a suspension of mammalian capping strain cells on another plate. A matrix of test compounds is applied to the two plates in parallel, e. g. , via deposition of a solution of each compound within a small hole created in the agar. The compound solution will diffuse radially from the point of application and, if growth-inhibitory, will result in a gradient of no-growth or slow-growth emanating radially from the point source. Hence, after the plates are incubated for several days at permissive growth temperature (30°C for the fungal capping strain and mammalian capping strain yeast strains), a halo of no-growth or slow- growth will be apparent against a background lawn of yeast cells. Compounds that inhibit growth of fungal capping strain and mammalian capping strain to the same extent are presumed to be nonspecific inhibitors of yeast growth. Capping-specific inhibitors are those that inhibit growth of one of the two test strains, but not the other. For example, a compound that inhibits fungal capping strain but not mammalian capping strain is a presumed to exert its effects via the fungal capping apparatus. Delineation of which of the three fungal capping enzymes is targeted by a candidate anti-fungal capping strain compound identified in step 1 is accomplished by a second round of screening for differential growth inhibition using strains which contain a mixture of fungal and mammalian capping components. Derivatives of the triple- knockout yeast strain YBS52 (Δcetl Δcegl Δabdl ) have been generated that contain: (i) fungal triphosphatase and guanylyltransferase plus mammalian cap methyltransferase, (ii) mammalian triphosphatase- guanylyltransferase plus fungal cap methyltransferase. Compounds that inhibited the fungal capping strain strain by targeting the fungal triphosphatase or guanylyltransferase will inhibit strain (i), but not strain (ii). Compounds that targeted the fungal methyltransferase will inhibit strain (ii), but not strain (i). Anti-FCS compounds found during step 2 to target the fungal triphosphatase and/or guanylyltransferase are subjected to an additional screen to gauge which of the fungal enzymes is responsible for selective growth inhibition. For this purpose, a yeast strain was constructed that contains fungal triphosphatase and mammalian guanylyltransferase. EXAMPLE 18 Tn Vitro Screening for Inhibitors of Fungal RNA Triph osph ata se Cetlp displays robust nucleoside triphosphatase (NTPase) activity in the presence of manganese or cobalt as the divalent cation cofactor. The NTPase function of Cetlp is mediated by the same catalytic moieties on the enzyme that carry out the RNA triphosphatase reaction. Hence, assay of the hydrolysis of NTP by Cetlp offers a much more convenient assay than RNA triphosphatase for conducting large scale testing of Cetlp inhibitors. This is because NTPs are commercially available (including radioactively labeled NTPs), whereas the synthesis of triphosphate-terminated RNA is technically complex. Detection of ATP hydrolysis in the experiment in Figure 7 entailed the use of radiolabeled ATP and product analysis by thin layer chromatography. However, the assay is easily adapted to a non- radioactive colorimetric method of detection of Pi release from ATP [49]. A colorimetric assay of Cetlp activity is especially conducive to high-throughput screening of candidate inhibitors. EXAMPLE 19 Isolation of the Gene Encoding Candida albicans Cap Methyltran sferase by Selection for Complementation of abά -ts Mutants The cap methyltransferase gene from the pathogenic yeast Candida albicans was identified by screening for complementation of the conditional growth defect of S. cerevisiae abdl -ts mutants after transforming three different ts mutant strains (abdl -5, abdl -8, and abdl - 15) with a C. albicans genomic DNA library that had been cloned into an S. cerevisiae 2μ URA3 plasmid vector (50). Ura + transformants were selected for growth at the nonpermissive temperature of 34°C; positive isolates were then rescreened for growth at the even higher restrictive temperature of 37°C. 2μ plasmid DNA recovered from 9 individual isolates that grew at 37°C was amplified in E. coli and then the plasmid inserts were analyzed by digestion with a battery of restriction endonucleases that cut in the polylinker flanking the library insertion site. An apparently identical 6.6 kbp insert was present in the 2μ plasmids from all 9 isolates that had been rescued to grow at 3 7°C. This 2μ plasmid (named pCCMl -6.6; Figure 14A) retested faithfully in complementation of the ts growth phenotype when retransformed into the abdl -5 strain (Figure 14B). Hence, it was presumed that the plasmid contains the Candida cap methyltransferase gene (which was named C CMl). Excision of a 2.7 kbp Sphl fragment from the right end of pCCMl-6.6 yielded the 2μ plasmid pCCM l-3.9 (Figure 14A), which was as effective as the original isolate in rescuing growth of abdl -5 at 37° C (Figure 14B). Removal of a 1 kbp £cσRI fragment from the left end of the insert (to yield the 2μ plasmid pCCMl-ΔEco) resulted in loss of complementing activity, which implies that the CCMl gene spans the EcoRl site of the insert. Removal of a 0.3 kbp H-rcdIII fragment from the right end of the insert had no effect on complementation. Restriction fragments derived from the insert of pCCMl-3.9 were subcloned into pBluescript and then sequenced. The CCMl gene comprises a continuous open reading frame of 1422 nucleotides extending from left to right across the EcoRl site (Figure 14A). C CMl encodes a 474-amino acid polypeptide (Figure 14C). The genomic insert in pCCM 1-6.6 extends 254 bp upstream of the translation start site; this element apparently includes a C. albicans promoter element which can drive C CM l expression in S. cerevisiae. The predicted amino acid sequence reveals Ccmlp to be a homologue of S. cerevisiae Abdlp (209 positions of identity or similarity) and the human cap methyltransferase Hcmlp ( 140 positions of identity or similarity). The 8 amino acid residues defined by alanine-scanning mutagenesis as essential for function of the human cap methyltransferase (22) are conserved in the C. albicans protein (Figure 14C). Ccmlp is the first example of a cap methyltransferase from a fungal genus that is pathogenic for humans. EXAMPLE 20 Deletion Analysis of Candida Cap Methyl tran sfera se The complete C CM l coding sequence was cloned into a yeast CEN TRPI plasmid such its expression was under the control of the constitutive S. cerevisiae TPIl promoter. The C CM l plasmid was introduced into a yeast strain in which the chromosomal A B D l locus was deleted. Growth of the Δabdl strain is contingent on maintenance of an extrachromosomal ABDl gene on a CEN URA3 plasmid. Trp + transformants were plated on medium containing 5-FOA to select against the URA3 ABDl plasmid. Control cells transformed with a TRPI ABDl plasmid grew on 5-FOA, whereas cells transformed with the 77?-°-? vector were incapable of growth on 5-FOA (Figure 15). Cells bearing the CCMl plasmid grew on 5-FOA (Figure 15). Thus, the C. albicans cap methyltransferase was functional in vivo in lieu of the endogenous S. cerevisiae enzyme. Several N-terminal deletion mutants of CCMl were cloned into the CEN TRPI vector and tested for function in vivo by plasmid shuffle. CCMl (137-474) complemented growth of the Δabdl strain on 5-FOA (Figure 15). However, the more extensively truncated alleles CCMl(151 -474) and PCMl (175-474) were lethal in vivo (Figure 15). Based on an alignment of the amino acid sequences of the Ccmlp and Abdlp polypeptides, the viable N-terminal Δ 136 deletion of Ccmlp would be analogous to a deletion of 108 amino acids from the N- terminus of Abdlp. The lethal N-terminal Δ 150 and Δ 174 deletions of Ccmlp correspond to Δ 122 and Δ 146 deletions of Abdlp. Prior studies showed that deleting 109 amino acids from the N-terminus of Abdlp had no effect on yeast cell growth, whereas deletion of 142 or 155 residues was lethal (51,52). Thus, the N-terminal margins of the functional domains of the Candida and Saccharomyces cap methyltransferases are fairly similar. EXAMPLE 21 In Vivo Deletion Analysis of the Candida albicans and Saccharomyces cerevisiae RNA Triphosphatases The Candida albicans CaCETl gene encodes a 520-amino acid RNA triphosphatase (53). The C-terminal segment of CaCetlp from residues 186 to 520 displays extensive sequence similarity to the carboxyl portion of S. cerevisiae Cetlp (a 549-amino acid polypeptide) from positions 225 to 538 (95 identical residues and 64 positions with side-chain similarity). In contrast, the N-terminal segments of CaCetlp and Cetlp are not conserved. To delineate the margins of the minimal functional domain of the Candida triphosphatase, the wild type CaCETl gene and a series of N-terminal deletion alleles of CaCETl were cloned into CEN vectors under the control of the yeast TPIl promoter. These plasmids were transformed into a Δcetl strain YBS20, in which the chromosomal CETl locus has been deleted and replaced by L E U 2. Growth of YBS20 is contingent on the maintenance of a wild type CETl allele on a C E N URA3 plasmid. Hence, YBS20 is unable to grow on agar medium containing 5-fluoroorotic acid (5-FOA) unless it is transformed with a biologically active CETl allele (Figure 16) or a functional homologue from another organism. The instructive finding is that growth on 5- FOA was complemented by full-length CaCETl and by deletion mutants CaCETl (1 79-520) , CaCETl (196-520), and CaCETl (203 -520), whereas CaCETl (217 -520) was lethal (Figure 16). This experiment delineates a short peptide segment between residues 203 and 216 that is required for CaCetlp function in vivo when the triphosphatase is expressed from the TPIl promoter. N-terminal deletion alleles of the S. cerevisiae CETl gene - CETK211 -549), CETl (221 -549), CETl (231 -549), and CETl (241 -549) - were cloned into a CEN TRPI vector so as to place them under the control of the CETl promoter. These plasmids were tested by plasmid shuffle for complementation of the Δcetl strain YBS20. Growth on 5- FOA was complemented by CET 1 (201 -549), CET 1 (211 -549), CET 1 (221 - 549) , CET 1 (231 -549), and CETl (241 -549)(Figure 17: Δ200, Δ210, Δ220, Δ 230, Δ 240), but not by the more extensively truncated alleles CETl (246-549) or CETl (276-549) (Figure 17, Δ245 and Δ275). This experiment delineates a short peptide segment between residues 241 and 245 that is required for Cetlp function in vivo when the triphosphatase is expressed from its own promoter. According to the sequence alignment shown in Figure 17, the viable Δ 195 and Δ202 deletions of CaCetlp correspond to deletions of 235 and 242 amino acids from the N-terminus of Cetlp, whereas the lethal Δ216 mutation corresponds to a deletion of 256 residues from Cetlp. These results show that the upstream functional borders of the two fungal RNA triphosphatases are quite similar. EXAMPLE 22 Complete Replacement of the Sar.chamm.yr.es Capping Apparatus by Candida Enzymes A plausible strategy for capping-specific antifungal drug discovery is to identify compounds that inhibit the growth of yeast cells containing fungus-encoded capping activities without affecting the growth of otherwise identical yeast cells bearing the mammalian capping enzymes. Ideally, the fungal capping enzymes that sustain growth of the tester yeast cells would be those encoded by a clinically significant fungal pathogen. To achieve this scenario, S. cerevisiae strains were constructed in which the entire S. cerevisiae capping apparatus was replaced by enzymes from the pathogenic fungus Candida albicans. It has been shown that expression of Candida albicans CaCetlp (RNA triphosphatase) or Cgtlp (RNA guanylyltransferase) in S. cerevisiae can complement the growth of singly deleted Δc e g l or Δcetl strains (53,54). Figure 18 shows that coexpression of CaCetlp and Cgtlp in S. cerevisiae can complement a Δcetl Δceg l double- deletion strain (YBS50). Growth of YBS50 depends on maintenance of a CEN URA3 CETl CEGl plasmid. Transformation with a CEN TRPI CETl CEG l plasmid permitted the cells to grow on 5-FOA, whereas a CEN TRPI plasmid containing only CETl or only CGTl was unable to rescue growth on 5-FOA (Figure 18). Transformation with a CEN TRPI CaCETl CG Tl plasmid complemented the Δ cetl Δ c e g l strain (Figure 18). Complementation was also evident when the cells were transformed with a plasmid containing C G Tl plus the truncated triphosphatase allele CaCETl (179-520). Having cloned the CCMl gene encoding the C. albicans c ap methylating enzyme, it became feasible to test whether the entire S. cerevisiae capping system could be replaced by Candida albicans enzymes . A CEN TRPI expression plasmid (pCan-CAP- 1 ) was constructed that contained three C. albicans genes under the control of S. cerevisiae promoters, as follows: CaCET 1 (179-520) controlled by the TPIl promoter; CG Tl driven by the GPD1 promoter; CC 7 controlled by the TPIl promoter. Another CEN TRPI expression plasmid (pCan- CAP-2) was constructed that contained: CaCETl (179-520) controlled by the TPIl promoter; C G Tl driven by the GPD1 promoter; CCM1 (137- 474) controlled by the TPIl promoter. pCan-CAP-1 and pCan-CAP-2 were transformed into the S. cerevisiae Δcetl Δceg l Δabdl triple- deletion strain (YBS52), growth of which is sustained by a CEN URA3 CETl CEGl ABDl plasmid. Plasmid shuffle experiments showed that YBS52 cells transformed with either pCan-CAP- 1 and pCan-CAP-2, or with an "all-Saccharomyces" capping enzyme expression plasmid (CEN TRPI CETl CEGl ABDl ) grew on 5-FOA, whereas plasmids containing only CETl , only CEGl , or only ABDl did not complement growth on 5- FOA. 5-FOA-resistant isolates were then streaked on YPD plates at 3 0°C. Using colony size as a rough estimate of growth, it is surmised that cells containing either "all-Candida" capping apparatus grew as well as the cells containing an "all-Saccharomyces" capping apparatus (Figure 19). Also shown on this plate is the growth of cells containing an "all-mammalian" capping system encoded by CEN MCEl plus 2μ HCMl (121 -476) plasmids (Figure 19). EXAMPLE 23 Expression and Purification of Recombinant CaCetl p Full-length CaCetlp and the amino-terminal deletion mutants CaCetl (203-520)p (Δ202) and CaCetl(217-520)p (Δ216) were expressed in bacteria as His-tagged fusions. The Ca CETl, CaCETl(203- 520) , and CaCETl (217-520) genes were cloned into pET-based bacterial expression vectors; the pET-CaCET plasmids were transformed into E. coli BL21(DE3). Cultures (100 ml) of single ampicillin-resistant transformants were grown at 37°C in Luria Bertani medium containing 0.1 mg/ml ampicillin until the A 60 o reached 0.5. The cultures were adjusted to 0.4 mM isopropyl-β-D-thiogalactopyranoside (IPTG) and incubation was continued at 37°C for 3 h. Cells were harvested by centrifugation and the pellet was stored at -80°C. All subsequent procedures were performed at 4°C. Thawed bacteria were resuspended in 5 ml of lysis buffer (50 mM Tris HCl [pH 7.5], 0.15 M NaCl, 10% sucrose). Cell lysis was achieved by addition of lysozyme and Triton X- 100 to final concentrations of 50 μg/ml and 0.1 %, respectively. The lysate was sonicated to reduce viscosity and insoluble material was removed by centrifugation in a Sorvall SS34 rotor at 18,000 rpm for 45 min. The soluble lysates were applied to 1 ml columns of Ni-NTA agarose that had been equilibrated with lysis buffer. The columns were washed with 5 ml of lysis buffer containing 0.1 % Triton X- 100 and then eluted step-wise with a buffer solution (50 mM Tris-HCl [pH 8.0] , 50 mM NaCl, 2 mM DTT, 10% glycerol, 0.05% Triton X-100) containing 50, 100, 200, 500, and 1000 mM imidazole. SDS-PAGE analysis showed that the recombinant Candida proteins were recovered predominantly in the 200 mM imidazole eluate fractions. The peak fractions containing the recombinant proteins were pooled, adjusted to 5 mM EDTA and dialyzed against P-Cell column buffer (50 mM Tris-HCl [pH 8.0], 2 mM DTT, 5 mM EDTA, 10% glycerol, 0.05% Triton X-100) containing 50 mM NaCl. The dialysates were applied to 0.5 ml columns of phosphocellulose that had been equilibrated in the same buffer. The columns were washed with buffer containing 50 mM NaCl and then eluted step-wise with P-Cell buffer containing 0.1 , 0.2, 0.5, and 1.0 M NaCl. The recombinant proteins were recovered predominantly in the 0.5 M NaCl eluate fractions. Protein concentrations were determined by the Biorad dye-binding method with bovine serum albumin as the standard. The phosphocellulose enzyme preparations were stored at -80°C. SDS-PAGE analysis of the polypeptide compositions of the Ni-agarose preparations and the phosphocellulose elution profiles are shown in Figure 20. The Ni-agarose eluate fraction of CaCetlp contained, in addition to the full-length 66 kDa recombinant protein, a large number of smaller polypeptides that are presumed to correspond to proteolytic fragments of CaCetlp that retain the N-terminal His-tag (Figure 20). Similar breakdown products were also noted during Ni- agarose purification of recombinant S. cerevisiae Cetlp (55). The recombinant Δ202 and Δ216 polypeptides were substantially pure at the Ni-agarose step (Figure 7, lane Ni); the absence of smaller breakdown products in these preparations suggests that the N-terminal domain of CaCetlp is especially susceptible to proteolysis when expressed in bacteria. Further purification of CaCet lp was achieved by phosphocellulose column chromatography. CaCetlp adsorbed to phosphocellulose and was step-eluted with buffer containing 0.5 M NaCl. Many of the lower molecular weight contaminant polypeptides were removed at this step (Figure 20). The Δ202 and Δ216 proteins also adsorbed to phosphocellulose and were recovered in the 0.5 M NaCl eluate fractions (Figure 20). EXAMPLE 24 Manganese- and Cobalt-Dependent NTPase Activity of CaCetl As documented in the initial patent application, S. cerevisiae Cetlp has an intrinsic capacity to hydrolyze nucleoside triphosphates to nucleoside diphosphates and inorganic phosphate in the presence of a divalent cation cofactor - either manganese or cobalt. The divalent cation specificity of the NTPase is distinct from the RNA triphosphatase activity of Cetlp, which is supported by magnesium. The utility of the ATPase-based assay for in vitro screening of inhibitors of fungal RNA triphosphatases is underscored by a demonstration that NTPase activity is a general property of fungal RNA triphosphatases (especially in pathogenic fungi like C. albicans) and not merely restricted to the enzyme from S. cerevisiae. Experiments illustrated in Figure 21 demonstrate that the phosphocellulose preparations of recombinant CaCetlp, Δ 202, and Δ216 catalyzed the release of 3 2 Pi from [γ 3 2 P]ATP in the presence of manganese chloride. The extent of ATP hydrolysis increased as a function of input enzyme (Figure 21). The specific activities of Δ202 and Δ 216 were virtually identical. A turnover number of 14 s " 1 for Δ202 was calculated from the slope of the titration curve in the linear range. This value for the Candida protein is similar to the turnover number of 25 s " 1 determined for manganese-dependent ATP hydrolysis by S. cerevisiae Cetlp. The activity of full-length CaCetlp (on a per nanogram basis) was slightly less than that of the truncated proteins (Figure 21 ); this is most probably attributable to the presence of inactive contaminating polypeptides in the preparation. Further characterization of the biochemical properties of CaCetlp was performed with the Δ202 enzyme. The metal specificity of CaCetlΔ202 was tested in reaction mixtures containing 1 mM ATP and 2 mM divalent cation (Figure 22 A). Cobalt was even more effective than manganese in activating the ATPase, whereas magnesium did not support ATP hydrolysis. Calcium and copper did not activate the ATPase; zinc was weakly active. (The divalent cation specificity of the ATPase activity of recombinant full- length CaCetlp was the same as that shown for Cetl Δ202). The failure of previous investigators to appreciate the ATPase activity of CaCetlp (53) can be attributed to their reliance on magnesium as the divalent cation cofactor. Cofactor titration experiments showed that hydrolysis of 1 mM ATP was optimal at 1 to 2.5 mM manganese or cobalt and declined slightly as the divalent cation concentrations were increased to 5 and 10 mM (Figure 22B). The titration curves were sharply sigmoidal at manganese or cobalt concentrations below the level of input ATP. The RNA triphosphatase activity of CaCetlΔ202 was gauged by the release of 3 2 Pi from 2 μM [γ 32 P]poly(A) in the presence of 1 mM magnesium (Figure 23). The specific activity, determined from the slope of the titration curve, was 32 pmol Pi released per nanogram of protein in 15 min. This corresponds to turnover number of -1.4 s " 1 (a value that is quite close to the Vmax value of 1 s " 1 for the hydrolysis of [γ 32 P]poly(A) by S. cerevisiae Cetlp). The following referenes were cited herein: 1. S human, S. (1995) Prog. Nucleic Acid Res. Mol. Biol. 50, 101-129. 2. Schwer, et al., (1994) Proc. Natl. Acad. Sci. USA 91, 4328-4332. 3 . Fresco, et al., (1994) Proc. Natl. Acad. Sci. USA 91, 6624-6628. 4. Shibagaki, et al., (1995) J. Biochem. 1 1 8, 1303-1309. 5 . Shuman, et al., (1994) Proc. Natl. Acad. Sci. USA 9 1, 12046- 1 2050. 6. Wang, et al., (1997) Proc. Natl. Acad. Sci. USA 94, 9573-9578. 7. Mao, et al., (1995) Mol. Cell. Biol. 1 5, 4167-4174. 8 . Mao, et al., (1996) Mol. Cell. 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Biol. 18, 4612-4619. 43. S human, et al., (1980) J. Biol. Chem. 255, 11588- 1 1598. 44. S human, S. (1990). J. Biol. Chem. 265, 1 1960- 1 1966. 45. Yamada-Okabe, et al., (1998) FEBS Lett. 435, 49-54. 46. Mao, X., & S. Shuman (1996) Biochemistry 35, 6900-6910. 47. Cho, et al., (1997) Genes Dev. 1 1, 3319-3326. 48. Shuman, S. (1997) Proc. Natl. Acad. Sci. USA 94, 12758- 12760. 49. Lanzetta, et al., (1979) Anal. Biochem. 1 0 0, 95-97. 50. Janbon, G., et al., (1997) Yeast 1 0, 985-990. 5 1 . Mao, X., et al., (1996) Mol. Cell. Biol. 1 6, 475-480. 52. Wang, S. & Shuman, S. (1997) J. Biol. Chem. 272, 14683- 14689. 53. Yamada-Okabe, T., et al., (1998) FEBS Lett. 435, 49-54. 54. Yamada-Okabe, T., et al., (1996) Microbiology 142, 2515-2523. 55. Ho, C. K., et al., (1998) Mol. Cell. Biol. 1 8, 5189-5198. Any patents or publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. Further, these patents and publications are incorporated by reference herein to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. One skilled in the art will appreciate readily that the present invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those objects, ends and advantages inherent herein. The present examples, along with the methods, procedures, treatments, molecules, and specific compounds described herein are presently representative of preferred embodiments, are exemplary, and are not intended as limitations on the scope of the invention. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention as defined by the scope of the claims.

Claims: WHAT IS CLAIMED IS: 1 . A method of screening for a compound that inhibits formation of an organism's 5' mRNA cap structure, comprising the steps of: a ) replacing a host organism' s genes encoding 5' mRNA capping functions with genes encoding 5 ' mRNA capping functions from a first organism, thereby producing a host organism expressing the first organism' s capping apparatus; b ) replacing a host organism' s genes encoding 5' mRNA capping functions with genes encoding the 5' mRNA capping functions from a second organism, thereby producing a host organism expressing the second organism' s capping apparatus, wherein said first organism and said second organism are not the same; c) treating said host organism expressing the first organism' s capping apparatus and said host organism expressing the second organism's capping apparatus with a test compound; and d ) comparing growth inhibition of the host organism expressing the first organism' s capping apparatus with growth inhibition of the host organism expressing the second organism' s capping apparatus, wherein growth inhibition of only the host organism expressing the first organism' s capping apparatus relative to the host organism expressing the second organism' s capping apparatus indicates that the test compound inhibits the 5 ' mRNA capping functions of the first organism, whereas growth inhibition of only the host organism expressing the second organism' s capping apparatus relative to the host organism expressing the first organism' s capping apparatus indicates that the test compound inhibits the 5 ' mRNA capping functions of the second organism. 2. The method of claim 1 , wherein the host organism is selected from the group consisting of viruses, fungal cells, insect cells, plant cells, and mammalian cells. 3 . The method of claim 2, wherein when said host organism is a virus, comparison of growth inhibition of the virus expressing the first organism' s capping apparatus with the virus expressing the second organism' s capping apparatus is selected from the group consisting of the number of viral plaques formed on viral- permissive host cells and the viral plaque size formed on viral- permissive host cells. 4. The method of claim 2, wherein said fungal cells are Saccharomyces cerevisiae. 5 . The method of claim 1 , wherein said first organism and said second organism are selected from the group consisting of viruses, fungi, protozoa, plants, insects and mammals. 6. The method of claim 1 , wherein said test compound is selected from the group consisting of chemicals, drugs and proteins. 7. The method of claim 6, wherein said protein is encoded by a DNA expressed in said host cells, wherein said DNA is operably linked to an inducible promoter. 8 . The method of claim 1, wherein said 5' mRNA capping function is selected from the group consisting of RNA triphosphatase, RNA guanylyltransferase, and RNA (guanine-N7)-methyltransferase. 9. A method of screening for a compound that inhibits formation of an organism' s 5 ' mRNA cap structure, comprising the steps of: a ) replacing genes encoding 5' mRNA capping functions from Saccharomyces cerevisiae with genes encoding 5' mRNA capping functions from a first organism, thereby producing S. cerevisiae cells expressing the first organism's capping apparatus; b ) replacing genes encoding 5' mRNA capping functions from Saccharomyces cerevisiae with genes encoding the 5' mRNA capping functions from a second organism, thereby producing S. cerevisiae cells expressing the second organism' s capping apparatus, wherein said first organism and said second organism are not the same; c) treating S. c e re v is ia e cells expressing the first organism' s capping apparatus and S. cerevisiae cells expressing the second organism' s capping apparatus with a test compound; and d ) comparing growth inhibition of the S. cerevisiae cells expressing the first organism' s capping apparatus with growth inhibition of the S. cerevisiae cells expressing the second organism' s capping apparatus, wherein growth inhibition of only S. cerevisiae cells expressing the first organism' s capping apparatus relative to S. cerevisiae cells expressing the second organism' s capping apparatus indicates that the test compound inhibits the 5 ' mRNA capping functions of the first organism, whereas growth inhibition of only S. cerevisiae cells expressing the second organism' s capping apparatus relative to S. cerevisiae cells expressing the first organism's capping apparatus indicates that the test compound inhibits the 5' mRNA capping functions of the second organism. 1 0. The method of claim 9, wherein said first organism and said second organism are selected from the group consisting of viruses, fungi, protozoa, plants, insects and mammals. 1 1 . The method of claim 9, wherein said test compound is selected from the group consisting of chemicals, drugs and proteins. 1 2. The method of claim 1 1 , wherein said protein is encoded by a DNA expressed in said S. cerevisiae cells, wherein said DNA is operably linked to an inducible promoter. 1 3 . The method of claim 9, wherein said 5' mRNA capping function is selected from the group consisting of RNA triphosphatase, RNA guanylyltransferase, and RNA (guanine-N7)-methyltransferase. 14. A method of screening for a compound that inhibits formation of an organism's 5 ' mRNA cap structure, comprising the steps of: a ) replacing genes encoding 5' mRNA capping functions from Saccharomyces cerevisiae with genes encoding 5' mRNA capping functions from a fungal organism, thereby producing S. cerevisiae cells expressing the fungal organism's capping apparatus; b ) replacing genes encoding 5' mRNA capping functions from Saccharomyces cerevisiae with genes encoding the 5' mRNA capping functions from a mammalian organism, thereby producing S. c e re vis ia e cells expressing the mammalian organism' s capping apparatus ; c ) treating S. c e revis iae cells expressing the fungal organism' s capping apparatus and S. cerevisiae cells expressing the mammalian organism' s capping apparatus with a test compound; and d ) comparing growth inhibition of the S. cerevisiae cells expressing the fungal organism' s capping apparatus with the S. c e re vis ia e cells expressing the mammalian organism' s capping apparatus, wherein growth inhibition of only S. cerevisiae cells expressing the fungal organism' s capping apparatus relative to S. c e re vis ia e cells expressing the mammalian organism' s capping apparatus indicates that the test compound inhibits the 5 ' mRNA capping functions of the fungal organism, whereas growth inhibition of only S. cerevisiae cells expressing the mammalian organism' s capping apparatus relative to S. c e re v is iae cells expressing the fungal organism' s capping apparatus indicates that the test compound inhibits the 5' mRNA capping functions of the mammalian organism. 15 . The method of claim 14, wherein said test compound is selected from the group consisting of chemicals, drugs and proteins. 16. The method of claim 15, wherein said protein is encoded by a DNA expressed in said S. cerevisiae cells, wherein said DNA is operably linked to an inducible promoter. 1 7. The method of claim 14, wherein said 5 ' mRNA capping function is selected from the group consisting of RNA triphosphatase, RNA guanylyltransferase, and RNA (guanine-N7)- methyltransf erase. 1 8. The method of claim 14, wherein said fungal organism is Candida albicans. 1 9. A method of screening for a compound that inhibits the catalytic activity of fungal RNA 5' triphosphatase, comprising the steps of: a ) contacting said fungal RNA 5' triphosphatase with a nucleoside triphosphate and a divalent cation cofactor in the presence and absence of a test compound; and b ) detecting hydrolysis of said nucleoside triphosphate, wherein a lack of hydrolysis of said nucleoside triphosphate or a decrease in hydrolysis of said nucleoside triphosphate indicates inhibition of said fungal RNA 5' triphosphatase by said test compound. 20. The method of claim 19, wherein said divalent cation cofactor is selected from the group consisting of manganese and cobalt. 2 1 . The method of claim 19, wherein said detection of hydrolysis is selected from the group consisting of a radioisotope assay and a colorimetric assay. 22. The method of claim 18 wherein said fungal RNA triphosphatase is an RNA triphosphatase from Candida albicans. 23. A transformed host organism, wherein said host organism' s genes encoding 5' mRNA capping functions are replaced with replacement genes encoding 5' mRNA capping functions. 24. The transformed host organism of claim 23, wherein said host organism is selected from the group consisting of viruses, fungal cells, insect cells, plant cells, and mammalian cells. 25. The transformed host organism of claim 23, wherein said replacement genes are from an organism selected from the group consisting of viruses, fungi, protozoa, plants, insects and mammals. 26. The transformed host organism of claim 24, wherein said fungi is Saccharomyces cerevisiae. 27. The transformed host organism of claim 26, wherein the strain of Saccharomyces cerevisiae is YBS52 (Δcetl Δcegl Δabdl ) . 28. The transformed host organism of claim 27, wherein said replacement genes are from an organism selected from the group consisting of a fungus and a mammal. 29. The transformed host organism of claim 28, wherein replacement genes from said fungus are ABDl, CETl and CEGl. 30. The transformed host organism of claim 29, wherein the resultant genotype is MA T a leu2 αde2 trpl hi 3 urα3 cαnl ceglr. hisG cetl::LEU2 αbdlrKAN p360-CETl /CEGl /ABDl. 3 1 . The transformed host organism of claim 28, wherein said replacement genes from said mammal are selected from the group consisting of an HCMl gene and an MCEl gene. 32. The transformed host organism of claim 25, wherein said fungus is Candida albicans. 33. The transformed host organism of claim 32, wherein said replacement genes are the Ca CETl, CGTl , and CCMl genes. 34. A transformed host S. cerevisiae, wherein said host organism' s ABD l gene is replaced with the Candida albicans CCMl gene. 35 . A transformed host S. cerevisiae, wherein said host organism' s CETl and CEGl genes are replaced the Candida albicans CaCETl and CGTl genes. 36. An isolated C C M l gene from Candida albicans, wherein said CCMl gene encodes an mRNA cap methylating enzyme AdoMet:RNA(guanine-N7)-methyltransf erase.

Inventors: SHUMAN STEWART, SHUMAN, STEWART
Applicants: SLOAN KETTERING INST CANCER, SLOAN-KETTERING INSTITUTE FOR CANCER RESEARCH
Application year: 1999
Main classification: C12Q1
IPC codes: C12N9, C12N15, C12Q1