INTERACTIONS OF SKN7 GENE AND ITS USE IN ASSAY METHODS

Abstract: The invention relates to the finding that the yeast protein Skn7 acts as a transcription factor and is involved in direct binding to DNA. Transcription by Skn7 involves cooperation with other cellular factors, including the proteins Yap1, Hsf1 and Mbp1. The binding of Skn7 to specific DNA sequences, and its interaction with other factors, provides targets for assays for screening putative inhibitors of fungal growth.

Description: INTERACTIONS OF SKN7 GENE AND ITS USE IN ASSAY METHODS Field of the invention. The present invention relates to assay methods and means, and substances identified using assays. In particular, the present invention is based on demonstion that the yeast polypeptide Skn7 acts as a transcription factor and that this is in cooperation with other cellular factors including Yapl and Hsfl. Experimental evidence disclosed herein indicates that Skn7 up-regulates expression of genes involved in response to stress by binding a specific nucleotide sequence motif. Substances which disrupt the function of Skn7, by interfering with its interaction with Yapl, Hsfl, or its interaction with the specific nucleotide sequence motif, may be used as anti- fungal agents. Background to the invention. Skn7 was identified originally as a gene which when over- expressed could alleviate a particular defect in the cell wall of the budding yeast, suggesting a role in intracellular biochemical pathways that control cell wall structure (Brown et al . , (1993) J " . Bacteriol 175: 6908-6915). Krems et al . (1996) Curr. Genet . 29: 327-334) observed that strains lacking the Skn7 gene are extremely sensitive to oxidative stress, indicating a role in the cellular response to this toxicity. This is consistent with the previous results in that stress such as heat shock leads to a specific arrest of yeast cells in the Gl phase of the cell cycle and cells respond to some stresses by altering cell wall structure. Morgan et al . (EMBO J. , 1995) report the involvement of Skn7 in the activation of promoter elements known as SCB and MCB which regulate activation of Gl cyclin gene expression. The authors failed to identify any direct interaction between Skn7 and these promoter elements, and proposed that Skn7 acts upon a novel MCB/SCB activating factor to bring about Gl cyclin expression. Disclosure of the invention. It has now been found that in addition to being sensitive to oxidative stress, strains lacking Skn7 are senstive to acute heat shock and that Skn7 interacts directly with DNA as well as with several other transcription factors. In particular, the present invention has arisen from identification of two genes with key roles in the cellular response to oxidative stress, TRX2 and TRR1, as being directly controlled by Skn7 , with Skn7 binding directly to the promoter. The specific binding site for Skn7 has been found, within a sequence of 23 nucleotides. Skn7 is thus shown to be a transcription factor. It has further been found that Skn7 binds to regulatory elements of heat shock proteins and acts as a regulator of these proteins in response to oxidative stress. Accordingly, in a first aspect the invention provides an assay for a putative inhibitor of fungal growth comprising: a) bringing into contact Skn7 polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding to a Skn7-polypeptide-specific nucleotide sequence; b) providing a nucleic acid molecule which includes a Skn7- polypeptide-specific binding nucleotide sequence to which the polypeptide is capable of binding to activate transcription of a sequence operably linked to the Skn7- polypeptide-specific binding nucleotide sequence; and c) measuring the degree of inhibition of binding of Skn7 to the Skn7-polypeptide-specific binding nucleotide sequence or transcriptional activation caused by said inhibitor compound . It has also been found that the transcription factor Yapl binds to the TRX2 promoter with ablation of either the Skn7 or Yapl genes abolishing the stress-dependent induction of TRX2 and TRR1. While not wishing to be bound by any one particular theory, the evidence suggests that Skn7 and Yapl bind to different sites of the TRX2 and TRR1 promoters, and bring about a conformational change to the promoter which is dependent upon an interaction between the two transcription factors. Thus in a second aspect, the invention provides an assay for a putative inhibitor of fungal growth comprising: a) bringing into contact a Skn7 polypeptide " and a Yapl polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding a Skn7-polypeptide- specific nucleotide sequence and cooperating with Yapl to activate transcription; b) providing a nucleic acid molecule which includes a Skn7- polypeptide-specific binding nucleotide sequence to which the Skn7 polypeptide is capable of binding to activate transcription of a sequence operably linked to the Skn7- polypeptide-specific binding nucleotide sequence; and c) measuring the degree of inhibition of interaction of Skn7 polypeptide and Yapl polypeptide or of Skn7 polypeptide and the Skn7-polypeptide-specific binding nucleotide sequence or transcriptional activation caused by said inhibitor compound. In another aspect of the invention, it has also been found that Skn7 interacts with the yeast Heat Shock Factor, Hsflp (referred to herein as Hsfl) , and that the direct physical interaction between these two proteins provides a maximal response to oxidative stress in yeast cells. Thus in a further aspect of the present invention, there is provided an assay for a putative inhibitor of fungal growth comprising: a) bringing into contact a Skn7 polypeptide, and a Hsfl polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding to the Hsfl polypeptide; and b) measuring the degree of inhibition of binding between the Skn7 and Hsfl polypeptides caused by said inhibitor compound . These and further aspects of the invention are set out in further detail in the accompanying description and examples. Description of the Drawings. Fig.l. Skn7 and Yapl regulate the expression of TRX2 and TRR1 in response to hydrogen peroxide. (A) Quantitation of RNA relative to actin from a Northern blot analysis of RNA isolated from different mid-log yeast strains prior to (tracks 1, 3, 5, 7, 9) and following treatment with lmM hydrogen peroxide for 1 hour at 25°C (tracks 2, 4, 6, 8, 10) . Strains used were W303-la (tracks 1 and 2) , skn7Δ (tracks 3 and 4) , W303-lb (tracks 5 and 6) , yaplΔ (tracks 7 and 8) , and skn7ΔyaplΔ (tracks 9 and 10) . (B) Comparison of potential Yapl binding sites (Kuge and Jones, 1994) with a possible Yapl binding site in the TRR1 promoter. Fig.2. Skn7 is required for induction by tetra-butyl -hydrogen peroxide of LacZ fused to the TRX2 promoter. Mid-log cultures of W303-la, skn7Δ and yaplΔ strains, transformed with the TRXLACZ plasmid (Kuge and Jones, 1994) , were treated with tetra-butyl-hydrogen peroxide for 1 hour, β- galactosidase assays were performed on the untreated and treated cultures. Fig.3. Skn7 and Yapl regulate the expression of TRX2 and TRR1 in response to diamide. Quantitation of a northern blot analysis of RNA isolated from different mid-log yeast strains prior to (tracks 1, 3, 5, 7, 9) and following treatment with 1.5mM diamide for 1 hour at 25°C (tracks 2, 4, 6, 8, 10) . Strains used were W303-la (tracks 1 and 2) , skn7Δ (tracks 3 and 4) , W303-lb (tracks 5 and 6) , yaplΔ (tracks 7 and 8) , and skn7ΔyaplΔ (tracks 9 and 10) . The panels below the data show quantitation of the RNA levels relative to the actin transcript . Fig.4. Skn7 and Yapl bind the TRX2 promoter directly. Crude extracts prepared from W303-la (track 1) , skn7Δ (track 2) , yaplΔ (track 3) , skn7ΔyaplΔ (track 4) and skn7ΔyaplΔ transformed with pBAMl (track 5) were analysed by EMSA using the probe TrxΔl (Materials and Methods) . Bands which are sensitive to the presence or absence of the Skn7 and Yapl proteins are indicated as 1-4. Fig.5. Skn7 binding region in the TRX2 promoter. Schematic diagram of the TRX2 promoter and the probes used to identify the Skn7 binding. Near match MCB and SCB elements are indicated. Probes are described in the Materials and Methods. Fig.6. Alignment of Skn7 DNA binding domain, amino acids 87-150 (SEQ ID NO: 18) with DNA binding domains of S . cerevisiae Hsflp (SEQ ID N0.19), the fisson yeast Heat Shock Factor (Hsflp sp, SEQ ID NO: 20), and the human Heat Shock Factor 2 (Hsf2hs, SEQ ID NO: 21) Highly conserved residues which may directly contact the DNA and have diverged in Skn7 are indicated by * . Fig.7. Map of Skn7 showing the HSF DNA binding domain, the potential coiled-coil structure, the homology to the bacterial response regulator domain and a region rich in gluatmine that is commonly found in eukaryotic transcription factors. Detailed Description of the Invention. In various aspects the present invention is concerned with the provision of assays, in particular assays for substances which inhibit interaction between Skn7 polypeptide and the specific nucleotide sequence motif it binds to. Further assays are for substances which inhibit interaction between Skn7 and Yapl polypeptides or Skn7 and Hsfl polypeptides. Substances which inhibit either or both of these interactions may be used in inhibit the response of a fungal cell to stress, and are thus anti -fungal agents. In a further aspect, the invention provides compounds obtainable by the above-described assays, for example peptide compounds or other small molecules. Peptide compounds may be based on the portions of Skn7 polypeptide which interact with the Yapl or Hsfl polypeptides, or the corresponding portions of said polypeptides which interact with each Skn7. Similarly, molecules identified using an assay according to the present invention as being able to inhibit interaction between Skn7 polypeptide and Yapl polypeptide and/or Skn7 polypeptide and its specific polypeptide binding nucleotide sequence, e.g. where the Skn7 and Yapl are the S . cerevisiae polypeptides (or fragments thereof) , may be used in attacking any of a wide range of fungi, including ascomycete fungi and the others mentioned herein, such as, for example, fungi responsible for conditions such as Candidiasis, Farmers' Lung, Cyrptococcosis and opportunistic fungal infections, e.g. as are prevalent in immuno-compromised individuals, such as transplant patients and AIDS sufferers,. Defini tions . 1.- Skn7 Polypeptide The term "Skn7 polypeptide" includes a polypeptide including the amino acid sequence for Saccharomyces cerevisiae wild-type Skn7 shown in Brown et al . , 1993, and Morgan et al . , 1995b and variants thereof (which may be naturally occurring or synthetic) , as discussed below, in particular showing a characteristic of S . cerevisiae Skn7 polypeptide, such as binding to the specific nucleotide sequence motif bound by S . cerevisiae Skn7, discussed below, or a variant thereof, transcription factor activity, particularly ability to activate transcription in cooperation with Yapl polypeptide, and/or a role in activation of a cellular response to stress, such as oxidative stress and/or heat shock. Preferred Skn7 polypeptides have a DNA binding domain which includes a sequence of amino acids with at least about 60%, or 70%, or 80%, or 90%, or 95% identity with the following amino acid sequence, which is 80% identical in the homologue/analogue in the fission yeast S.pombe (as we have determined by comparison of the S . cerevisiae Skn7 polypeptide sequence with sequence information available to the public in the Fission Yeast Genome Sequencing Project, Sanger Centre, Cambridge, UK): LPNHFKHSNFASFVRQLNKYDFHKV (SEQ ID NO:l). Variants include homologues and analogues which are likely to exist in all fungi, as evidenced by the very close structural identity, close sequence identity in the DNA binding domain, and high level of homology of the homologues/analogues in S. pombe and S . cerevisiae, which yeast are not closely related. (The S.pombe protein has the Hsfl-related DNA binding domain found in Skn7, a receiver domain and also a coiled-coil region and a glutamine-rich domain that occur in Skn7. ) Accordingly, the present invention may be applied to any fungus, including pathogens such as Cryptococcus neoformans, Candida albicans and Aspergillus and others including those mentioned above. Part of the Candida albicans Skn7 is available on the Internet published by the Stanford DNA Sequence and Technology Centre, at http://candida.stanford.edu. This confirms that the skn7 sequence of Candida is conserved with that of S .pombe and S. cerivisiae, in that there is greater than 75% identity in the region between amino acid residues 377 to 456 of the C. albicans and S . cerivisiae sequences. The homologue or analogue of Skn7 of any of these organisms may be used in the present invention. 2.- Yapl Polypeptide. The term "Yapl polypeptide" includes a polypeptide including the amino acid sequence for Saccharomyces cerevisiae wild-type Yapl shown in Moye-Rowley et al . (1989) Genes and Dev. 3: 283- 292, and variants, thereof (which may be synthetic or naturally occurring) , as discussed below, in particular showing a characteristic of S . cerevisiae Yapl polypeptide, such as binding to its specific nucleotide sequence motif, or a variant thereof, ability to activate transcription in cooperation with Skn7 polypeptide, and/or a role in activation of a cellular response to stress, such as oxidative stress and/or heat shock. Preferred Yapl polypeptides may include a DNA binding domain with at least about 50%, 60%, 70%, 80%, 85%, 88%, 90% or 95% identity with that of S . cerevisiae Yapl. Papl, the analogue of Yapl in fission yeast, has in its DNA binding domain a region of 88% identity with the S . cerevisiae Yapl and 50% identity over the 50 C-terminal amino acids (Toda et al . , (1991) Genes Dev. 5: 60-73) . 3.- Hsfl polypeptide. The term "Hsfl polypeptide" includes a polypeptide including the amino acid sequence for Saccharomyces cerevisiae wild-type Hsfl shown in Wiederrecht et al , 1988 and variants thereof (which may be synthetic or naturally occurring) , as discussed below, in particular showing a characteristic of S . cerevisiae Hsfl polypeptide, such as binding to the Skn7 ' polypeptide and/or cooperating with it in the induction of heat shock gene expression in response to oxidative stress. Preferred Hsfl polypeptides may include a DNA binding domain with at least about 50%, 60%, 70%, 80%, 85%, 88%, 90% or 95% identity with that of S. cerevisiae Hsfl. This DNA binding domain is shown in Figure 6. 4. - Variants Variants of the above-described polypeptides may be mutants, such as temperature sensitive mutants, alleles such as sequence variants of the proteins described above from S . cerevisiae which demonstrate a substantially similar phenotype, homologues and analogues, such as found in other species, or synthetic variants and derivatives, which retain the functions of the above described proteins to the extent necessary for the paricular assay format being utilised. Those of skill in the art will appreciate that not all the functional features of the wild-type polypeptides will be required in all the assay formats described herein. Thus instead of using the wild-type Skn7 polypeptide, Yapl polypeptide or Hsfl polypeptide employed in various aspects and embodiments of the present invention may include an amino acid sequence which differs by one or more amino acid residues from the wild-type amino acid sequence, by one or more (e.g. from 1 to 20, such as 2 , 3, 5 or 10) of addition, insertion, deletion and substitution, preferably substitution, of one or more amino acids. Thus, variants, derivatives, alleles, mutants and homologues, e.g. from other organisms, are included. Preferably, the amino acid sequence of a variant shares homology with the S. cerevisiae Skn7, Yapl or Hsfl sequences, as the case may be. Preferably the homology is a degree of amino acid identity of at least about 30%, or 40%, or 50%, or 60%, or 70%, or 80%, or more preferably at least about 90% or 95% identity. Variants also include fragments of wild type or variant Skn7 , Yapl and Hsfl proteins, since it is not necessary to use the entire proteins for assays of the invention. Fragments may be any suitable size, for example from 20 to 300 amino acids, for example from 100 to 200 amino acids. Fragments may be generated and used in any suitable way known to those of skill in the art. Suitable ways of generating fragments include, but are not limited to, recombinant expression of a fragment from encoding DNA. Such fragments may be generated by taking encoding DNA, identifying suitable restriction enzyme recognition sites either side of the portion to be expressed, and cutting out said portion from the DNA. The portion may then be operably linked to a suitable promoter in a standard commercially available expression system. Another recombinant approach is to amplify the relevant portion of the DNA with suitable PCR primers. Small fragments (up to about 20 or 30 amino acids) may also be generated using peptide synthesis methods which are well known in the art. The ability of suitable fragments of Skn7 polypeptide to bind to Yapl or Hsfl polypeptides (or fragment thereof) , or suitable fragments of Yapl or Hsfl polypeptides to bind to Skn7 polypeptide (or fragment thereof) , may be tested using routine procedures such as those illustrated in the accompanying examples . Fragments include those which are an "active portion", which means a peptide which is less than said full length polypeptide, but which retains biological activity. In context, for Skn7 polypeptide this is the ability to interact with Yapl or Hsfl polypeptides and/or the specific nucleotide sequence, and for Yapl and Hsfl polypeptides the ability to interact with Skn7 polypeptide. Such portions may be used to interfere with interaction between Skn7 polypeptide and Yapl or Hsfl polypeptides and/or Skn7 polypeptide binding to its specific nucleic acid motif, with anti-fungal potential. The Skn7 , Yapl .and Hsfl polypeptides and their variants, including fragments, may also comprise additional sequences, usually located at the N- and/or C-termini, which are useful for the provision of the assays described herein. For example, the additional sequences may comprise a polyhistidine or epitope (e.g. HA) tag to serve as a tag for pull -down or similar assays. The additional sequences may alternatively be a functional domain, such as a domain for use in a two-hybrid assay. As another alternative, the domain may be a marker domain, such as a beta-galactosidase, chloramphenicol acetyl transferase, luciferase or green fluorescent protein sequence. 5.- Amino acid sequence homology and identity. As is well -understood, homology at the amino acid level is generally in terms of amino acid similarity or identity. Similarity allows for "conservative variation", i.e. substitution of one hydrophobic residue such as isoleucine, valine, leucine or methionine for another, or the substitution of one polar residue for another, such as arginine for lysine, glutamic for aspartic acid, or glutamine for asparagine . Similarity may be as defined and determined by the TBLASTN program, of Altschul et al . (1990) J " . Mol . Biol . 215: 403-10, which is in standard use in the art. This program may also be used to determine DNA homology. Amino acid identity may also be determined by reference to the Smith-Waterman algorithm, currently used by the United States Patent and Trademark Office. Homology, i.e. identity, may be over the full-length of the relevant polypeptide or may more preferably be over a contiguous sequence of about 15, 20, 25, 30, 40, 50 or more amino acids, compared with the relevant wild-type amino acid sequence . Assay " formats . The identification of the Skn7 interactions described herein gives rise to the various assays of the invention. Those of skill in the art will be able to provide any number of different assay formats which are based upon these interactions, depending upon a number of circumstances, such as cost, convenience and objectives. For example, some assay formats will be more suited to high throughput methods which are generally designed to be used for primary screens for putative inhibitor compounds. Preferred putative inhibitors identified in such a way may then be subject to other assay formats, for example in vivo formats. Although the relevant polypeptide may be provided in free from it may also be used in the form of a fusion protein linked to a marker or reporter protein. Assays of the invention maybe conducted in the following ways, which are provided by way of illustration and are not limiting: 1. Two hybrid assays. One assay format which is widely used in the art to study the interaction of two proteins is a two-hybrid assay. This assay may be adapted for use in the present invention. A two-hybrid assay comprises the expression in a host cell of the the two proteins, one being a fusion protein comprising a DNA binding domain (DBD) , such as the yeast GAL4 binding domain, and the other being a fusion protein comprising an activation domain, such as that from GAL4 or VP16. In such a case the host cell will carry a reporter gene construct with a promoter comprising a DNA binding element compatible with the DBD. The reporter gene may be a reporter gene such as chloramphenical acetyl transferase, luciferase, green fluorescent protein (GFP) and β- galactosidase, with luciferase being particularly preferred. Two-hybrid assays may be in accordance with those disclosed by Fields and Song, 1989, Nature 340; 245-246. In such an assay the DNA binding domain (DBD) and the transcriptional activation domain (TAD) of the yeast GAL4 transcription factor are fused to the first and second molecules respectively whose interaction is to be investigated. A functional GAL4 transcription factor is restored only when two molecules of interest interact. Thus, interaction of the molecules may be measured by the use of a reporter gene operably linked to a GAL4 DNA binding site which is capable of activating transcription of said reporter gene. Thus two hybrid assays may be performed in the presence of a potential modulator compound and the effect of the modulator will be reflected in the change in transcription level of the reporter gene construct compared to the transcription level in the absence of a modulator. Host cells in which the two-hybrid assay may be conducted include mammalian, insect, yeast and bacterial cells, with mammalian and yeast cells (such as S. cerivisiae and S .pombe) being particularly preferred. In the case of the present invention, the Skn7 or Yapl polypeptide may be fused to a heterologous DNA binding domain such as that of the yeast transcription factor GAL 4. The GAL 4 transcription factor includes two functional domains. These domains are the DNA binding domain (DBD) and the transcriptional activation domain (TAD) . By fusing Yapl polypeptide or Skn7 polypeptide to one of those domains and the respective counterpart, i.e. Skn7 polypeptide or Yapl polypeptide, to the other domain, a functional GAL 4 transcription factor is restored only when two proteins of interest interact. Thus, interaction of the proteins may be measured by the use of a reporter gene probably linked to a GAL 4 DNA binding site which is capable of activating transcription of said reporter gene. In an alternative embodiment, the Yapl polypeptide may be replaced by the Hsfl polypeptide. 2. Pull-down and Immunoprecipitation assays. The interaction between the Skn7 and Yapl or Hsfl polypeptides may be studied in vi tro in a "pull -down" format by labelling one with a detectable label and bringing it into contact with the other which has been immobilised on a solid support, or carries a tag allowing it to be immobilised. Suitable detectable labels include 35 S-methionine which may be incorporated into recombinantly produced Skn7 and/or Yapl and/or Hsfl polypeptides. The recombinantly produced Skn7 and/or Yapl and/or Hsfl polypeptide may also be expressed as a fusion protein containing an epitope which can be labelled with an antibody. The protein which is, or is to be, immobilized on a solid support may be immobilized using an antibody against that protein bound to a solid support or via other technologies which are known per se . A preferred in vi tro interaction may utilise a fusion protein including glutathione-S-transferase (GST) . This may be immobilized on glutathione agarose beads. In an in vi tro assay format of the type described above the putative inhibitor compound can be assayed by determining its ability to diminish the amount of labelled Skn7 , Yapl or Hsfl polypeptide which binds to the immobilized GST-Yapl or GST-Hsfl polypeptide on the one hand or GST-Skn7 polypeptide on the other, as the case may be. This may be determined by fractionating the glutathione-agarose beads by SDS- polyacrylamide gel electrophoresis. Alternatively, the beads may be rinsed to remove unbound protein and the amount of protein which has bound can be determined by counting the amount of label present in, for example, a suitable scintillation counter. Similarly, such an assay according to the present invention may also take the form of an in vivo assay wherein the interaction is studied in by way of immunoprecipitation of Skn7 or one of Yapl or Hsfl. The amount of the Yapl or Hsfl, or both, in Skn7 immunoprecipitates, or vice versa, may be examined by any suitable means, for example Western blotting of the immunoprecipitate and probing with the appropriate antibody. The in vivo assay may be performed in a cell line such as a yeast strain in which Skn7 polypeptide and/or Yapl or Hsfl polypeptides are expressed from a vector introduced into the cell. Where the assay of the invention relates to the interaction of Skn7 with Yapl, it is preferred that the assay is conducted in the presence of DNA with one or more binding sites for each factor. Where, such an assay is used, the interaction of the two polypeptides may be examined by the use of electrophoretic mobility shift assays, for example as described in the accompanying examples. Similarly, DNA comprising Skn7 and/or Hsfl binding sites may be included in assays which measure the interaction between these two factors. 3. DNA Binding Assay. The binding of Skn7 to a specific nucleotide sequence may be utilised to provide further assays of the invention, by assaying for antagonists of such binding. For example, a nucleotide sequence comprising the specific nucleotide sequence may be immobilised and Skn7 together with a putative inhibitor compound may be brought into contact with the sequence, and the degree of binding measured, for example by labelling the Skn7 and detecting the amout of labelled Skn7 bound to the immobilised sequence. Alternatively, the Skn7 may be immobilised and the amount of congnate sequence, optionally labelled, which binds to it in the presence or absence of inhibitor may be determined. The DNA binding assays of the type described above may be conducted in the presence or absence of an additional factor, particularly Yapl or Hsfl, which interacts with Skn7 in binding to a specific nucleotide sequence. A further way of identifying interaction with of Skn7 with its binding sequence is a bandshift assay, such as the EMSA assay described in the accompanying examples . 4. Transcription assays. In another embodiment, a reporter gene construct including a Skn7-polypeptide-specific binding nucleotide sequence operably linked to a reporter gene may be introduced into an expression system such as a cell or cell free expression system together with an expression vector or vectors capable of expressing Skn7 or one of Yapl or Hsfl. Two or more Skn7 binding sites (for example 3, 4 or 5) may be present in the nucleic acid construct and this may enhance sensitivity of the assay. The expression of the reporter gene may be determined in the presence and absence of the putative inhibitor, such that a reduction in expression of the reporter gene indicates a putative inhibition of the interaction of Skn7 with Yapl or Hsfl, as the case may be. To facilitate cooperation between Skn7 and Yapl polypeptide, one or more Yapl-polypeptide-specific binding nucleotide sequences may be included in the construct . The reporter gene may be any suitable reporter gene used in the art. Such reporter genes include reporter genes mentioned above, for example -galactosidase or luciferase. The expression vector (s) will include DNA encoding Skn7 and/or one of Yapl and Hsfl polypeptide operably linked to a promoter capable of expressing the gene in the host cell . Suitable promoters include yeast promoters such as GAL or ADH promoters . Cell lines. The cell lines used in assays of the invention may be used to achieve transient expression, although in a further aspect of the invention cells which are stably transfected with constructs which express Skn7 polypeptide and, where required, Yapl or Hsfl polypeptide may also be generated. Means to generate stably transformed cell lines are well known in the art and such means may be used here . Where the cell line does not express Yapl polypeptide, a construct capable of expressing this protein may also be introduced into the cell operably linked to a suitable promoter. Systems for cloning and expression of a polypeptide in a variety of different host cells are well known. Suitable host cells include bacteria, mammalian cells and yeast, and baculovirus systems. A common, preferred bacterial host is E. coli . Preferred for performance of aspects of the present invention are yeast cells. Suitable vectors can be chosen or constructed, containing appropriate regulatory sequences, including promoter sequences, terminator fragments, polyadenylation sequences, enhancer sequences, marker genes and other sequences as appropriate. Vectors may be plasmids, viral e.g. 'phage, or phagemid, as appropriate. For further details see, for example, Molecular Cloning: a Laboratory Manual : 2nd edition, Sambrook et al . , 1989, Cold Spring Harbor Laboratory Press. Many known techniques and protocols for manipulation of nucleic acid, for example in preparation of nucleic acid constructs, mutagenesis, sequencing, introduction of DNA into cells and gene expression, and analysis of proteins, are described in detail in Current Protocols in Molecular Biology, Ausubel et al . eds., John Wiley & Sons, 1992. Thus a further aspect of the invention provides cells which comprise an expression construct comprising a Skn7 polypeptide- encoding sequence operably linked to a heterologous promoter, together with an expression construct comprising one of a Yapl or Hsfl polypeptide-encoding sequence operably linked to a heterologous promoter. The two constructs may be present on separate vectors, or the same vector. The cells may further comprise a reporter gene which comprises a promoter capable of being transcriptionally activated by the presence of Skn7 polypeptide when in the presence of one of Yapl and Hsfl polypeptides. As is apparent from the above description, the reporter gene is generally one with an easily assayable expression product, and in any event is one which is heterologous to the promoter. Inhibitor Compounds . The amount of putative inhibitor compound which may be added to an assay of the invention will normally be determined by trial and error depending upon the type of compound used. Typically, from about 0.01. to 100 nM concentrations of putative inhibitor compound may be used, for example from 0.1 to 10 nM. Inhibitor compounds which may be used may be natural or synthetic chemical compounds used in drug screening programmes. Extracts of plants which contain several characterised or uncharacterised components may also be used. Inhibitor compounds may be provided by way of libraries of compounds may by combinatorial chemistry. A further class of putative inhibitor compounds can be derived from Skn7 polypeptide, Yapl polypeptide or Hsfl polypeptide. Peptide fragments of from 5 to 40 amino acids, for example from 6 to 10 amino acids from the region of the relevant polypeptide responsible for interaction between these proteins, or interaction with nucleic acid, may be tested for their ability to disrupt such interaction. Antibodies directed to the site of interaction in either protein form a further class of putative inhibitor compounds. Candidate inhibitor antibodies may be characterised and their binding regions determined to provide single chain antibodies and fragments thereof which are responsible for disrupting the interaction. Antibodies may be obtained using techniques which are standard in the art. Methods of producing antibodies include immunising a mammal (e.g. mouse, rat, rabbit, horse, goat, sheep or monkey) with the protein or a fragment thereof . Antibodies may be obtained from immunised animals using any of a variety of techniques known in the art, and screened, preferably using binding of antibody to antigen of interest. For instance, Western blotting techniques or immunoprecipitation may be used (Armitage et al . , 1992, Nature 357: 80-82). Isolation of antibodies and/or antibody-producing cells from an animal may be accompanied by a step of sacrificing the animal. As an alternative or supplement to immunising a mammal with a peptide, an antibody specific for a protein may be obtained from a recombinantly produced library of expressed immunoglobulin variable domains, e.g. using lambda bacteriophage o.r filamentous bacteriophage which display functional immunoglobulin binding domains on their surfaces; for instance see WO92/01047. The library may be naive, that is constructed from sequences obtained from an organism which has not been immunised with any of the proteins (or fragments) , or may be one constructed using sequences obtained from an organism which has been exposed to the antigen of interest. Antibodies according to the present invention may be modified in a number of ways. Indeed the term "antibody" should be construed as covering any binding substance having a binding domain with the required specificity. Thus the invention covers antibody fragments, derivatives, functional equivalents and homologues of antibodies, including synthetic molecules and molecules whose shape mimicks that of an antibody enabling it to bind an antigen or epitope. Example antibody fragments, capable of binding an antigen or other binding partner are the Fab fragment consisting of the VL, VH, Cl and CHI domains; the Fd fragment consisting of the VH and CHI domains; the Fv fragment consisting of the VL and VH domains of a single arm of an antibody; the dAb fragment which consists of a VH domain; isolated CDR regions and F(ab')2 fragments, a bivalent fragment including two Fab fragments linked by a disulphide bridge at the hinge region. Single chain Fv fragments are also included. A hybridoma producing a monoclonal antibody according to the present invention may be subject to genetic mutation or other changes. It will further be understood by those skilled in the art that a monoclonal antibody can be subjected to the techniques of recombinant DNA technology to produce other antibodies or chimeric molecules which retain the specificity of the original antibody. Such techniques may involve introducing DNA encoding the immunoglobulin variable region, or the complementarity determining regions (CDRs) , of an antibody to the constant regions, or constant regions plus framework regions, of a different immunoglobulin. See, for instance, EP184187A, GB 2188638A or EP-A-0239400. Cloning and expression of chimeric antibodies are described in EP-A-0120694 and EP-A- 0125023. Hybridomas capable of producing antibody with desired binding characteristics are within the scope of the present invention, as are host cells, eukaryotic or prokaryotic, containing nucleic acid encoding antibodies (including antibody fragments) and capable of their expression. The invention also provides methods of production of the antibodies including growing a cell capable of producing the antibody under conditions in which the antibody is produced, and preferably secreted. Other candidate inhibitor compounds may be based on modelling the 3 -dimensional structure of Skn7 polypeptide and/or Yapl polypeptide and using rational drug design to provide potential inhibitor compounds with particular molecular shape, size and charge characteristics. An inhibitor compound identified using the present invention has therapeutic potential . Budding yeast that are deleted for Skn7 are alive but compromised in their response to stress. Invading pathogenic organisms will respond to attack by the host defences through their own defence systems, the intracellular stress response. Neutralising Skn7 function will render the pathogen more sensitive to bodily defences. As noted, the conservation of Skn7 in fungi provides indication that any substance identified with the requisite inhibitor activity will be of therapeutic value against a wide spectrum of fungal pathogens. Furthermore, since Skn7 does not occur in human cells, drugs active against Skn7 function should not harm human cells. Anti-fungal treatment is useful against Candidiasis, Farmers' Lung, Cyrptococcosis and opportunistic fungal infections, e.g. as are prevalent in immuno-compromised individuals, such as transplant patients and AIDS sufferers. Inhibitor compounds may also be used in combination with any other anti-fungal compounds, e.g. azole compounds such as fluconazole. In such a case, the assay of the invention, when conducted in vivo, need not measure the degree of inhibition of binding or transcriptional activation caused by the inhibitor compound being tested. Instead the effect on fungal cell growth and/or viability be measured. It may be that such a modified assay is run in parallel or subsequent to the main assay of the invention in order to confirm that any effect on cell growth and/or viability is as a result of the inhibition of binding or transcriptional activation caused by said inhibitor compound and not merely a general toxic effect. Skn7 binding sequence . A 23 nucleotide sequence within the TRX2 promter has been identifed as a binding sequence for Skn7 polypeptide. This sequence is identifed in Figure 5 at nucleotides -164 to -142 of the TRX2 promoter. This sequence comprises: 5' TTTCCAGCCAGCCGAAAGAGGGA (SEQ ID NO : 2 ) . Within this sequence is the motif CGAAA which has previously been identifed as an SCB element (see examples) . Mutation of the element to ATAAA lowers Skn7 binding 20 -fold (see examples) . A further binding sequence for Skn7 is present in the 26 bp sequence from the HSE2 region of the SSA1 promoter (see examples) . This has the sequence: 5' TGCATTTTCCAGAACGTTCCATCGGC (SEQ ID NO: 3) Experimental evidence provided below shows that particular alterations of these sequences abolish Skn7 binding. The minimal sequence to which Skn7 binds may be identified by using synthetic oligonucleotides of these regions as competitor DNA in a gel mobility shift assay. Systematic mutation of the nucleotide sequences will define key residues. In another approach, randomly generated oligonucleotides may be passed over immobilised, pure Skn7 protein. Elution " of bound oligonucleotides followed by DNA sequencing will directly reveal the sequences . The invention thus provides in a further aspect an oligonucleotide which consists essentially the sequences SEQ ID NO: 2 and SEQ ID NO: 3, or has a sequence which is a mutant, variant, derivative or homologue sequence by way of addition, deletion, substitution and/or insertion of one or more base pairs, which retains Skn7 binding, preferably a shorter sequence than these 23 and 26 base pair sequences shown. Such a variant, mutant, derivative or homologue may have at least about 50%, 60%, 70%, 80%, 90% or 95% homology with the sequences shown. In particular, the core triplet GAA appears to be a potential core recognition sequence. While not wishing to be bound by any one particular theory, preferred oligonucleotides of the invention comprise derivative of SEQ ID NO: 2 or SEQ ID NO: 3 which comprise this core, are at least 15, preferably 18, nucleotides in length, and are at least 75% homologous and preferably no more than three, more preferably no more than two nucleotides different from either of the above sequences when aligned to the GAA core. The invention also provides a promoter construct, able to activate transcription of an operably linked sequence, including SEQ ID NO: 2 or SEQ ID NO: 3 or a said mutant, variant, derivative or homologue thereof, other than a naturally occurring promoter sequence. Thus, an Skn7-binding site may be used in construction of a promoter that contains one or more other regulatory motifs, transcription factor elements, and promoter elements to produce a promoter which contains a heterologous Skn7-polypeptide-binding site. The invention further provides a nucleic acid construct which ω U t to μ> LΠ O LΠ o LΠ O LΠ > d ft Ω μ- tr Hi CD *d rt Ό £1 μ- cr SD 3 ■ d CO 25 rt O tr 3 ft ft rt to Ό CO SD H LQ Φ μ- ri Φ tr a X 0 *d ii 77 i a a μ- Ω SD 0 77 a tr ii Φ 0 ό 0 0 ii X ϋ ?r tr Φ X 0 rt SD cr i Φ 0 Φ 0 SD μ- a a rt a ^ μ^ a Ω Φ 0 *d a ≤ SD rt a ft φ a S Ω ft Φ μ- SD H, Ω ft ft a rt ft μ- a 3 ii a μ- Φ μ- (i SD u rt μ- μ- < cr a -0 3 μ- φ Φ tr a φ a = SD -0 φ O 0 - a Ω i CQ CQ Ό rt o μ- • ^ ϋ Ω 3 CO ft X Hi Ω 3 Ω μ- SD a μ- Φ Φ O μ ii rt μ- C SD φ rt Ω ii μ- Ό μ a ft rt - ri Ό μ- SD μ- φ 0 Φ Hi rt CQ rt •d o Ω 0 Φ rt μ r 1 i i 0 <J o μ- <; Φ φ μ- a φ φ CQ 3 Ω ft Si tf μ- φ . X Φ rt 3 ri μ- φ CQ Φ μ- 3 Φ LQ Φ CQ 0 SD SD Ω φ O a SD tr rt cr tr rt 0 3 μ- Ό ft a SD 0 ϋ ~ X rt SD T3 o Ω rt μ- rt ϋ 3 rt rt μ- 3 0 a SD a rt Ω μ- a rt CQ • μ- rt Hi S φ 2 X > <; H =; Hi Φ Φ μ- Φ co Φ CO SD μ- Φ a Φ a μ- tr ϋ 0 μ- μ- μ- *2z 0 Φ SD rrj 0 t •d ft i ft si μ- a ,-- 0 i tr O Ω 0 Φ Ω a 0 Ω rt si > rr Hi rt a ii rt X tr rt • = μ- a ft μ- a μ- rt a rt tr a 77 CD H ii Hi CQ 0 tr Φ μ- μ- Ω ϋ Hi rt 77 φ . tr μ- Φ φ 0 SD μ- > d SD O SD 0 Ω 3 Φ ft a ft o a ii SD φ ft φ 3 μ- μ- ft o r~h . ii SD Hi 3 rt a ϋ a i a ii i 0 ft Φ a ft 0 cr ft • SD CQ rt Φ rt a , 0 •d tr Q Φ CQ 0 CD μ- rt O μ- μ- CQ Φ 3 = D X 0 - μ- i μ •d O CJ Φ CQ Ω rt Ω μ- O Φ ii a a cr rt ii X 25 μ- 3 ii ft rt 0 ii H CQ μ- D φ ϋ Φ ii a rt 0 CQ μ- ii rt 3 > a tr φ Hi Φ tr CQ o 0 Φ Hi 3 d a μ- μ- μ- CQ μ- 3 CQ a si rt tr 5 Φ Φ SD Ω 0 CD φ 0 3 3 μ- 5) Φ rt •d a O rt 0 SD 0 Ω 3 ft Ω ϋ Φ 0 SD • rt a 0 H i a o 0 0 Ω z Ω rt rt SD a Ω rt ii 0 μ- rt SD ω a tr μ- rt 3 μ- O rt rt Hi SD > μ- μ- μ- >d μ- a rt Φ φ rt a Q a Ό μ- CQ φ a >d Φ a *d Φ Φ rt Hi a 0 ii 0 Ω μ- i φ μ- CQ CQ ii rt μ- SD o X i 0 ii ri SD μ- SD μ- a 0 a φ < a Hi • > Ω Ω 0 μ- LO rt a SD SD 0 i 0 a Ω SD ft H μ- cr μ- 0 ii 3 0 0 - μ- CO a 3 a <! O SD ri a ft a Hi a Hi tr tr rt • - a tr μ- 0 a μ- SD Φ ft O μ- Hi d . Ω Φ 0 ii Ω H X Φ X Q SD X •d 5 rt Φ a ft rt Λ SD ft 0 Φ rt •d ri s- φ 0 rt 0 =; μ- a ϋ rt Φ ft φ μ- Φ si Φ CD Φ ii ii μ- 0 φ tr μ- K 3 3 SD a Hi ft < μ- μ- ϋ ft i ft φ CD CO Φ SD <! ii SD μ- CQ V •d CQ Φ μ- rt o μ- CQ 0 . SD Φ a μ- SD ' μ- q μ- rt Ω Ω 0 i rt ii rt CQ <! CQ Φ M SD li μ- a a SD Ω 0 Ω a X rt CQ μj rt Φ rt tr a Φ tr 0 tr Φ φ ii rt a SD ft CQ rt Hi Φ tr a , X ri μ- si CD Φ ft Φ CQ ϋ Hi SD tr a CQ d μ- rt cr Φ 0 SD Ω CD a CQ CQ φ Φ Φ 5 0 Ό 0 D o tr 0 CQ cr LQ a i μ- V Ω O 3 0 Φ ri CQ rt SD μ- i SD Φ a 25 Hi Φ C Φ μ- ^ Φ Φ 0 ii Φ i Φ Hi ft Φ si Φ o a μ- i > ft D Λ ti a CQ 0 a 0 ft o a a cr H Hi O μ- 0 φ rt 0 a Ό φ Φ c X i φ si μ< rt 3 3 rt O rt Ω CQ 0 ii rt O a a O a ii rt φ φ Φ Ω SD μ- CQ 0 cr 0 i O Φ rt μ- 0 μ- Φ rt 0 O Ω Φ Φ 0 a Hi Hi t tr tr ft *<: rt • ; rt O 0 SD O a 3 SD H Φ Hi rt f Φ CQ ii Hi Ω φ SD Φ Φ CQ Φ Φ Hi 3 i ≤ a CQ Ω 0 rt SD ft tr Φ Φ 3 Φ rt ri Ω SD CD rt i rt i 0 Φ μ- Ω 0 rt μ- cr rt Φ SD 0 a μ- rt 0 Φ μ- CQ Φ • ϋ rt rt Hi rt φ ϋ si Φ 0 Hi tr q rt Ω ts tr CQ a Φ ≤ 3 SD 0 t Φ Φ tr CQ ft ri a X o Φ CQ μ- Φ φ Φ c SD Ω μ- rt ft tr a ii Φ i H SD μ- i Φ X ft Ω Φ rt 77 μ- 3 ] CD O tr CQ a ri i CO a Φ SD rt SD tr CQ SD φ 0 Ω SD tr tr SD SD rt Φ CQ φ Φ μ- rt SD CQ CQ ts tr σ φ rt rt r ' X Φ SD X 3 Ω 0 a φ a CQ Q a ϋ 3 φ μ- ft φ 0 SD iP 0 Φ Ό rt 0 rt -j Φ SD rq ϋ rV SD Φ SD a Hi <; CQ c CO O 3 SD cr SD μ- Si μ- SD 1 rt 0 Φ 0 ,-- CO a SD a CQ 77 i SD q Φ φ •d SD i Φ 3 0 CQ a < cr >d ft CO a rt Φ 0 σ μ- SD US ri 1 ^ φ 0 a CQ rt μ- μ- 0 ii rV CQ rt Ω a i ft 3 CQ a Ω Ω 0 ft a Φ rt 3 Φ a μ- μ- rt i Ω SD Φ ft 0 h{ Φ rt φ cr SD Φ a 0 CQ O X μ- O •d 1 <: O 0 μ- a s: s μ- a 1 Φ Φ i Φ tr rt rt Hi CQ Hi rt ft 0 1 φ a O φ ft ^ Ω a O O μ- SD Φ 3 μ- X Φ si ii = SD rt μ- μ- Φ LQ rt 0 Hi a CQ C Φ i 0 Φ 3 CQ *d rt rt μ- Ω 0 Ω ii ri 0 φ CQ 3 Hi a rt rt SD Φ 0 0 0 Hi tr 0 0 φ rt μ- rt 0 μ- rt i Hi a SD Hi a rt i Hi Ω φ ft 0 a SD > ft Φ Ω rt i φ 0 φ rt μ- SD SD CQ ft Hi O ft Generally, the gene may be transcribed into mRNA which may be translated into a peptide or polypeptide product which may be detected and preferably quantitated following expression. A gene whose encoded product may be assayed following expression is termed a "reporter gene", i.e. a gene which "reports" on promoter activity. A reporter gene preferably encodes an enzyme which catalyses a reaction which produces a detectable signal, preferably a visually detectable signal, such as a coloured product. Many examples are known, including -galactosidase and luciferase. jS-galactosidase activity may be assayed by production of blue colour on substrate, the assay being by eye or by use of a spectrophotometer to measure absorbance . Fluorescence, for example that produced as a result of luciferase activity, may be quantitated using a spectrophotometer. Radioactive assays may be used, for instance using chloramphenicol acetyl - transferase, which may also be used in non-radioactive assays. The presence and/or amount of gene product resulting from expression from the reporter gene may be determined using a molecule able to bind the product, such as an antibody or fragment thereof. The binding molecule may be labelled directly or indirectly using any standard technique. Those skilled in the art are well aware of a multitude of possible reporter genes and assay techniques which may be used to determine gene activity. Any suitable reporter/assay may be used and it should be appreciated that no particular choice is essential to or a limitation of the present invention. For therapeutic purposes, e.g. for treatment of a yeast or other fungal infection a substance able to down-regulate expression of the promoter may be sought . A method of screening for ability of a substance to modulate activity of a promoter may comprise contacting an expression system, such as a host cell, containing a nucleic acid construct as herein disclosed with a test or candidate substance and determining expression of the heterologous gene. co > to t μ 1 μ 1 Lπ o LΠ o LΠ o LΠ SD SD Ω rt φ Ω 0 tr Ό 3 rt ι-3 μ] CQ K) Hi SD ^ rr Φ Ω •d Ω CQ SD > CQ Ki CQ 3 0 q 3 X ft ti 0 ii 0 ii Φ 0 0 0 tr tr a ii a Hi 0 W 0 0 Φ μ- a a i Φ 0 H tr SD tr 3 ft 3 Φ Q 3 i ft si φ Ω Φ ϋ Hi Φ Ό SD a a X Ki σ 0 a ft Φ X Φ μ- •d SD Ki 0 Φ - CQ 3 Hi μ- tr a a a SD CO CQ tr Ki rt Φ i Ki rr Ki 0 Φ ϋ CQ 3 Φ a q X SD 0 q ϋ rt μ- - φ SD tr Ω 0 Ω Φ 0 Φ 0 3 q 0 q 0 tr q q μ φ SD CO SD ii Φ rt ≤ Φ CQ a CO a φ Ω Φ 3 SD q Ω SD Φ Φ Φ Φ ts 3 μ- Φ 0 co Φ •d rt a rt 3 CQ SD ϋ CQ μ- CQ a a • 0 Ω 0 a Φ 0 q CQ <! Q Φ q a ii μ- K rt 0 tr tr SD rt • a μ- ft q ft a tr q Ω i 3 Φ q q Ω Φ rt μ- q tl tr μ- ii O φ X SD μ- •d Q CQ 0 CD 0 CO q a Φ φ 'd Φ 0 CO tr 0 - SD CQ Ω a ft rt 0 ii a • μ] SD μ- ii SD D CQ CQ CO 3 si 0 ti a 0 rt Φ 0 SD >d cr 3 a ^ 0 μ- s- Hi Hi tr μ- & Hi CQ ii Φ q a Ki 0 Ω ft si rt SD 3 Hi a i Φ φ ~ si 3 ft μ- SD SD 0 Φ a a Ω 0 φ a q SD Hi tr Hi O μ- ft Ό SD cr Ω φ ft ii 0 φ a >-3 Ω ϋ μ- Φ 0 ti ft ft Φ CQ CQ ri 0 0 φ 1 3 0 μ- rq Φ O SD μ- μ- rt rt a SD tr μ- Ω a a μ- a q φ Φ SD Hi ri Hi μ- CQ CO a ii φ ft Ω • tr Φ q Φ < Φ CQ Ki CQ 3 CQ s; Φ ■ q SD ft X 0 a P μ- 0 ft 0 μ- a 3 SD Φ φ ti μ- q- X μ- SD q 0 Ω μ- X ' O a Ki CD 3 SD Hi a μ- rt μ- 3 ft q μ- 3 • ii Hi Φ CD Ω q ii q Φ M ft q Ki q Ω c ii a SD ft CQ CQ μ- SD a 0 φ a Φ 3 SD μ- Φ φ SD X CQ tr a a 0 tr i SD φ μ- Φ tr 77 3 CQ SD rt 0 CQ rt a μ- μ- a 3 0 Ω Ω ft q μ- φ μ- Ω CQ Φ a • q CO CQ μ- μ- a ii a Ki Φ rt a co rt SD ti rt SD X φ a 3 0 q SD φ Φ CD Ω tr CQ rt t! tJ Ω SD tr ii μ- <! rt a Φ μ- rt CO CD CQ SD ϋ a q ft X CQ φ μ- SD CQ μ- tr ft rt Ω SD μ- Hi Φ φ a - < μ- φ rr X φ CO 3 μ- K> μ- > rr 0 ti O ii μ- 0 ϋ a CD μ- a ii Ki Hi μ- 0 ft •fl 3 tr rr *d SD 0 tr H 0 μ- tr a Ω SD rt SD φ 0 3 3 Ω φ rt Φ tr SD μ- rt a 0 SD Φ μ- q o X ^ a φ Φ a a ft Φ φ ii SD a Ki SD SD rt ft μ- ft SD Ω rt X rt ϋ X 3 Φ ϋ i CQ • ft μ- μ- •d SD CQ rt ii Ω Ω μ- 0 i rt ^ 0 tr SD φ q ft tr o φ CQ μ- Hl a ≤, tr rt a 3 0 μ- φ Ω <! a a q 3 ϋ 3 Hi φ CO rr a CQ CQ Φ Φ Φ Hi μ- μ- Ω Hi Φ μ- SD μ- cr Φ Hi co a 0 μ- CD 0 SD ii SD rt a 0 tr SD • i ii CQ a 0 > SD φ <; rr q ii 0 CD a μ- q ii rt μ- Φ Ω Φ X tr SD 3 φ 0 Ω μ- q a a q H Φ tr tr 3 a rt rt CQ a μ- ft X a X μ- Φ Φ SD Φ a X Ξ Ω i a tr μ- Φ φ φ SD SD ^ a CQ Ω SD CQ rt a a 0 tr CQ a CO H a 0 μ- q Φ a 0 μ- CQ a SD Ω ti Ω SD a cr Φ ft q ti Φ Q a SD ϋ σ μ- tr a q i ft Hi a Ω 0 Ki Φ Φ Ω ϊ=; tr CD μ-> Ω X SD a μ- μ- a tr Φ Φ CQ Φ SD CQ φ 0 Ki μ- Ω SD φ Hi j Ό a φ tr a Φ ft <! Ω Hi 3 tr CQ X Ki a q ft ft ii Ω SD i q Φ tr rt CQ μ- SD Ω Ω μ- a CQ μ- SD 0 SD CO rt Ul 0 s; 0 Ω ft ϋ a 0 SD a Φ μ- μ- Φ O SD μ- μ- Ω tr 0 ≤ a a ft ϋ a rt SD Φ h (D Hi Φ a q Ω 3 q 0 SD a X Φ i Ω a tr Ω 3 μ- q Ω μ- a 3 a SD a ft rr CO μ- Ω 0 Φ 0 Φ q CQ μ- q a 3 (D Hi 0 SD V q Φ a SD Ω a Hi a Ω • Φ tr a q ft q CQ tr Φ q φ i Ω SD 0 3 3 3 0 tr n Φ 0 SD Ω φ ii Φ SD CO Ω Ki Φ Φ ^ X Φ Φ Ω ϋ SD SD *d CO CQ Hi μ- <! CQ 3 SD Ω φ CO ft ii a μ- ii μ- ti CQ i μ- K CD Φ Ω a ^ 0 a μ- Φ Ω SD • Ki rt rt ≤ - CQ 3 q a 0 a • Ki a 0 H CD 0 a 0 SD a CQ cr q tr ii 0 μ- Si 3 tr SD Φ SD SD q ft q φ CQ Hi Φ μ- Hi rt 3 a Hi μ- μ- D μ- SD Φ SD μ- CQ 0 i SD μ- CQ a X q Φ a 0 Ω Φ Φ CO o μ- r§ rt SD a rt rt 0 rt a Ω 0 μ- a Φ X Ω a ft a Φ CQ Ω μ- a X q CO a q Ω 0 μ- Ω Ω μ- SD a Ω μ- a rr ^ ft tr Ω a q ft H Φ SD Hl Φ •q tr μ- tr SD CQ 1 rt 0 a μ- φ ft CQ μ- q tr rt 3 Φ SD μ- ii Φ 0 μ- φ μ- Hi a a a Ω CQ 0 0 SD Φ 3 tr q £ q SD Ω Ω a Φ a a q a ϋ ft φ 3 s; 3 SD a Hi a o SD Φ Φ CQ μ- Φ Φ μ- 0 CQ CD q X μ- a φ φ rt ^ Φ ft μ- O CQ ft μ- ft CQ ϋ ii q ft CQ μ j q q CQ a μ- q Φ 0 LQ 0 ft μ- rt Ki 0 SD \ a a i O O tr q X μ- q a tr CQ SD a SD 0 Ό (D μ- CQ tr Φ Φ i 0 <! φ s; μ- (D 0 CQ Φ q Ω Hi ii SD Ω Ω Ω Ω Ω ii Φ SD a Ki 0 a q a q 0 μ- a q q Ω Ω φ 3 (D O a q ft 0 CO rt Ω 0 Hi Φ q μ- Hi a SD tr SD CQ Φ 0 rt SD <J Ki Φ 3 0 77 CD ii 3 a rt Φ μ- ii μ- CO O φ a 77 0 a φ , q a Ki 3 ii φ SD rt φ CQ a φ a Ki CQ μ- CQ Hl q a q φ q φ Hi Ω CQ q rt •5 Φ a rt tr a Φ ~0 a CQ 0 Φ Q φ Φ q Ω tr ft Φ Ki φ Q SD i SD rr μ- 0 rt ft SD o • CQ ft SD 0 i i VD a" Φ a Φ q i a q tr μ- rt SD Φ ft - CQ rt μ- rt i SD Φ CO q tr q Φ Ω (D CQ 3 rt a rt μ- Φ CO CQ μ- 0 Φ 0 CQ Φ a φ μ- φ μ- rt ft SD 0 μ- a ft φ Φ a Φ Ω a a ϋ a 0 a Φ CQ SD a 0 φ CQ rt φ a μ j a Φ Ω Hi CQ X φ excipient, vehicle or carrier, and optionally other ingredients . Also encompassed within the scope of the present invention are functional mimetics of peptide fragments of Skn7 polypeptide or Yapl or Hsfl polypeptides which interfere with interaction between these polypeptides or with binding of Skn7 polypeptide to its specific nucleic acid motif. The term "functional mimetic" means a substance which may not contain an active portion of the relevant amino acid sequence, and probably is not a peptide at all, but which retains the relevant interfering activity. The design and screening of candidate mimetics is described in detail below. A substance identified using the present invention may be peptide or non-peptide in nature. Non-peptide "small molecules" are often preferred for many in vivo pharmaceutical uses. Accordingly, a mimetic or mimick of the substance (particularly if a peptide) may be designed for pharmaceutical use. The designing of mimetics to a known pharmaceutically active compound is a known approach to the development of pharmaceuticals based on a "lead" compound. This might be desirable where the active compound is difficult or expensive to synthesise or where it is unsuitable for a particular method of administration, e.g. peptides are not well suited as active agents for oral compositions as they tend to be quickly degraded by proteases in the alimentary canal. Mimetic design, synthesis and testing may be used to avoid randomly screening large number of molecules for a target property. There are several steps commonly taken in the design of a mimetic from a compound having a given target property. Firstly, the particular parts of the compound that are critical and/or important in determining the target property are determined. In the case of a peptide, this can be done by systematically varying the amino acid residues in the peptide, e.g. by substituting each residue in turn. These parts or residues constituting the active region of the compound are known as its "pharmacophore" . Once the pharmacophore has been found, its structure is modelled to according its physical properties, e.g. stereochemistry, bonding, size and/or charge, using data from a range of sources, e.g. spectroscopic techniques, X-ray diffraction data and NMR. Computational analysis, similarity mapping (which models the charge and/or volume of a pharmacophore, rather than the bonding between atoms) and other techniques can be used in this modelling process. In a variant of this approach, the three-dimensional structure of the ligand and its binding partner are modelled. This can be especially useful where the ligand and/or binding partner change conformation on binding, allowing the model to take account of this the design of the mimetic. A template molecule is then selected onto which chemical groups which mimic the pharmacophore can be grafted. The template molecule and the chemical groups grafted on to it can conveniently be selected so that the mimetic is easy to synthesise, is likely to be pharmacologically acceptable, and does not degrade in vivo, while retaining the biological activity of the lead compound. The mimetic or mimetics found by this approach can then be screened to see whether they have the target property, or to what extent they exhibit it. Further optimisation or modification can then be carried out to arrive at one or more final mimetics for in vivo or clinical testing. Mimetics of substances identified as having ability to interfere with the interaction of Skn7 polypeptide with its binding site motif and/or its interaction with Yapl or Hsfl polypeptide in a screening method as disclosed herein are included within the scope of the present invention. Whether it is a polypeptide, antibody, peptide, nucleic acid molecule, small molecule, mimetic or other pharmaceutically useful compound according to the present invention that is to be given to an .individual , administration is preferably in a "prophylactically effective amount" or a "therapeutically effective amount" (as the case may be, although prophylaxis may be considered therapy) , this being sufficient to show benefit to the individual. The actual amount administered, and rate and time-course of administration, will depend on the nature and severity of what is being treated. Prescription of treatment, e.g. decisions on dosage etc, is within the responsibility of general practioners and other medical doctors . A composition may be administered alone or in combination with other treatments, either simultaneously or sequentially dependent upon the condition to be treated. Pharmaceutical compositions according to the present invention, and for use in accordance with the present invention, may include, in addition to active ingredient, a pharmaceutically acceptable excipient, carrier, buffer, stabiliser or other materials well known to those skilled in the art. Such materials should be non-toxic and should not interfere with the efficacy of the active ingredient. The precise nature of the carrier or other material will depend on the route of administration, which may be oral, or by injection, e.g. cutaneous, subcutaneous or intravenous. Pharmaceutical compositions for oral administration may be in tablet, capsule, powder or liquid form. A tablet may include a solid carrier such as gelatin or an adjuvant. Liquid pharmaceutical compositions generally include a liquid carrier such as water, petroleum, animal or vegetable oils, mineral oil or synthetic oil. Physiological saline solution, dextrose or other saccharide solution or glycols such as ethylene glycol, propylene glycol or polyethylene glycol may be included. For intravenous, cutaneous or subcutaneous injection, or injection at the site of affliction, the active ingredient will be in the form of a parenterally acceptable aqueous solution which is pyrogen-free and has suitable pH, isotonicity and stability. Those of relevant skill in the art are well able to prepare suitable solutions using, for example, isotonic vehicles such as Sodium Chloride Injection, Ringer's Injection, Lactated Ringer's Injection. Preservatives, stabilisers, buffers, antioxidants and/or other additives may be included, as required. A polypeptide, peptide or other substance able to interfere with the interaction of Skn7 polypeptide with its binding site motif and/or its interaction with Yapl polypeptide or Hsfl polypeptide according to the present invention may be provided in a kit, e.g. sealed in a suitable container which protects its contents from the external environment . Such a kit may include instructions for use. Further aspects and embodiments will be apparent to those of ordinary skill in the art upon consideration of the above disclosure and the following examples. All documents mentioned in this specification are hereby incorporated herein by reference. In this specification, the terms "comprising" and "comprises" are used synonymously with "including" and "includes" . EXAMPLE 1 . This example shows that deletion of the bacterial two component response regulator homologue Skn7 results in sensitivity of yeast to oxidizing agents indicating that Skn7 is involved in the response to this type of stress. Following oxidative stress Skn7 regulates the induction of two genes, TRX2 , encoding thioredoxin, and a gene encoding thioredoxin reductase. TRX2 is already known to be induced by oxidative stress dependent on the Yapl protein, an API-like transcription factor responsible for the induction of gene expression in response to various stresses . The thioredoxin reductase gene has not previously been shown to be activated by oxidative stress and, significantly, we find that it too is regulated by Yapl. The control of at least TRX2 by Skn7 is a direct mechanism as Skn7 binds to the TRX2 gene promoter in vitro. This shows Skn7 to be a transcription factor, at present the only such eukaryotic two component signalling protein. Our data further suggests that Skn7 and Yapl co-operate on the TRX2 promoter, to induce transcription in response to oxidative stress. The data of Example 1 may also be found in Morgan et al , 1997, published after the priority date of the present application, whose contents are incorporated herein by reference . Reactive oxygen species (ROS) are responsible for a wide range of intracellular damage to DNA, proteins and cellular structures (for reviews see Moradas-Ferreira et al . , 1996; Ruis and Schuller, 1995) . Free radicals are a normal by-product of respiring cells and are also produced by a wide range of different environmental chemicals. Hence cells have developed mechanisms to respond to this type of stress termed the oxidative stress response (OSR) . Although a number of the cellular proteins functioning in the OSR have been identified in eukaryotes little is known regarding the signal transduction pathways which detect and respond to oxidative stress. Much of our knowledge in this field has come from work in the budding yeast Saccharomyces cerevisiae. In yeast, overexpression of two transcription factors, Yapl and Yap2 , were found to confer resistance to a wide range of drugs and oxidizing agents (Moradas-Ferreira et al . , 1996). Yapl and Yap2 are members of the c-jun family of proteins which constitute part of the API transcription factor of higher eukaryotes. Both Yapl and Yap2 contain a basic leucine zipper domain (bZIP) adjacent to a DNA binding domain located at the N-terminus of the proteins. Null mutants of both YAP1 and YAP2 result in sensitivity of S. cerevisiae to the oxidizing agent hydrogen peroxide (Stephen et al . , 1995) . Further, Yapl binds directly to the promoter, and regulates the expression of the TRX2 gene (Kuge and Jones, 1994) encoding thioredoxin which acts in the oxidative stress response to reduce protein disulphides. Yapl also regulates GSH1, encoding glutamyl cysteine synthetase, in response to hydrogen peroxide . The connection between Yapl and Yap2 with API of higher cells is further strengthened by the observation that the API itself is regulated by the oxidative stress status of the cell (Moradas-Ferreira et al . , 1996) . Other work in S. cerevisiae has also demonstrated that there are distinct signalling pathways to hydrogen peroxide and superoxides though the transcription factor (s) and signalling pathway for the superoxide response have not been characterised (Collinson and Dawes, 1992; Jamieson, 1992; Flattery-O' Brian et al.,1993 and Stephen et al . , 1995) . In particular, the GSH1 gene is induced by both hydrogen peroxide and menadione, a superoxide generating drug (Stephen et al . , 1995) . Although YAP1 mutants abolish the hydrogen peroxide induced expression of GSH1 they have little effect on menadione induced expression (Stephen et al . , 1995) . Thus yeast mounts a complex defence to oxidative stress involving several different transcription factors and signalling pathways. Recently it was shown that disruption of the SKN7 gene (also known as POS9 (Krems et al . , 1996) and BRY1 (Morgan et al . , 1995b) ) resulted in yeast cells becoming sensitive to hydrogen peroxide (Krems et al , 1996) . SKN7 has been implicated in the regulation of cell wall biosynthesis and the cell cycle (Brown et al., 1993, 1994; Morgan et al . , 1995b). Overexpression of SKN7 suppresses the cell wall defect associated with mutation of the KRE9 gene (Brown et al . , 1993) and additionally, the growth defect associated with deletion of PKC1 (Brown et al . , 1994) . Furthermore, deletion of the SKN7 gene was found to be synthetically lethal in a pkclΔ background (Brown et al . , 1994; Morgan et al . , 1995b) . As the PKC1 MAP kinase pathway is involved in cell wall biosynthesis (rev. in Herskowitz, 1995; Igual et al . , 1996) this supports the notion that SKN7 might play a role in the expression of cell wall genes. In addition, overexpression of Skn7 suppresses the lethality associated with loss of the Gl transcription factors SBF and MBF, suggesting a cell cycle role (Morgan et al . , 1995b) . These transcription factors recognise the sequences CACGAAAA (an SCB element) and ACGCGT (an MCB element) , respectively, in the promoters of genes they regulate. These include the Gl cyclin and DNA synthesis genes (for reviews see Johnston, 1992; Koch and Nasmyth, 1994) . The lethality associated with loss of SBF and MBF has been shown to be due to the absence of Gl cyclin expression. High copy SKN7 was shown to restore Gl cyclin expression through the MCB and SCB elements present in the cyclin promoters (Morgan et al . , 1995b) . SKN7 does not appear to bind directly to MCB and SCB elements (Morgan et al . , 1995b) and is more likely regulating a MCB/SCB binding factor other than MBF and SBF. The sequence of SKN7 revealed homology to the DNA binding domain of heat shock factor (HSF1) (Brown et al . , 1993; Morgan et al . , 1995b) hence it is possible that SKN7 binds to a sequence related to the HSE elements that HSF1 recognises . The Skn7 protein contains a potential receiver domain found in the two component signal transduction family of proteins in prokaryotes (Brown et al . , 1993; Morgan et al . , 1995b). These signal transduction systems are a common method of detecting and responding to the environment in bacteria (for reviews see Bourret et al . , 1991; Parkinson, 1993). Generally, the first component, a homodimer histidine kinase present in the cell membrane, detects the signal, and phosphorylates a conserved histidine residue on its partner. This phosphate is then transferred to a conserved aspartic acid residue within the 120 amino acid receiver domain of the second component, the response regulator protein. Response regulator proteins are generally transcription factors that are activated by the phosphorylation. In eukaryotes only a few potential two component signal transduction proteins have been identified (reviewed in Morgan et al . , 1995a) . In no case, including Skn7, have genes regulated by these systems been identified. In Saccharomyces cerevisiae genetic screens have identified one histidine kinase, Slnl, and two response regulator proteins Sskl and Skn7. Slnl and Sskl were shown to act in the regulation of the response of yeast to osmolarity by regulating the Hogl MAP kinase pathway (Maeda et al . , 1994, 1995). However, deletion of SKN7 does not result in any osmolarity defect. The only stress defect known for skn7 mutants is in oxidative stress as mentioned above, although the role of Skn7 in the OSR remains unclear. Recently Krems et al (1996) proposed that the receiver domain was essential for the ability of Skn7 to confer cellular resistance to oxidizing agents suggesting, perhaps, that an upstream histidine kinase could be important for the activity of Skn7 in the OSR. Here we have identified a direct role for Skn7 in the OSR through the induction of TRX2 and also a gene encoding thioredoxin reductase. The Skn7 protein binds directly to the TRX2 promoter and cooperates with the Yapl protein to induce gene expression. This is the first example of a gene known to be directly regulated by a response regulator protein in eukaryotes. However, in contrast to Krems et al (1996) we find that signalling through the receiver domain of Skn7 appears to have no role in the oxidative stress response. RESULTS Skn7Δ strains are sensitive to a range of oxidizing agents skn7Δ strains are sensitive to hydrogen peroxide (Krems et al . , 1996) . We have confirmed this result and find that skn7Δ strains are sensitive to a range of oxidizing agents including t-butyl-hydrogen peroxide, cadmium, menadione but not significantly sensitive to diamide. Thus the Skn7 protein is required for the cellular response to a variety of free radicals. Deletion of the YAP1 gene also results in sensitivity of yeast cells to many of these agents (Kuge and Jones, 1994) . The sensitivity of yaplΔ strains has been shown to be due to the role of the Yapl protein in the induction of several genes which function in the OSR. Significantly, deletion of the SKN7 gene does not enhance the sensitivity of a yaplΔ strain to diamide, hydrogen peroxide, cadmium or menadione (Krems et al . , 1996; our unpubl . obs . ) suggesting that SKN7 and YAP1 are epistatic, functioning in the same pathway. The basis for the sensitivity of the skn7Δ strain to oxidising agents is unknown but a likely explanation, based on the epistasis of SKN7 and YAP1, is that Skn7 affects the expression of a, set of genes also regulated by Yapl in response to oxidative stress. Skn7 is required for the induction of thioredoxin and thioredoxin reductase gene expression by the oxidative stress response Expression of the TRX2 gene is induced in response to several oxidizing agents including hydrogen peroxide and this induction is under the control of the Yapl protein which binds the TRX2 promoter (Kuge and Jones, 1994) . To examine the role of Skn7 in TRX2 gene expression various yeast strains were treated with hydrogen peroxide, total RNA was isolated and probed with a TRX2-specific gene probe (Figure 1A) . In yaplΔ strains the induction of TRX2 is much reduced, though some residual induction remains, as observed previously (Kuge and Jones, 1994) . Significantly, deletion of the SKN7 gene mimics the deletion of the YAP1 gene. TRX2 induction is almost abolished but once again residual induction is observed. The skn7ΔyaplΔ double mutant is no more defective in TRX2 induction than either single mutant, consistent with the genetic epistasis described above. The residual induction of TRX2 that occurs even in the skn7ΔyaplΔ double mutant suggests the existence of a further minor induction mechanism. However, the important result is that skn7Δ and yaplΔ mutants appear to have identical phenotypes with respect to TRX2 induction consistent with their functioning in the same pathway. Next, the expression of a gene encoding thioredoxin reductase (Chae et al . , 1994) was tested. No name has yet been assigned to this gene and we will refer to it as TRR1. Thioredoxin reductase recycles the oxidized form of thioredoxin to the reduced form to scavenge more ROS suggesting that TRR1 expression may also be induced by ROS. In addition, the TRR1 promoter contains a potential Yapl binding site, identical to the SV40 AP-1 site (Figure IB) . Northern blot analyses CQ 0 μ- ft q ] O 0 0 n ii Hi q Hi q ii O μ] CQ Φ q SD > 3 μ- μ- •d tϋ q q >d ii tr cr ti μ- tr t μ- Hi Hi δ 0 Φ 0 a 1 SD 0 tr Φ ii pd Φ CO t CQ trj a a ii 0 SD 0 ^ Φ Φ 0 q ft SD Φ a SD Ki Φ CO ii Φ CQ Φ a *d. tr f Φ ft SD j ft ft 0 ≤ CQ μ- ii <! ≤ SD a 3 CQ to 3 μ- 1-3 CQ 0 H Ό <! to CO 3 ii CO μ- μ- to a a a φ SD SD LQ 0 Φ Φ μ- Ω μ- Φ Φ μ- a pd μ- ii 0 μ- q φ 0 μ- μ- Φ φ Φ q a - Ω Ω 0 <! Ω a X SD ft a q ft Hi CQ ft ft xj o q ≤ SD a tr Hi a 0 Ki q a CQ μ- q q a Φ q H μ- φ μ- Φ Hi SD Φ Φ a to q Φ μ- q q φ 0 CQ a ii μ- q CO Ki 0 q CQ μ- μ- a ii Φ ft Φ SD ft 0 Φ a a - Ω Φ H a q ti ii φ CQ 0 <! μ- μ- ii a tr P7 0 0 Ω Φ O Φ ft a q Ω ft Φ q Φ a CQ Φ 0 ii Φ 3 Φ 0 0 0 SD Φ a a a Φ i M £ ii q SD μ- X q Φ H ft 0 q ^ 0 a a q -j ft q Φ μ- SD q 0 μ- Ω Φ <H Φ 0 Ki X q a a 0 q q Φ • Φ Q 1> 0 μ- tr a a tr CQ q SD 0 0 X q a ϋ ≤ •d ii CQ Ω Φ CO q Φ tr ft μ- rV Hi a SD Φ O Ω . ft SD CQ q 1 Φ q Hi a Ki tr Φ μ- ii Φ q Φ Φ q 0 ϋ SD a 0 a SD CQ μ- q SD 3 Φ ϋ μ- 0 CO q Φ SD ii ft 0 ft tr a SD CQ CQ • J a q SD μ- Ki q 0 q μ, Φ q CO Φ 0 Hi O tr CO q SD Hi μ- Ω 1 tr CQ r> ft tr 1-3 a tr , SD ^ tr q Hi a tr - O μ- CO 3 μ- μ- q CQ 0 0 s- X Φ X d ft μ-ι a SD tr Φ ^ q Φ O " CO 0 q μ- φ a 3 CO tr a IT 1 μ- < Φ 0 SD X SD P^ a Ω ii q φ μ- μ> μ- 0 1 a tr 0 a q 77 μ- q Hi SD Φ H Hi Ό SD ] •d to Ω tr X q a H3 ft ≤ O D 0 X LQ Φ a q q 0 a CQ ϋ μ- Ω ft Φ μ> Ki π μ» q tr q ι-3 ft *> μ- SD 3 vo a μ- SD tr -J Φ ii !S1 tr ι-3 to > tr μ- £ 0 φ tr to a pd ft CQ 25 .r* ft q ft μ- £ Φ CQ Ω SD 3 q 0 SD O 1 a 0 Φ a ii μ- to Ω μ 1 ] — • SD SD SD tr a a μ- i rV μ- 0 q q Ω f ft CQ q a ii 3 CQ μ 1 q 0 . a a Ω φ SD q φ a a a 3 q Ii ^ a" q μ» 0 CQ LQ q Φ q 0 μ- CQ Ω Φ LQ ft q q SD a 1 -j CQ Φ tr Φ Φ μ- a q Φ i q 0 tr ϋ μ- LQ O Φ Ω CD Φ 0 CQ s: > ι-3 q a SD O q a Φ q ii a SD tr Hi ii Φ Φ a Φ a 3 a CQ a s: q ,— - CO ?r Hi s: q SD w ii q q 0 CO tr CQ X SD Φ μ- Φ Φ - ft a Φ ii Φ Φ tr rrj μ- a si μ- 1 O SD (xj a ii q Φ Ki Φ μ- a ii t-3 ft μ- a φ . o • 3 μ- ft -j a tr a t Ω ~ - H q i μ- H a SD • Φ IΛ a a μ- 1 Ω Hi _- SD 0 Ω CD a SD > Ω ft a a ft q 3 sd Φ SD CQ a φ 3 CQ a to Ω ft μ- q μ- ^ H a 0 3 CQ q q 0 φ a P^ ii μ- ft CQ a ft s- μ\ Φ a 77 μ- CD H μ- ?? a φ 3 φ CO μ- q ^ X X q LQ to a ti φ ω q SD ^ CO SD ft Ω Φ 0 s H-3 CQ φ H Hi X φ q o 3 μ- SD Ki 0 Φ Ω O Ό μ- SD ► d CD LQ Φ a μ- tr a ft B μ- Ki i Hi ti a a Ki ii φ 0 > φ 0 a • μ- 55 Φ q ft X a to Φ ii H ri 0 SD Φ a- ft M Φ •d SD a q a a q Φ a -J SD a X μ- ft Φ to 3 φ Φ μ- 0 X a > CQ ii a CO 0 SD ft SD Φ p^ CQ φ SD ft SD φ CD q a !~tι - ft Ω CQ 0 ft ti q a Φ SD CQ ii SD μ- < Ul Φ to φ a 0 LO φ ft CD 0 SD ii q CQ μ- <: Φ ϋ q ft a CQ tr LQ CQ Φ Φ 0 < ii ft ft a — q X μ- SD CQ K! Ω 0 μ- q o μ- X CQ a 1 q 0 μ- ti ii X 3 Φ φ φ q • tr Ό q 0 μ- tr SD ?r Ό 0 ti a ft 0 CQ Ω q X £ s; ft Φ SD ^ Ό Φ X CQ μ- H o ii tr a • a o >d CQ Φ a SD • φ a μ- q a SD ; 0 CQ φ Φ H CQ Ό ■ a a t Φ > a Φ μ- ^ LQ £ μ 1 H ii μ- Φ o q Ό a a 3 CQ CD Φ LO ii ft po CQ CD ω μ- Φ 0 0 0 a μ- CO a tr ^ > q μ* a q Φ 0 CQ 0 0 ■ Φ SD a CQ CO tr CD CQ H 0 ft SD a X Hi C Q 3 a - SD 0 ft 0 μ- a μ- CQ o\° Hi CQ q Ω 77 μ- μ- ft O X a a μ- tr ft μ- CD Ω q ii CQ μ- CQ Φ Φ CQ ti 3 CQ 0 SD K! ∞ μ- SD ft ft ft t7 SD Φ φ μ 1 CO a; CQ ϋ q CO SD Φ tr 0 μ- H μ- ft ft μ- -j μ- 0 a q SD M ft Φ SD a Ki D q : q Φ φ μ- ≤ Λ q a a ftf 0 3 > 3 SD K3 — SD CO O ,— - Ω ft ϋ φ LD μ- Φ ft ii LQ LQ μ- a PO a q q μ- SD Φ Hi μ 1 *• q μ- P7 ^i ϋ N Φ a ιfc> a Φ ii SD a φ a . q μ- ϋ s; φ 1 X X X ri 0 tr 1 μ- 3 ti μ- Φ SD rC ft ft a 0 μ- X μ- ^ tr ϋ Φ SD SD SD SD a ft SD <i μ- -J LQ SD CO 0 Φ • a -d a SD r Hi μ- φ Q CQ Ω CD K q tr i O ii 0 q Φ I- 1 Φ a SD Ω ii a Ω a Φ CO q Φ μ- LQ q ft μ- tr a SD tr X μ> Φ CQ 0 •d CO SD μ- ii q q φ q q φ O i Φ Ω Ω a tr ft a ft Ki φ ft q > CD CQ $ Hi Φ 0 CO ϋ a Φ μ- μ- X Φ S ft φ 0 SD q SD ti Φ Hi a 0 Ω ri 0 (i μ- 0 a q <J <! ► d ii Φ Φ ft X ii X Φ a Φ O SD q SD a κ| 0 0 a a ft μ- ii SD Ω to φ μ- Φ <; μ- μ- φ W ft q ft i CQ 0 SD ' > LQ q CQ q Hi LQ a Φ a Φ q 0 — q Ω O μ- a ft W μ> SD SD Φ q Ki rrj Φ t q ft a ft Q CO a • q X q ft φ 3 μ- > q q CQ • K) μ* a SD i ≤ q a Q φ q 0 φ SD Φ 0 Hi SD tr q φ q SD SD q Ω ii ft CO a i LQ Φ tr SD LQ μ- CQ tr Φ Ω 0 3 Ω Φ Φ Hi φ ii Φ a SD CQ Φ Hi μ- Φ > a 0 CQ q 0 ft Φ i Φ ii Ki q SD Ki 2 ϋ Cd X ii SD S H Ω CQ ft CQ s; Ki CO CO cr O q s: q a ϋ •d ii q H O > SD CQ 0 0 tr tJ μ, Φ 35 0 Φ 3 SD 3 0 q Φ ?r SD i a 77 SD ?r £, Φ tr •d 25 ii φ tr tr ;^ <£> q 77 q φ SD Φ q μ- O SD ft CQ 3 i O a CQ 0 CQ ti 3 3 0 ri Φ CO > Φ LQ Φ Φ a LD 0 3 CQ X •d a 1 co > ft Ό SD q SD Φ ! q CQ -J ft -J Φ q Ki a ι < -J Φ Ω K! • ; SD 0 Φ φ Ό q q LQ SD μ- 3 t> Ki Φ Φ > tr μ- ii Hi SD O O ~— ' Φ > i SD CD i ft ft € 3 Φ tr ii li μ- CQ ft ti μ- CO - Ki SD SD 3 Φ ϋ t SD ?r μ- q — <! 0 •d φ Φ CD SD 0 SD Φ SD Φ 3 tr Φ 0 φ 3 q - i a a 3 SD SD Φ q a 3 μ- SD CQ Φ a μ> ft ft — SD CQ a ii a Φ CQ q a ii CO CD μ- • ; 0 ft SD φ 3 CQ ft Φ -J Ki a a ≤ q ft SD . LQ O CD Ω CQ 3 Hi q φ • a SD z q CQ μ j ft 3 ft ft tr i •d tr Hi SD •d q ii Hi Φ tr 0 CQ a CQ Ki SD Φ X μ- ii φ Hi q O Φ CQ Φ SD s; 0 X ■i μ- φ i ,--. si 0 3 0 ti 0 77 q H CQ μ- s- Ul SD Φ a ii ii ii CO μ- ii μ- μ- SD 0 H3 a ii SD trj ft i q q £ 3 3 q > 3 φ φ q CQ q 0 SD 0 q q 3 Φ 3 q a ^ a 3 tr O Hi q μ- Φ 3 tr SD -J μ- 3 tr Ό ii ft Φ μ- 3 a q tr ft SD O tr q Ω 3 Φ q 0 Φ CQ Φ ≤ ii q > 3 SD 0 ϋ . — . φ 0 Hi CQ Φ Φ Φ μ- CD μ- co 2! ii a φ ft μ- O ft tr X Ω q ii Φ ► A q a a ii SD Ω μ- X ,Ω CQ X CQ 0 > CO Ki q 3 q ti X q P7 Φ SD q 0 CQ μ- Hi X 5 0 ii 3 Ki X , SD X tr SD Φ 3 o ii tr Φ tr Φ q 3, tr a μ- r tr a q q φ LQ 0 CO Hi 3 < μ- po q SD 0 •d ii SD LO 0 Φ ft SD ii μ- ^] 3 Ό μ> Φ q SD tr 3 a a Φ q ϋ SD Ki ft SD X Φ Ki ≤ μ 1 a 1 q q ^ SD q q q ii ti 3 φ q ii a i 0 q ii q μ- a t CD μ> > 3 m μ > Φ O SD • —^ Ω tr tr SD Φ φ £ SD ft φ ft φ Φ 3 tr μ- μ- ii SD > a SD tr SD ?r CQ O . q μ- 3 ϋ CQ Φ μ- CQ CQ 0 Ω SD Φ φ 0 Φ q d CQ 0 SD a Φ H3 a rV CD SD CQ ft Ki φ X μ- 1 q μ> ? a lfc» q q 3 q q a Ω μ- ii 0 CO q ri -j SD a ii 3 rr ft - 3 — 0 ii tr Hi X q 0 0 3 q CQ 0 μ] μ- CQ CD X tr ~o £ Q O K! Φ Ω ii μ- ^J ^ • 0 CQ Φ pd 3 3 φ ii μ- a tr CQ SD ? Ki 0 Φ M a SD q Φ SD q SD o > tr O Φ 0 X 0 SD LQ Φ φ Φ 3 a μ 1 i <j cr i 25 CQ •d SD Ω a X μ- Φ tr 3 Ki to Hi H3 Hi q O μ- a ii Φ l 0 Φ μ- Φ CQ q 0 q Hi ft SD 3 μ] 0 Φ 0 0 q 0 Φ φ 3 μ- S! a > £ q 3 •d φ μ- Hi CQ Φ SD φ Ki tr CD ii q *d CQ μ- i ii Ki CQ Hi LO 3 Ω «. SD Φ Φ ft SD CQ 5 » 0 ^ CQ Ki H ≤ Φ Φ Φ Φ ii q Φ a SD μ- H O μ, CO μ- X CO a SD q tr 3 CO SD q Φ > μ- a q S! a 0 ti X ft q ii SD Ω tr LO SD a X 3 Ω SD CD μ- μ- 77 CQ 3 Φ a 3 CD μ- tr q 3 SD SD CQ tr SD i SD μ- 1 3 ft SD 3 φ ft M 3 0 0 3 a ft i ft ft ft 0 μ- Ω Φ μ- 0 μ- 3 Φ q Φ a CO ft Φ Ό μ- ≤ r μ- X μ- a Q Hi -J CQ Φ 0 CO q a SD q a μ- q μ- q SD μ> X SD q ϋ CQ ft q CQ ^ 3 a a q q μ- 3 Φ CQ 3 q ft 0 CO ft CQ Φ > φ CD Φ Φ q SD φ SD μ- q tr < 0 q ii Φ φ SD a φ O SD SD Φ X ^ ft q Ω ft Ω cr tr a CQ rd CQ r^ tr Φ q tr H SD q a Φ q a •d q μ- tr q SD tr SD μ- ft q μ> ii 0 φ Φ μ- μ- Φ : Ω a q H SD 0 O a SD ft SD ii SD , 3 SD q ii Φ a co μ- > SD a Q q O t O ft tr Hi •<: ii a ft a 3 q 3 Φ tr 0 μ- Φ B μ- CQ ϋ SD a μ- Ω q a μ- ft q μ- 3 X SD Ω ft q μ- 1 X ft 0 CQ Φ μ- q a X q CQ q co to φ CO μ- q μ- SD q q tr a SD q ta μ- q to SD q X 3 0 q q ^ q ^ O q μ- tr - CQ ?r < μ- LQ 3 Φ CD CQ ft 0 3 a tr q q q CQ q SD ti tr ii 0 co Φ Ό Ω a φ <! tr μ» 77 Ω Φ •d O CQ Φ Ω SD X tr ii tr a SD ii Φ CQ 0 q a Ω ii ii -j μ- q > X ti ii ft 0 q 0 fD SD SD tr q Ω Φ tr q q μ- ii 0 μ- ^ --. q Φ -J a tr 0 O a μ- ► d q Ω SD q q q d μ- Φ tr a μ- a 3 tr SD 3 X •d Φ > ft ,--. X Φ a ?r q CQ μ> q 3 tr Φ CD SD ii Hi μ- Φ CQ 3 ft 0 φ a si' SD X μ- φ X φ s Ω 0 3 SD • cr CQ ft Φ X ii Φ q a Ω Φ q ft ft CQ q ii q q SD SD >-3 a X -J μ- q SD ^ - q SD ft CQ " \ CD 0 a Φ φ 0 q ii tr SD •d φ q 0 B > 3 tr a CO ri a Φ φ SD α q Hi q Φ H SD X SD μ- 1 μ> X Φ a ^ rr X CQ SD H ft cr q 77 SD ft ft a CQ 5 0 SD X tr SD SD ft X Ω q O > q ϋ SD to tr SD q tr a Φ 3 Ω Ω CQ > q q q μ- 0 Ki a μ- q tr 0 - ii μ- tr φ O 0 Φ to q H O q 3 tr Φ SD tr Φ ii 3 < μ> ft SD Φ O Φ SD SD O SD a q 3 CD 0 SD X Ω Ω Φ ft SD Φ 3 X 3 SD Ω SD ii a CO > SD 3 μ- 3 Φ SD - q a 0 Ul 0 0 Ω 0 Hi C| μ- Ki Z CO μ- 3 q CQ a 0 ft 3 X a 0 CQ 0 ft 3 ft Hi 3 ft Φ €. q ti μ- a o ft μ- 1 Φ 77 X ft CQ q 3 5 ω ft CQ q ft μ- - .— ^ ► d 0 a μ- CQ ft co SD μ- 0 to -J q a a φ SD i ti Φ C q tr 3 q M 3 q q Φ Ω Φ μ- φ - l ft a cr q ri ?r H q SD K; q tr S φ μ- tr 3 > ii q q CO ^ - 3 ft 0 Φ CQ SD a tr a D q Φ Φ CO X 0 ii Φ tr μ- - CQ s- ft ϋ Φ μ- -j Φ ft r tr H < Φ Hi Φ SD ii q SD 0 Φ ≤ μ- X 3 3 • μ> Φ 3 — ω Φ 0 q 3 Φ tr q SD Φ to φ X a- a - ft separately incubated with preimmune serum at the same dilution as the polyclonal antisera. Using the probe TrxΔl, the yaplΔ crude extract was analysed by EMSA. In trackl no antiserum was added, in track 2 the Skn7 polyclonal antiserum, and in track 3 the preimmune serum at the same dilution as the Skn7 polyclonal antiserum. EMSA revealed that bands 1 and 2 were clearly supershifted by the Skn7 antibody. To confirm this, the experiment was repeated using only yaplΔ extracts and longer autoradiograph exposures. The antibody supershifted band 2 completely and the preimmune serum was without effect. Thus Skn7 must bind directly to the TRX2 promoter. The Yapl protein also binds directly to the TRX2 promoter at two sites termed site 1 and site 2 (Kuge and Jones, 1994; Figure 5) . In our EMSA two bands in addition to band 1, bands 3 and 4, are sensitive to the presence of the YAP1 gene. To determine whether all these Yapl -dependent bands contain the Yapl protein various crude extracts were mixed with a polyclonal antibody against Yapl and the effect on the bands examined. The wild type extracts clearly show a prominent supershifted band and essentially recreate the pattern obtained from yaplΔ extracts. Hence all the Yapl-dependent bands contain the Yapl protein. In agreement with the suggestion above that bands 1 , 3 and 4 , but not band 2 , contains the Yapl protein, only band 2 appears not to be supershifted by the Yapl antibody. This has been confirmed by protracting the EMSA electrophoresis step to ensure complete separation of band 1 from the supershifted Yapl band. Hence band 1 contains at least the Skn7 and Yapl proteins, band 2 contains Skn7, and bands 3 and 4 contains Yapl. Another important conclusion from this data is that significant binding of Skn7 and Yapl proteins can be detected in the absence of oxidative stress and, furthermore, that Skn7 can bind in the absence of the YAP1 gene and vice versa. To map the TRX2 promoter site responsible for Skn7-specific binding a series of overlapping probes for EMSA were constructed from the upstream region of the gene (Figure 5A) To map the Skn7 binding site the probes TrxΔ2 , TrxΔ3 and TrxΔ6 were mixed with crude extracts prepared from yaplΔ and skn7ΔyaplΔ strains, and analysed by EMSA.. Band 2, containing the Skn7 protein, could only be detected with the TrxΔ3 probe. The probes TrxΔl, TrxΔ3 , TrxΔ4 , WTMCB, and MUTMCB (see Methods) containing the Skn7 binding site, gave a single band which was not detectable in skn7Δ extracts. In contrast, no Skn7 sensitive band could be detected with probes TrxΔ2 and TrxΔ6. As described above, Skn7 binds within a 23 nucleotide region between the Yapl binding site 2 (Kuge and Jones 1994) and the potential TATA sequence. The potential DNA binding domain of Skn7 has homology to HSFl and whilst the 23 nucleotide sequence does not contain a complete HSE element, there is some limited homology to one. Within the 23 nucleotides is also some limited homology to an SCB element. Mutation of the CG to TA within this lowers Skn7 binding to this region 20-fold, as determined by comparing the binding of Skn7 to the TrxΔ4 and TrxΔ5 probes using EMSA. However, the addition of a large molar excess of a fragment from the HO gene promoter containing SCB elements did not compete with Skn7 binding. Thus, although the CG nucleotides are important for Skn7 binding, this sequence is unlikely to constitute a functional SCB element. There is also one potential MCB element in the TRX2 promoter. To test whether this sequence had any role in Skn7 binding, the essential CG core was mutated to TA and a fragment with the mutation tested in EMSA. There was no effect on Skn7 binding, as determined by comparing binding of Skn7 to the MUTMCB and WTMCB probes. Thus neither of the potential MCB nor SCB sites appear to be relevant for Skn7 activation of TRX2. Next, we determined whether any significant changes in binding to the TRX2 promoter occurred following treatment of the W303- la and a yaplΔ strain with hydrogen peroxide. Extracts were prepared from the cells following 1 mM H 2 0 2 treatment for 1 hour at 25 * C. This is at a time when maximal induction of the TRX2 and TRR1 genes had been observed. The extracts were mixed with a probe from the TRX2 promoter. No obvious effect on the binding of the Yapl or Skn7 proteins was observed under these conditions. This implies that, at least in vitro, DNA binding per se is not the major regulatory step in the induction of gene expression and that another mechanism is involved. In addition, no new bands were evident suggesting that the induction does not involve the binding of a new protein to the TRX2 promoter. Kuge and Jones (1994) demonstrated that Yap 1 binding in vitro is induced significantly by diamide. To test whether Skn7 responded in a similar fashion, crude extracts were prepared from the W303-la and yaplΔ strain treated with 1.5mM diamide for 1 hour at 25 'C and analysed for the binding of Yapl and Skn7. Like Kuge and Jones (1994) we observed induction of Yapl binding following diamide treatment. However, in contrast no significant induction of Skn7 binding occurred. Signalling through the receiver domain of Skn7 plays no role in the OSR. Recently it was suggested that the receiver domain of Skn7 was necessary for the resistance of yeast cells to oxidative stress (Krems et al . , 1996) . Hence we tested whether the receiver domain of Skn7 was required for the induction of TRX2 following oxidative stress. In Skn7 , Asp427 is the phosphorylable amino acid (Brown et al . , 1994; Morgan et al . , 1995b) and previously we demonstrated that mutation of this residue to an asparagine residue prevented the ability of Skn7 to rescue the temperature-sensitivity of a swi4ts swi6D strain (Morgan et al . , 1995b) . Hence CEN plasmids containing either the wild type SKN7 gene or with a D427N mutation were introduced into the skn7Δ strain and the sensitivity to hydrogen peroxide tested. A 20 ml of a fresh overnight culture of the strains indicated were streaked radially on a YPD plate and allowed to dry. A filter disc saturated with hydrogen peroxide was placed in the centre of the plate. Following incubation at 30 'C the extent of inhibition of growth of each strain from the disc was measured. pBAMl contains wild type Skn7 and pBAM2 contains Skn7 D427N. Both plasmids clearly behave identically, they fully complemented the sensitivity of the skn7Δ strain to hydrogen peroxide . Next the same strains were grown in minimal medium and treated with ImM hydrogen peroxide for 1 hour. Northern hybridisation revealed that Skn7Δ427N behaves identically to wild type and TRX2 expression is induced normally. Finally, the CEN plasmids were introduced into the skn7ΔyaplΔ strain. When crude extracts of these strains were characterised by EMSA the wild type and D427N version of Skn7 demonstrated similar binding abilities to a TRX2 promoter fragment. Thus signalling by means of the receiver domain has no role in the induction of TRX2 in response to oxidative stress or on the binding of Skn7 to the TRX2 promoter. Summary . The receiver domain as a whole, however, is essential for the response to oxidative stress. A C-terminal truncation of Skn7 , missing amino acids 353-623 containing the receiver domain and the glutamine rich region, was isolated by transposon mutagenesis (Morgan et al . , 1996). This construct was unable to rescue the sensitivity of skn7Δ to hydrogen peroxide (data not shown) . Nonetheless this truncated protein, which contains the domain with homology to the HSFl DNA binding domain, was still able to bind the TRX2 promoter. In summary, the receiver domain may play a structural role that is necessary for the oxidative stress response but phosphorylation of Asp 427 seems unimportant . The Yapl transcription factor is required for the OSR through the induction of gene expression (rev. in Moradas-Ferreira et al., 1996) . Deletion of the SKN7 gene also results in the increased sensitivity of yeast to oxidative stress. Since response regulator proteins like Skn7 in bacteria are transcription factors it seemed likely that Skn7 , in addition to Yapl, was also required for the induction of gene expression. This is indeed the case; two genes normally induced by oxidative stress, TRX2 and TRR1 , require the SKN7 gene for induction. This is a direct mechanism as Skn7 was shown to bind at least the TRX2 promoter. This is the first gene that Skn7 has been shown to directly regulate and is the first example of a direct gene target for any eukaryotic two component signal transduction protein. The binding of Skn7 to a specific region within the TRX2 promoter also, of course, confirms that it is a transcription factor. Despite response regulators being almost entirely transcription factors in prokaryotes, the two component systems so far characterised in eukaryotes lie upstream of MAP kinase pathways (Morgan et al . , 1995a) and appear to function in regulating MAPKKK activity. This is certainly true of the only other eukaryotic response regulator protein, Sskl in budding yeast, which controls the Hogl MAP kinase pathway (see above) . Thus our demonstration that Skn7 is a transcription factor makes it the only example known at present that functions as a classical prokaryotic response regulator. The precise role of Skn7 in the regulation of TRX2 and TRR1 expression is unclear at present. The fact that binding of Yapl is not significantly affected by the absence of Skn7 and vice versa suggests that no interaction is required for binding to occur. However, EMSA analysis revealed one band that clearly contained both the Yapl and Skn7 proteins. Attempts to co-immunoprecipitate Skn7 with Yapl failed to identify an interaction. Moreover, Krems et al (1996) could detect no association using the two-hybrid technique. Possibly the interaction of Yapl and Skn7 may require prior binding to DNA hence a direct contact between Yapl and Skn7 cannot be excluded. Examples 2 and 3 show that Skn7 interacts with other transcription factors. Apart from Skn7 and Yapl function in TRX2 expression, other transcriptional events are also taking place as residual TRX2 induction occurs even in the absence of both proteins. Nonetheless they are responsible for bulk induced levels of the gene. The dual role of Skn7 and Yapl in control of TRX2 is consistent with their genetic epistasis and also the Northern hybridisation data on TRX2 induction. In response to hydrogen peroxide, skn7Δ strains behave identically to yaplΔ strains, the skn7ΔyaplΔ_double mutant strain behaving like the single deletion of either SKN7 or YAP1. However, when the oxidizing agent used was diamide, deletion of the SKN7 gene has a less deleterious effect than deletion of the YAP1 gene for sensitivity to the agent and for both TRX2 and TRR1 induction. The double delete combination in this case behaves identically to the deletion of YAP1 alone. Moreover, Yapl binding increases detectably in response to diamide in contrast to Skn7. This increase in Yapl binding occurs even in the absence of Skn7. Thus there must be some separation in function of the two proteins and some activation of Yapl which is Skn7 independent . This is not observed with hydrogen peroxide treatment and indicates that Yapl is regulated differently by these treatments. The existence of the receiver domain in Skn7 suggests that it responds to phosphorylation from a histidine kinase. Asp427 is the residue within the receiver domain which would normally be phosphorylated by the histidine kinase (Bourret et al . , 1991; Parkinson, 1993) . In general, a mutation of this aspartate to asparagine is used to test functionality of the receiver, the similarity of the two amino acids ensuring the structural integrity of the domain. We and others have used a D427N mutation to show that signalling to the receiver domain is essential for the function of Skn7 both in cell wall biosynthesis (Brown et al . , 1994) and in Gl cyclin expression (Morgan et al . , 1995b) . Thus the normal behaviour of Skn7 D427N in the OSR is surprising. Krems et al (1996) used D427A and D427R mutations in examining the Skn7 response to oxidative stress and concluded that the receiver domain was essential . These particular mutations are rarely used in studying the phosphorylable aspartate residue and conceivably they might have disrupted the tertiary structure of the domain producing the conflicting results. Certainly when we deleted the receiver domain, this destroyed the function of Skn7 in the OSR. In any event, our result with the D427N mutation indicates that the dephosphorylated form of Skn7 is active in the OSR, which does not, of course, necessarily preclude a role for a histidine kinase. The only known histidine kinase in budding yeast is Slnl, with a role in the response to osmotic stress (Maeda et al . , 1994; 1995) . The recent completion of the S. cerevisiae genomic sequence has revealed no other likely histidine kinases, implicating Slnl in control of Skn7. Deletion of SKN7 does not lead to osmotic sensitivity, although whether mutation of SLN1 results in increased sensitivity to oxidative stress is not yet clear. Interestingly, Slnl appears to be constitutively active, the kinase activity becoming inactivated in high osmolarity. Thus, its cognate response regulator, Sskl, is activated when dephosphorylated. If Slnl controlled Skn7 activity and responded to oxidative stress as it does to osmotic stress, this might lead to dephosphorylation of the Skn7 receiver domain by analogy with Sskl. Since the Asp427 is not essential for regulation of OSR genes, this could direct Skn7 to these genes rather than cell wall or cyclin genes. Note, however, that Skn7 is also phosphorylated on serine and/or threonine residues (Brown et al . , 1994) so that control of the protein could involve alternative pathways without any histidine kinase involvement. Some form of modification of Skn7 may be required for its DNA binding activity, as we were unable to detect binding to the TRX2 promoter of bacterially produced Skn7. It is clear that Skn7 regulates at least two groups of genes, those requiring phosphorylation of Asp427, which include cell wall and Gl cyclin genes, and the OSR genes which do not require the phosphorylation. The role of Skn7 in regulation of this disparate group of genes remains obscure. The functional receiver domain argues that the protein transduces signals rather than acting as a non-specific transcription factor. The common feature to the genes mentioned above might be an involvement in stress. Apart from the OSR, skn7Δ strains are lethal in combination with pkclΔ, the protein kinase C gene (Brown et al . , 1994; Morgan et al . , 1995b). PKC1 regulates a MAP kinase pathway that responds to hypotonic stress (rev. in Herskowitz, 1995) and that regulates the synthesis of cell wall genes (Igual et al . , 1996) . There is also the Skn7 DNA binding domain which is, related to that of the heat shock factor. Indeed we have recently found that skn7Δ strains are sensitive to specific heat stresses. Concerning the Gl cyclin genes, certain stresses lead to a cell cycle delay in Gl (Rowley et al . , 1993) and we are currently exploring the possibility that Skn7-induced cyclin expression is necessary for recovery from this arrest. MATERIALS AND METHODS Strains and growth conditions The strains of S. cerevisiae used in this study were as follows : W303-la (a ade2-l trpl-1 canl-100 leu2-3,112 his3-ll ura3), skn7Δ (a ade2-l trpl-1 canl-100 leu2-3,112 his3-ll ura3 skn7::HIS3), W303-lb (a ade2-l trpl-1 canl-100 leu2-3,112 his3-ll ura3) , yaplΔ (a ade2-l trpl-1 canl-100 leu2-3,112 his3-ll ura3 yapl::TRPl), skn7ΔyaplΔ (a ade2-l trpl-1 canl-11 leu2-3,112 his3-ll ura3 skn7::HIS3 yapl::TRPl). Minimal and rich media used for yeast growth has been described previously (Sherman et al . , 1986) . Chemicals used in the sensitivity assays were obtained from the following manufacturers: 1.76M hydrogen peroxide from BDH, 7.7M tetra-butyl -hydrogen peroxide, diamide, menadione and cadmium sulphate from Sigma. The chemicals were used at the concentrations indicated in the relevant experiments. Yeast techniques The skn7ΔyaplΔ strain was constructed by disrupting the SKN7 gene in the yaplΔ strain. This was performed by transforming a restriction fragment carrying a HIS3 insertion in the wild type SKN7 gene into the his3- yaplΔ strain and selecting for his-t- (Morgan et al . , 1995) . Possible double mutants were then tested by PCR to confirm the disruption. The yeast transformations were performed by a derivation of the lithium acetate technique described previously (Schiestl and Gietz, 1989). /3-galactosidase assays were performed on mid-log phase cells as described previously (Guarente, 1983) . Activities are given in OD units at 420 nm per minute per mg of protein. Values represent the averages for duplicate samples in three independent experiments. Plasmid constructs An Sstl-Xbal fragment was isolated from plasmids pB-BRYl and pB-BRYl Asn427 respectively (Morgan et al . , 1995b) and ligated with the URA3 CEN vector YCplac33 (Gietz and Sugino, 1988) digested with Sstl-Xbal to create plasmids pBAMl (wild type SKN7) and pBAM2 (skn7 D427N) respectively. The C-terminal truncation derivative of Skn7 , was constructed by transposition of a derivative of the bacterial transposon TnlOOO (Morgan et al., 1996) into the plasmid YEp24/BRYl (Morgan et al . , 1995b). RNA analysis RNA was extracted from yeast strains grown under described conditions, Northern blotted and probed with various gene-specific probes as described previously (Morgan et al . , 1995b) . Equal amounts of RNA were loaded in all tracks. This was confirmed by probing all Northerns with the ACT1 gene encoding actin. The probes for the TRX2 and TRR1 genes were obtained by PCR from the yeast genome using gene-specific oligonucleotides. Oligonucleotides used for gel mobility shift assays The DNA sequence of the oligonucleotides used were as follows :- Shift 1, 5' CATTATTGATGTGTTATTTAAAGATATCG 3' (SEQ ID NO : 4); Shift 2, 5' CATAACTTGAGTGCCAGTGAAT 3' (SEQ ID NO: 5); TrxΔl, 5' TATACTGATATCCCTCTTTCGG 3' (SEQ ID NO: 6); TrxΔ2 , 5' GAAAAGAGCCACCTTGTAGG 3' (SEQ ID NO: 7); TrxΔ3, 5' TTTCCAGCCAGCCGAAAGAGG 3' (SEQ ID NO : 8); TrxΔ4 , 5' TCCCTCTTTCGGCTGGCT 3' (SEQ ID NO: 9); TrxΔ5, 5' TCCCTCTTTATGCTGGCTGGAAACTG 3' (SEQ ID NO: 10); TrxΔ6, 5' GCTGGCTGGAAACTGAACGCGC 3' (SEQ ID NO: 11); WTMCB, 5' TCTTTTCTTACTAAGCGCGTTCAGTTTCCAGCCAGCCG 3' (SEQ ID NO: 12); and MUTMCB, 5' TCTTTTCTTACTAAGCTAGTTCAGTTTCCAGCCAGCCG 3' (SEQ ID NO : 13 ) . Probes were obtained by PCR using the combinations of these oligonucleotides described below. PCR was performed on the TRXLACZ plasmid described in Figure 2. The following oligonucleotides were used to obtain probes 1/2, oligonucleotides Shift 1 and Shift 2; TrxΔl, oligonucleotides TrxΔl and Shift 2; TrxΔ2 , oligonucleotides TrxΔ2 and Shift 2 ; TrxΔ3, oligonucleotides TrxΔ3 and Shift 1; TrxΔ4 , oligonucleotides TrxΔ4 and Shift 2; TrxΔ5, oligonucleotides TrxΔ5 and Shift 2; TrxΔ6, oligonucleotides TrxΔ6 and Shift 2 ; WTMCB, oligonucleotides WTMCB and Shift l;and MUTMCB, oligonucleotides MUTMCB and Shift 1. Protein extracts and gel mobility shift assays Yeast cell extracts were prepared from midlog cultures. Cell pellets were vortexed for 8 x 30s with glass beads in 100-200ml of 200 mM Tris-HCl pH 8.0 , 10 mM MgCl 2 , 10% glycerol and protease inhibitor mix (PI mix- 100 μg/ml phenylmethyl sulfonyl fluoride, 2 μg/ml aprotinin, 2 μg/ml leupeptin, 2 μg/ml pepstatin A, 50 μg/ml TLCK and 100 μg/ml TPCK) . After centrifugation, protein concentrations were determined and extracts were stored at -70 * C. EMSAs of DNA-protein complexes were conducted as follows. 7μg protein extract was incubated with 0.5 ng 32 P 5' -end labelled DNA fragment in 10 ml of 25 mM Tris-HCl pH 7.5, 50 mM NaCl, 2 mM EDTA, 7 mM MgCl 2 , 10% glycerol, PI mix, 1 mg/ l 3- [ (3 -cholamidopropyl) dimethyl- ammonio] -1-propanesulfonate (CHAPS) and 50 ng/ml poly(dI:dC) at room temp for 15 min and on ice for a further 15 min. Competition experiments included a 50-fold molar excess of unlabelled competitor DNA over labelled probe. In EMSA supershift experiments, anti-Skn7 or anti-Yapl polyclonal serum was added to binding reactions at a serum dilution of 10 "2 . Reaction mixes were loaded directly onto a 4% (37.5:1) non-denaturing polyacrylamide gel and electrophoresed at 200 volts in 0.6XTBE buffer at 7 * C until free DNA reached the bottom of the gel . The gel was dried onto Whatman 3MM paper and autoradiographed . EXAMPLE 2 . The yeast Heat Shock Factor, Hsflp, is also central to the induction of another set of stress-inducible genes, namely the heat shock genes. These two regulatory trans-activators, Hsflp and Skn7p, share certain structural homologies, particularly in their DNA-binding domains and the presence of adjacent regions of coiled-coil structure, known to mediate protein-protein interactions. Here, we provide evidence that Hsflp and Skn7p interact both genetically and physically. Furthermore, we show that Skn7p can bind to the same regulatory sequences as Hsflp, namely Heat Shock Elements, and that strains deleted for the SKN7 gene and containing a temperature-sensitive mutation in HSFl are hypersensitive to oxidative stress. A number of heat shock genes have been identified that are jointly regulated by Hsflp and Skn7p in response to oxidative stress. Our data suggests that Skn7p and Hsflp interact to achieve maximal heat shock gene induction in response specifically to oxidative stress . Oxygen, in the form of superoxide anion (02-) , hydroxyl ion (0H-) , and hydrogen peroxide, cause damage to nucleic acids, cell membranes and proteins (Halliwell and Gutteridge, 1984; Halliwell, 1994) . Yeast, in common with all other organisms, have evolved protective mechanisms to survive in the presence of these byproducts of aerobic metabolism and can mount distinct adaptive responses to different sources of oxidative stress (reviewed in Ruis and Schuller, 1995; Jamieson, 1992) . For example, the Cu,Zn-linked superoxide dismutase, encoded by the S0D1 gene, detoxifies superoxide anion to hydrogen peroxide and catalase, encoded by the cytosolic CTT1 gene, can catalyse the breakdown of H 2 0 2 . Other free radical scavengers in the cell include glutathione, ascorbic acid and thioredoxin. Of the many proteins which are induced under adverse environmental conditions, perhaps the best understood are the heat shock proteins (reviewed in Lindquist and Craig, 1988; Morimoto et al.,1990) . The major heat shock proteins have been classified according to their molecular weight- Hspl04, Hsp90, CO LO to to μ> μ> LΠ o LΠ o LΠ o LΠ *] H CO SD $. fd l SD ii cr SD Ω tr Φ Ki tr CQ q LQ 3 X Hi CO SD tn *d X SD μ- SD SD q a LO q Ω tr φ rr X Ω Φ Φ 3 0 φ X Φ φ 3 tr S 0 φ Φ tr a q a CQ o 0 tr q 3 Ω a q H LO i q μ- CQ ii -β- q CD Φ ft ti SD q SD 0 tr 3 ft Φ a i ft •d ii Φ ft q ft φ q CTi Φ μ- Ω Ki Φ 1—1. μ- μ- a ii q Φ tr q O Ω Φ q Φ μ- Ω Φ μ> SD a μ- tr a tr ~. CO <5 tr 0 O <! ft rrj Φ a SD • ?r ri tr SD μ- q CQ μ- o μ- i μ- Ω SD ^ tn Φ CO SD a CQ SD SD a Ki φ CQ CQ 3 CQ Φ φ ,---. q 3 μ- CO CQ > LQ φ LQ μ- CQ Φ CQ ii CO q q CQ 3 q Φ ϋ SD tr μ- tr Ω CQ ii Φ ft 0 - Ki •d tr q ϋ Ki Ω Ω Φ tr μ- M ft μ- CO Φ tr q 0 < 0 Φ Φ a Ki Φ ft a a Ω i Φ q ii μ- 1 3 ii 1 q tr 0 Φ <J Φ 0 CQ SD Φ Ω Φ μ> Ω ii a SD < Ω 0 Φ Ki q μ- μ- φ 0 φ o 0 3 3 CO φ CO a Φ 3 ft 77 LD rV Φ Φ μ- μ- q q ii a & ϋ ft 3 φ μ- μ- ft q N Ki SD 0 a Ki 00 <! CQ co μ- φ *< μ- φ ft a 0 SD Φ Ω <! μ- φ i Hi μ- q μ- ii 3 M 0 rV ft Ω q £ SD tr oo φ μ- 3 0 3 μ- μ- q CQ μ- φ CQ Ω SD SD q q q Φ 1 3 0 0 SD Φ μ 1 μ- 3 0 -• CQ ^ -- SD ft Φ X a SD q H Φ • •d 0 a ^ 3 CQ μ- CD Hi φ CQ q 3 q ft LO q ft CQ φ μ- rO a ii ft Φ μ- μ- φ Ω ^ SD SD CQ Hi μ- SD 0 CQ μ- 3 μ- CO SD Φ 00 tr Ki CO 3 SD Φ X 0 SD 0 3 0 ft q a ,0 SD ii 3 a 3 Φ < Φ μ- ft q CD q tr 0 φ Φ 3 μ- q Hi CO a CQ ft > i i LO X 0 ft CO CQ LQ a φ a 3 0 -• μ- tr 0 ti 3 - tr a μ- 3 CO Hi LO ii n 3 LD φ Φ q • CQ q q μ- H CQ q SD a Ω CO q a 0 q VO q 0 i CQ Ω LO Φ 3 μ- 3 ii CO LQ Φ CQ CO CQ CQ ^ Φ CQ X 3 Ω ω q tr a ^ ~- ii tr iF> tr SD - tr -~^ Ω 3 φ φ μ- q a 0 ii Ul • CO 0 0 Φ Ω CO Φ — φ Φ μ- SD . X μ- M ft q ^ --. 3 LD co s. LQ ii q Ki SD —-. Hi Φ φ ii Ω CO SD Ω • CQ LQ ≤ O Ul 3 3 φ φ ϋ CO Φ LO φ CQ CQ ii 0 q SD tn μ> μ> 0 Λ CQ P7 φ Φ a i Ω CQ ^ tr Φ Hi Ki q "d X SD ι-3 CQ Φ> μ- φ φ Φ SD l μ- a co Ki LO q si φ SD Ω 0 Φ > 0 μ- μ- ϋ tr en q Φ Φ •ϋ 3 o — - 3 3 CQ H 3 q 0 ft M oo μ- φ ii < - LQ ii tr CQ 3 μ > 0 Φ o tr Φ 3 ii CO M 3 <! q - CO 1 3 - μ- to ft 3 - SD φ • Φ co 3 a Ω LO CO a ~ φ - ft Φ Ω - φ 0 q Ω q 25 CQ CQ -• φ Ω Ω a N 1 Φ σ LO q Φ tn Φ CQ ϋ CQ 3 H tr q μ> ii ii 0 φ — — Φ H q Φ Ki ft g μ- si to CD CO tn . tr μ 1 3 CD μ- — q a < φ tr o μ- SD a Hi CO LO 0 CO Φ i a q ft ^ . 0 O CO Φ LO q μ- Ό μ- φ SD LO •d a CQ SD 0 0 Hi O LO i q 0 ii Φ μ- Φ q SD -J Ki SD LO 0 q ≤ 0 SD ft q q o q CQ CQ o CQ a ii 0 3 H ^ μ- q μ- i CQ ii q ts o t q n 0 a μ- μ- a ft ϋ • — μ- Φ a a CQ a a — a SD Φ 3 SD q ' φ Φ ft C Hi 0 q Φ ft μ- μ- tr • 0 SD ϋ SD ft Φ a P • φ Ω LQ 0 q q < ii Hi CD • μ- a tr ft μ- a 3 a q μ- μ > i ft L 3 i • q Hi μ- Φ μ- CO Ω SD ςu tr tn 3 μ- q Φ ^ X SD μ- 3 LD q Ω t? Hi Ω φ ~ μ- Φ 0 Φ 0 3 ti 0 CD Hi 3 SD μ- q ϋ Φ 0 0 Φ oo i 0 SD μ- tr 0 CQ 0 X q a q Ξ SD ft Ω Hi a μ- ii SD q 0 tr μ- ii a LD μ- a a 3 a Φ ft •d μ> a Ki a- ii Φ ii q μ- 1 ?r μ> Φ Ω q 0 a Φ Q Ki φ SD SD SD — 3 CQ ft Hi Φ 0 LD 0 φ 0 SD ft φ SD X tn •d D q tr <: SD tr <1 Ω a • Φ q q H tn ft a LD SD l Hi 3 μ- CQ i *d 0 Φ Φ q ft q 0 Φ q ft ii μ- rO tr n CQ CO to CQ a tn CO μ- φ a 0 •d Φ O i μ- 0 Φ μ- CQ ii μ- 3 Hi μ- q φ Φ a Hi q Φ — ' SD i CO ft Ki a X Hi 1 CO 3 3 CQ i SD SD 0 •d •» <! 0 q tr Ω a a H X μ- Φ Ki Φ 0 Ki SD to Ki SD CD tn Ω a tr S ft tr Φ q tr O Ω q q 3 μ> a ti po ti rr tr 0 ϋ Φ 3 Ki CQ co q 0 Φ q μ- i Ω μ- SD H •d q 0 tr CQ q o SD q SD φ Φ Λ a q CQ i 0 M μ- (D ii <! μ- Hi Φ Ki 0 <! 3 0 ,— -. ii tr 0 q O o q CD CQ D o CD μ\ q 3 0 - < a * 5 μ- 0 μ- 0 ii Φ 3 CO 0 φ tr a a SD CD O Φ q ii φ 3 0 to 3 Ω CQ SD ft ft a Ω a 0 Ki 0 M q Φ Q SD Hi i a 0 φ SD SD - Φ rd 0 0 - q SD φ SD μ- q ► : μ> q Φ φ SD tr Φ SD Φ ft £. ft CO O μ- N SD q a μ- o q a Hi q a Φ Φ vo Φ μ- CQ q < q 3 ft q tr LQ X 1 SD Φ q CO 0 1 μ- Ω a μ- Φ μ- X cr oo i H a CQ μ- SD q μ- Hi φ Ω 0 l rO < 3 ϋ SD 0 a q 0 Φ 3 0 3 0 -o. CO ϋ φ CQ q ii SD Ό 0 q 0 Hi Ω Hi SD SD ft CQ μ- 3 Φ a Ω a ii - a ~. ^ —- a tr CO μ- 3 Φ 0 ii tr q Φ a r-ι a 0 25 tn q 0 φ ^ ii a μ- rV H μ> CQ ϋ 0 ft SD CO < LD κ> q μ- 3 Hi 3 0 SD μ- tr CQ a SD ft s; Hi O CQ μ- Ω Φ q tr q μ- q Φ *d LO 0 Φ CQ q CO 0 μ- Hi CO 0 SD μ- ft 77 a • • Hi SD 77 X X Φ a Φ 3 . μ- ii to tr i q a tr q Hi X a 3 CD ft φ 0 q ~ 3 μ> (D SD SD μ- LQ ~ q tr • 0 SD X . ii ii Ul SD CQ SD a i cr 0 SD 1 O μ- Ω 0 q 3 3 a φ q CQ Φ •d 0 Φ q Hi φ ^ --. q Φ CQ CQ > a lfc> - — tn CQ q Hi Φ CQ O μ > q SD Φ 3 q 0 a CQ tr μ> ii CQ O Φ CQ 0 Φ q LO ^ — CO CO Hi O μ- q μ- SD SD Hi Ω CQ O φ O μ- 0 0 Φ φ 3 tr LO q ^ tr LO < 1 a CQ SD SD tr 3 ri a a CQ φ LΠ tr μ- μ> 0 -J Φ CQ Φ CQ a Φ LQ Ki SD CQ - — Φ 3 Ω ii 0 Φ 0 3 Φ SD -. ft PT X ii a ft q O been shown to protect the cell against heavy metals, such as copper and cadmium, through its activation of the copper metallothionein gene, CUP1 (Silar et al . , 199-1; Sewell et al . , 1995) . It was subsequently demonstrated that Hsflp becomes phosphorylated in response to the superoxide generator menadione, and this modification of Hsflp could be correlated to the transcriptional activation of CUP1 by oxidative stress (Liu and Thiele, 1996) . Hence, like Skn7p, Hsflp also plays a critical role in the cellular defense against oxidative stress. The Skn7p protein contains a region with a high degree of homology to the receiver domain of response regulator proteins found in bacterial two-component signal transduction systems (Brown et al . , 1993; Morgan et al . , 1995). In these systems, a membrane bound sensor histidine kinase can phosphorylate a conserved aspartate residue within the receiver domain of its cognate response regulator (reviewed in Parkinson, 1993; Stock et al . , 1989) . This phospho-aspartate form of the response regulator can then carry out a function appropriate to the incoming signal, usually the transcriptional activation of a specific set of genes. Toward the N-terminus of Skn7 there is a region of extensive homology to the DNA binding domain of Hsflp. This domain is separated from the receiver motif by a region of coiled-coil structure, again similar to the leucine zipper domain of yeast Hsflp. Given the degree of conservation in the structure of both the DNA-binding domain and the leucine zipper region of SKN7 and HSFl genes, it was of interest to determine whether Skn7p interacted with Hsflp, and to establish the significance of this potential interaction in the yeast stress response. Skn7Δ cells are sensitive to acute heat stress. Deletion of the SKN7 gene does not confer a heat shock sensitive phenotype when cells are shifted from 25 'to 37 'C. (Morgan et al . , 1997) . However, given the high degree of homology between the DNA binding domains of Skn7p and Hsflp, we investigated the effect of a skn7Δ mutation on cell viability under acute heat shock at 51 'C. Mid-log cultures of W303-la and isogenic skn7Δ cells were grown in YPD at 25°C and an aliquot shifted to 51°C. Samples were taken at 2 , 4, 6, 8, 10 and 14 hours, diluted and spread onto YPD agar to assess cell viability. Cells deleted for the SKN7 gene were found to be some ten times more sensitive to the lethal effects of acute heat stress than the isogenic wild type strain. It has been reported, however, that the main cause of cell death under these conditions is due to the generation of toxic intermediates of oxygen metabolism (Davidson et al . , 1997). Since SKN7 is required for cell survival under conditions of oxidative stress (Krems et al . , 1996; Morgan et al . , 1997) we determined whether SKN7 had a role, together with HSFl, in the induction of heat shock gene expression in response to oxidative stress. SSAl-lacZ induction by H 2 0 2 requires SKN7 To investigate the potential role of the SKN7 gene in the induction of heat shock protein expression we assessed expression of a Hsp70-LacZ reporter construct in wild type and skn7Δ cells. The SSA1 gene encodes a major isoform of the yeast Hsp70 protein which is abundant under non-stressed conditions and is strongly induced by heat shock (Craig et al . , 1985) . The plasmid pZJHSE2-137 contains a heat shock element, HSE2 , from the SSA1 promoter fused to the β-galactosidase coding sequence (Slater and Craig, 1987) . This HSE2 sequence is responsible for the majority of both basal and heat shock induced expression of SSA1 (Slater & Craig, 1987) . β- galactosidase assays were carried out on wild type W303-la and isogenic skn7Δ cells containing the reporter plasmid following treatment for one hour with hydroperoxide. In wild type cells this resulted in an eight-fold induction of -galactosidase activity (Table 1) . In marked contrast, in the skn7Δ strain induction in response to oxidative stress was abolished. However, induction of HSE2 driven LacZ activity in response to a temperature shift from 25 * C to 37 * C was unaffected in the skn7Δ strain compared to the isogenic wild type parent . (Table 1) . It has been proposed that Yapl, which has been shown to interact with Skn7 at the TRX2 promoter (Morgan et al . , 1997), is also required for induction of the HSE2-LacZ reporter in response to hydrogen peroxide (Stevens et al . , 1995) . However, we have found no evidence that yaplΔ affects the hydroperoxide induction of the SSA1 HSE2-LacZ reporter construct. Our data suggests that Skn7 can function through HSE2 of the SSA1 gene and, while dispensable for heat shock induction, may be required for full expression of the SSA1 gene under conditions of free radical stress . Table 1. Induction of SSAl-Lac Z in response to hydrogen peroxide requires Skn7. Wild type and isogenic skn7Δ cells containing pZJHSE2-137 were assayed for /3-galactosidase levels before (-) and after (+) the addition of butyl hydroperoxide to 0.6 mM for one hour. For the heat shock experiment, cells were grown at 25 * C and transferred to 37 * C for one hour. β-galactosidase activity is expressed as Δ OD420/min/mg protein. Values are averages of duplicate samples from two independent experiments. Skn7p can specifically bind the HSE2 element from the SSA1 promoter. The DNA-binding domain of Skn7p (residues 87-150) is highly homologous to that of Hsflp) . To determine whether Skn7p can recognise and bind specifically to the same recognition sequence as Hsflp, we performed electrophoretic mobility shift assays with E.coli expressed 6His-Skn7p. The 6His-Skn7 fusion protein was purified on a Ni2+-NTA agarose affinity column (see Materials and Methods) and added to a 26bp probe derived from the 137bp HSE2 region of the SSA1 promoter. Electrophoretic moblity shift assays were performed using E. coli -expressed and affinity purified 6His-Skn7 protein with a probe comprising the 26bp HSE2 region of the SSA1 gene (see Materials and Methods) . Specificity of binding was assessed by the addition of cold HSE2 probe at 5 ' , 10 and 50 -fold molar excess, compared to the addition of a mutated HSE (MUT-HSE) at 10, 50 and 100-fold molar excess. Either polyclonal anti-Skn7 antibody (c--Skn7) at 1/100 dilution, or pre-immune serum at the same concentration was added to the binding reaction 15 minutes prior to the addition of labelled probe. Free probe without addition of protein was used as a control. The 6His-Skn7 fusion protein clearly bound the HSE2 sequence. To confirm the presence of the 6His-Skn7 protein in the retarded complex, polyclonal antiserum to the protein was added to the reaction mix. The retarded complex formed by the HSE2 probe and 6His-Skn7 protein was super-shifted by antibody to Skn7p whereas no effect was observed by the addition of pre-immune serum at the same concentration . To demonstrate the specificity of this binding, competitive binding asays were performed with the native HSE2 oligonucleotide and a mutated probe, MUT-HSE2, in which the G and C positions of the consensus HSE sequence nnGAAnnTTCnn (SEQ ID NO: 15) were changed to A and T, respectively. These mutations within the consensus have previously been shown to abolish binding of the heat shock factor Hsflp (Park & Craig, 1989) . The native 26-mer heat shock element competes efficiently for the binding of Skn7p at a 10 -fold molar excess. However, the mutated version of the heat shock element does not compete for binding of the 6His-Skn7 protein even at a 100-fold molar excess. These results have been confirmed using an E. coli expressed truncation of the Skn7p containing just the SKN7 DNA-binding domain alone fused in frame to GST, and also with 6His-Skn7 protein purified from yeast. The above results demonstrate that Skn7p binds to heat shock elements with a specificity similar to that of Hsflp, as might be expected from the extent of homology within their respective DNA-binding domains . Genetic interactions between SKN7 and HSFl. Both Hsflp and Skn7p have previously been shown to play a role in the activation of stress-responsive gene expression under conditions of free radical stress (Liu and Thiele, 1996; Krems et al., 1996; Morgan et al . , 1997). To establish whether they may cooperate in the yeast oxidative stress response we initially examined the interaction of a deletion of the SKN7 gene with a mutation in the HSFl gene. HSFl is essential so that a temperature-sensitive allele, hsfl-m3 (Smith and Yaffee, 1991) , was used. A skn7Δ derivative of W303-la was crossed with the hsfl ts strain MYY385 and a hsfl ts spore clone containing the HIS+ marked skn7 deletion, strain DR20-2b, was selected for further study. Strains MYY385 and DR20-2b were then tested for growth at various temperatures. As expected both strains grew at the permissive temperature of 25 * C and neither grew at the restrictive temperature of 37 "C. However, at an intermediate temperature of 34 "C whilst the hsfl ts strain formed colonies, the double mutant failed to grow. Transforming strain DR20-2b with a CEN version of the SKN7 gene restores growth at 34 * C, indicating that the increased temperature-sensitivity of DR20- 2b is due specifically to the deletion of the SKN7 gene rather than genetic background effects. Thus, deletion of SKN7 exacerbates the growth defect of the hsfl ts mutant allele. We then assessed whether high copy expression of SKN7 could suppress the growth defect associated with the hsfl s allele at 37 * C. The hsfl ts strain was transformed with either the vector YexH-SKN7, which expresses high levels of Skn7p under the control of a galactose inducible promoter, or the empty vector alone. The plasmid YexH-SKN7 had previously been shown to rescue the hydroperoxide sensitivity of a skn7Δ strain, thus confirming the functionality of the fusion. The hsfl ts cells expressing high levels of Skn7p displayed strong growth at 35.5 * C, where the hsfl ts strain containing the empty vector alone could not form colonies (Fig. 4B) . However, over expression of SKN7 failed to rescue the hsfl ts strain at 37 * C which is not surprising given the pleiotropic nature of the hsfl-m3 allele (Smith and Yaffe, 1991) . Therefore, whilst there is clearly some overlap between the function of SKN7 and HSFl in the cell, the response regulator cannot completely substitute for HSFl and so fails to rescue the pleiotropic effects of the hsfl-m3 mutation at 37°C. An increased sensitivity to hydroperoxide was also evident in the skn7Δhsfl ts strain DR20-2b " relative to either the skn7Δ or hsfl s strains alone. Sensitivity to hydroperoxide was assayed for W303-la and isogenic skn7Δ; MYY290 (HSFl) and the hsfl ts derivative MYY385 and DR20-2b, the hsfl ts skn7Δ spore clone derived from a cross between W303-la skn7Δ and MYY385. Cell suspensions in saline were streaked on a YPD plate (Control) and a YPD plate on to which lμl 7.7M butyl -hydroperoxide was placed on a disc of Whatman 3MM paper positioned in the centre of the plate (+H 2 0 2 ) . The single copy plasmid YCplaclll (Gietz and Sugino, 1998) was used to introduce the SKN7 gene (+SKN7) or a D427N mutant (+SKN7-DN) in which the conserved aspartate D427 residue was mutated to asparagine. Plates were incubated for 2 days at 30 * C. Introduction of SKN7 on a CEN plasmid reversed this increased sensitivity, indicating that the phenotype was not due to a general effect of genetic background. Thus, when Hsflp activity is defective, survival of skn7Δ cells under stress is further compromised. Both Skn7p and Hsflp therefore contribute to cell survival during oxidative stress, perhaps through a related, or shared, pathway. Previous data on Skn7p indicated that phosphorylation of the conserved aspartate (D427) within the receiver domain was not required for survival under conditions of oxidative stress (Morgan et al . , 1997) . Significantly, the D427N version of the SKN7 gene was also found to reverse the hypersensitivity of DR20-2b to hydroperoxide. Furthermore, the D427N mutated form of SKN7 also restored HSE2-LacZ expression in skn7Δ suggesting that phosphorylation of D427 is not required for Skn7p function through heat shock elements. In contrast, Skn7D427N fails to activate CLN1 and CLN2 expression in a swi4tsswi6Δ background (Morgan et al . , 1995) or rescue the cell wall assembly defect of the kre9Δ mutant (Brown et al . , 1994) . Hence, phosphorylation of the receiver domain, presumably by the only histidine kinase in S. cerevisiae, Slnlp, can direct the activation of Skn7 function to different target genes (for further discussion of this point see Morgan et al . , 1997) . Skn7p and Hsflp interact physically To extend the genetic evidence suggesting some interaction between Skn7p and Hsflp, we undertook co-immunoprecipitation analysis of whole cell extracts to determine whether these proteins physically interact in vivo. Thus, 1 mg of whole cell extracts were incubated with polyclonal antiserum to Skn7p and subjected to Western analysis with antiserum to Hsflp. Immunoprecipitates from wild type extracts contained a protein which reacted specifically to Hsflp antibody and co-migrated with Hsflp. In contrast Skn7 antibody failed to immunoprecipitate Hsflp from the control skn7Δ extract. Consideration of the Hsflp levels in the input material suggest that only a small proportion of the total cellular content is bound to Skn7p. To confirm this result we examined associaton of Hsflp with 6His-tagged Skn7p . Extracts (1 mg) prepared for galactose grown cells transformed with YexH-Skn7, expressing the 6His- Skn7 fusion protein from the galactose promoter, were mixed with Ni2+-NTA affinity matrix. Subsequent Western analysis of the proteins associated with the nickel-bound Skn7p indicated that Hsflp co-purified with the 6His-tagged Skn7 protein. In a control no co-purification of Hsflp with Skn7p was detected when the procedure was repeated using a cell extract which did not contain the 6His-Skn7 protein. Once again these experiments suggest that only a small proportion of the input Hsflp is associated with Skn7p . However, in vivo Hsflp binds as a trimer and presumably Skn7p forms one of the subunits of this complex. We cannot exclude the possibility that Skn7p might readily be displaced from this complex by Hsflp molecules in the crude extract. Nonetheless the data does confirm their physical association which could form the basis for a cooperative interaction in the oxidative stress response. Induction of heat shock gene expression by hydrogen peroxide is dependent on both Hsflp and Skn7. We next explored the possibility that Skn7p, through its interaction with Hsflp, could also have a role in the induction of heat shock proteins in response to oxidative stress. Northern analysis was therefore carried out on a number of heat shock genes in the hsfl ts strain MYY385 and its wild type parent MYY290, skn7Δ and its isogenic parent W303-la, and the hsfl s skn7Δ strain DR20-2b, derived from a cross of the above strains. Because the temperature-sensitive hsfl ts mutation results in lethality at 37 * C, the experiment was carried out at an intermediate temperature of 30 * C. Total RNA was isolated from mid-log cultures, untreated and after exposure to hydroperoxide for up to one hour (see Materials and Methods) . In wild type W303-la cells HSP82 and SSA1 are induced two-fold after 45 minutes in hydroperoxide. However, this increased expression does not occur in the isogenic skn7Δ strain. The more dramatic ten-fold induction of HSP12 in W303-la is also abolished by the deletion of the SKN7 gene. The hsfl ts mutation at the semi-permissive temperature of 30 * C has the effect of lowering the level of HSP82 and SSA1 expression, relative to the HSFl strain, without dramatically affecting the overall induction of these genes by hydroperoxide. On the other hand, the hsfl ts lesion virtually eliminates the six-fold induction of HSP12 by hydroperoxide observed in the HSFl parental strain. When the double mutant strain DR20-2b was examined, a pronounced additive effect of the skn7Δ and hsfl ts mutations was apparent. The peroxide- induction of all three heat shock genes tested was essentially abolished. Indeed, in two cases the expression of heat shock genes in the double mutant is actually reduced 2 -3 -fold after one hour in the presence of hydroperoxide. Similar effects of the two mutations on the hydroperoxide induction of HSP26 and HSP104 were also evident. There appears, therefore, to be a synergistic effect of skn7 deletion with the hsfl ts mutation on heat shock gene expression following oxidative stress. To examine the specificity of Skn7p function in heat shock protein expression, we compared the heat shock induction of a number of heat shock genes " in W303-la and isogenic skn7Δ cells with their induction in response to oxidative stress. Whereas the skn7 deletion had no effect on heat shock induction of SSA1, HSP26, and HSP104, their induction by hydroproxide was significantly reduced. SKN7 is therefore specifically required for the oxidative stress induction of heat shock genes and is not required for their heat shock-mediated induction. This is in accord with Table 1 and our original observations that skn7Δ cells show no increased sensitivity upon a temperature shift from 25 * C to 37 "C when compared to the isogenic wild type strain (Morgan et al . , 1997). Both Skn7p and Hsflp have previously been shown to play important roles in the cellular response to oxidative stress. The response regulator Skn7 cooperates with the yeast AP-1 homologue Yapl on the promoter of the thioredoxin gene, TRX2 , and activates transcription of the gene in the presence of hydrogen peroxide (Morgan et al . , 1997). Similarly, Hsflp- dependent activation of the CUP1 metallothionein gene occurs in yeast cells treated with the superoxide generator menadione (Liu and Thiele, 1996) . Here, we provide evidence that in yeast Skn7p and Hsflp interact to activate heat shock gene expression in response to oxidative stress. We have shown that the two proteins physically interact and have identified target genes jointly regulated by these activators in response to free radical stress. Physical and Genetic interactions between Hsflp and Skn7. A genetic interaction between Hsflp and Skn7p was first indicated by the lowered restrictive temperature of a hsfl ts skn7Δ strain relative to the hsfl ts strain alone. Furthermore, the growth defect of the hsfl s strain could be partially suppressed by high copy expression of the SKN7 gene. That this suppression is partial indicates that Skn7 can fulfil some but not all of the functions of Hsflp in the cell. This is not surprising given the pleiotropic nature of the hsfl-m3 mutation (Smith and Yaffee, 1991) . Consistent with the above data we found that by reducing the activity of Hsflp through mutation, sensitivity of skn7Δ cells to hydroperoxide was greatly exacerbated. This hypersensitivity was reversed by simply restoring SKN7 function in the same strain. The above genetic data strongly indicates some form of in vivo association between Hsflp and Skn7p . Consistent with this conclusion, we found that Skn7p and Hsflp directly interact. Pull down assays showed that Hsflp physically associates with 6His-tagged Skn7p. This association was confirmed by co-immunoprecipitation analysis which demonstrated that Hsflp and Skn7p directly interact under physiological conditions in vivo. These transcription factors could therefore cooperate in a common pathway to activate a specific subset of genes in response to oxidative stress. Skn7 is required to activate heat shock gene expression specifically in response to hydroperoxide Preliminary studies using a SSAl-LacZ fusion construct indicated that Skn7 was required for HSE-mediated LacZ induction in response to hydroperoxide but was not required for the heat shock induction of the reporter (Table 1) . Northern analysis of heat shock gene expression in wild type and skn7Δ cells confirmed that SKN7 was not required for induction of these genes in response to heat shock. For example, when cells are shifted from 25 * C to 37 * C, the heat shock induction of HSP104 in skn7Δ cells was identical to that in the isogenic wild type strain. In contrast, the strong induction of this gene by hydroperoxide in the wild type was almost abolished in the isogenic skn7Δ strain. The skn7Δ mutation also greatly reduced induction of HSP26 and SSA1 by hydroperoxide without affecting their response to heat shock activation. Thus, our Northern analysis clearly shows that Skn7 has no role in the heat shock induction of gene expression, rather it is specifically required for activation of HSE-driven gene expression in response to oxidative stress. Northern analysis also indicated that the hsfl-m3 mutation alone causes a reduction in expression and a marked delay in the induction of HSP82, SSA1 and HSP12 in response to hydroperoxide without abolishing the induction completely. The skn7Δ mutation alone similarly has a dramatic effect on the activation of these genes in response to hydrogen peroxide with perhaps a greater impact on induction than the hsfl ts mutation. The effect of the skn7Δhsfl ts double mutation was to reduce basal expression of heat shock genes in addition to abolishing the induction response. For instance, whilst the induction of both HSP82 and SSA1 expression in response to hydroperoxide was almost eliminated in skn7Δ cells, in the hsfl ts skn7Δ strain expression of these genes actually decreased significantly after one hour in the presence of hydroperoxide . The two proteins therefore cooperate to achieve maximal heat shock gene expression in the oxidative stress response. This is in marked contrast to the Yaplp-Skn7p interaction which is clearly epistatic (Morgan et al . , 1997). The requirement of Hsflp and Skn7p for activation of the HSP12 and HSP26 genes by hydroperoxide is intriguing given that induction of these genes by a variety of other stresses has been shown to be mediated through STRE sequences by the H0G1 MAP kinase pathway (Varela et al . , 1995; Martinez-Pastor et al . , 1996). We note, however, that induction of these and other STRE-regulated genes specifically in response to hydrogen peroxide is not significantly affected by mutations in either Msn2 or Msn4 (Schϋller et al . , 1994) . It appears, therefore, that regulation of HSP12 and HSP26 in response to oxidative stress is mediated by Hsflp/Skn7p whereas activation in response to other stress conditions, such as osmotic shock, is regulated via the H0G1 pathway. In this context, it is interesting to note that the HOG1 pathway is itself regulated by the SLN1 histidine kinase, which, as the only such protein in S. cerevisiae, may well act upstream of the Skn7 response regulator. Although the HOG1 pathway is thought only to be involved in adapting to changes in extracellular osmolarity (Schϋller et al . , 1994), it is tempting to speculate that Slnlp could regulate Skn7p activity through HSE's in response to hydrogen peroxide, and independently regulate STRE-dependent activation via Msn2 and Msn4 in response to other stress conditions (for further discussion on the possible connection between Skn7p and Slnlp in the oxidative stress response see Morgan et al . , 1997) . Structural homology between Skn7 and Hsflp. With regard to the cooperative nature of the Skn7p-Hsflp interactions in the oxidative stress response, the structural similarities of the two proteins and specific regions of homology are of particular interest. Hence, the Skn7p DNA- binding domain, with over 50% identity to the corresponding 89 residue domain of Hsflp, is separated from its C-terminal trans-activation domain by a region of coiled-coil structure, similar to the general structure of the Hsflp protein. We have shown through electrophoretic mobility shift assays that both 6His-tagged Skn7 purified from yeast and E. coli expressed GST- Skn7p can bind to a 26bp probe derived from the HSE2 regulatory region of the SSA1 promoter. This binding is of a similar specificity to that of Hsflp, insofar as mutation of the GAAnnTCC sequence to AAAnnTCT ablates binding of both Hsflp and Skn7p . Previously we have identified a regulatory site within the TRX2 promoter through which Skn7 can act (Morgan et al . , 1997) . The site contains the sequence CCGAAA where mutation of the CG nucleotides to AT was found to reduce binding of Skn7p by 20-fold. The common motif between this regulatory sequence and HSE's being the GAA triplet, three inverted repeats of which in the sequence nnGAAnn constitute a consensus heat shock element. Although the exact consensus binding site for Skn7 has not been established, the triplet GAA evidently represents a potential core recognition sequence. In terms of sequence specificity of Skn7p recognition of HSEs versus that of Hsflp, there is one notable divergence between their otherwise highly conserved DNA-binding domains. This occurs at the last residue of the c-3 sequence recognition helix of Hsflp (Harrison et al . , 1994), where the invariant M58 and G60 residues are replaced by K58 and D60 respectively in the Skn7 protein (with residues numbered according to Fig. 2) . These substitutions may have a significant effect on DNA- binding specificity or stability of Skn7p relative to Hsflp since M60 of Hsflp has been proposed to contact the DNA (Damberger et al . , 1994) . All other residues proposed to contact the DNA are, however, conserved between Skn7p and Hsflp. The other region of structural homology between these two proteins lies between residue 222 and 257 of the Skn7 protein. This stretch contains five heptad repeats, with hydrophobic residues at positions 1 and 4, and polar residues elsewhere in the repeat units, characteristic of regions which form coiled- coil structures (reviewed in Lupas, 1996) . Hsflp contains six heptad repeats which have been shown to mediate trimerisation of the protein through the formation of triple-stranded a- helical coiled coils (Sorger and Nelson, 1989; Peteranderl and Nelson, 1992, Rabindran et al . , 1993). These coiled-coils are also known to mediate hetero- and homodimerisation, for example, of the yeast GCN4 member of the bZIP transcription factor family (Harbury et al . , 1993). We are currently exploring the possibility that it is through the formation of heterodimers and/or heterotrimers, that Skn7p and Hsflp mediate activation of heat shock genes, and possibly other sets of genes, through HSE-like sequences, specifically in response to oxidative stress. Yeast strains and growth conditions. The yeast strains used were as follows: W303-la (a ade2-l trpl- 1 canl-100 leu2-3,112 his3-ll ura3); W303 skn7Δ (a ade2-l trpl- 1 canl-100 leu2-3,112 his3-ll ura3 skn7::HIS3); MYY290 is a ura3 derivative of strain AH216 (a leu2 his3 phoC phoE) ; MYY385 (a leu2 his3 ura3 phoC phoE hsfl-m3) (Smith and Yaffe, 1991) ; DR20-2b was obtained as a haploid HIS+ and temperature- sensitive spore clone from a cross of MYY385 and W303 skn7Δ . Minimal and rich medium for yeast propagation has been described previously (Sherman et al . , 1986) . β-Galactosidase assays. The vector pZJHSE2-137 (a gift from E. Craig) containing a region of the SSA1 promoter inserted into the 2μm based LacZ fusion plasmid pLG660 was transformed into W303-la and an isogenic skn7Δ strain. Transformants were grown to mid-log phase in selective medium at 30 * C and harvested before or after the addition of butyl hydroperoxide to 0.6mM for one hour. For the heat shock experiment, cells were initially grown in selective minimal medium at 25 β C and cells harvested before and one hour after the culture was shifted to 37 "C, Cell extracts were prepared as described previously (Guarente, 1983) . Plasmid constructions. The CEN-SKN7 and D427N-SKN7 plasmids were constructed by inserting a 3.8Kb Xbal-Sacl fragment of either pBAMl or pBAM2 (Morgan et al . , 1997), containing the entire SKN7 coding and promoter regions, into the multiple cloning site of YCplaclll (Gietz and Sugino, 1988). pAKS80 (YEpHSFl) contains a 3.9Kb Eco RI fragment of the S. cerevisiae HSFl gene in YEplacl95. YexH is a modified derivative of the 2μm-based galactose-inducible expression plasmid pEMBLYex4 (Murray, 1987) in which a six histidine tag sequence was inserted upstream of the multiple cloning site to allow the N-terminal tagging of inserted genes. YexH-SKN7 was constructed by inserting a 2 Kb Bam HI fragment containing the SKN7 coding region into the Bam HI cloning site of the vector. DNA sequencing confirmed the reading frame and the construct allows galactose-inducible expression of 6His- SKN7. RNA analysis. Northern hybridisation was as previously described (White et al . , 1986) . In all cases, probes for hybridisation to the heat shock genes used in this study were derived from PCR amplification of an internal fragment of the coding sequence of the gene. The internal control used for mRNA quantitation in hydroperoxide treated cells was RPB4 (Choder, 1993) ; for heat shock experiments the actin gene was used. Protein extraction and immunoprecipitation. Yeast cells were broken by vortexing with glass beads for 5-x30 seconds with 30 second intervals on ice in breakage buffer: 150mM NaCl, 50mM Tris-HCl pH7.5 , ImM MgC12 , 1% NP40, 10% glycerol, ImM EDTA, lOmM BaF, 50mM -glycerol phosphate. At time of use, a protease inhibitor mix was added to a final concentration of lOOμg/ml phenylmethyl sulfonyl fluoride, 2 μg/ml leupeptin, 2 μg/ml pepstain A, 50 μg/ml TLCK and 100 μg/mlTPCK. Cleared lysates were prepared by centrifugation for 20 minutes at 18K rpm (Beckman SS34 rotor) and lmg of whole cell extract, initially pre-cleared with protein A Sepharose beads (Pharmacia) was incubated at 4°C with lμl polyclonal antiserum to the Skn7p under constant mixing for 1 hour. Protein A Sepharose beads (approx. 50μl of a 50% suspension in breakage buffer) were then added and mixing was allowed to continue for a further hour at 4°C. The beads were then harvested and washed four times in breakage buffer containing 200mM NaCl, followed by one wash in the same buffer with 50mM NaCl, and finally resuspended in an equal volume of 2X SDS sample buffer. Proteins were separated by SDS-PAGE through 6% acrylamide gels and transferred to nitrocellulose membranes via semi-dry transfer prior to ECL (Amersham) Western analysis with polyclonal antiserum to yeast Hsflp. ECL was performed in accordance with manufacturers guidelines and membranes were exposed to X-ograph XB-200 film for between 30 seconds and 5 minutes. For pull-down assays, lmg cell extract prepared as above (with the omission of EDTA and MgCl 2 in the breakage buffer) was added to 200μl Ni 2+ -NTA resin (50% slurry) equilibrated in breakage buffer. After incubation with mixing at 4 * C for one hour, the resin was washed four times in wash buffer (200mM NaCl, 50mM Tris-HCl, 1% NP40) , and once in wash buffer containing 50mM NaCl. Beads were then boiled for two minutes in 2X sample buffer and the supernatant subject to SDS-PAGE as described above. Purification of 6His-Skn7 and Mobility Shift asavs . A 2 kb Bam HI fragment containing the entire SKN7 open reading frame (Morgan et al . , 1995) was inserted into plasmid pQE-30 (Qiagen) . Transformed DH-5c- (minimum 2 litres) was grown to an OD600 0.5 at 37'C and then brought to 25 °C by briefly incubating on ice before induction by addition of isopropyl- 1- thio-3-D-galactoside (IPTG) to ImM for 5 hours at 25 'C. Cells were harvested by centrifugation, washed in cold distilled water, resuspended in 2 -5ml breakage buffer (150mM NaCl, 25mM Tris pH7.5, 10% glycerol, 0.5% Nonident P-40) . At the time of use lysozyme was added at lmg/ml and PMSF at ImM. The cell suspension was incubated on ice for 30 minutes and then passed twice through a chilled French press chamber. The clarified supernatant was then incubated with 5ml of a 50% slurry of Ni 2+ - NTA resin, equilibrated in binding buffer (250mM NaCl, 50mM Tris-HCl pH 7.5, 15mM immidazole) , and allowed to mix at 4 * C for one hour before preparing a 5ml column. After washing in binding buffer, tagged protein was eluted by a step gradient of binding buffer containing 50mM, lOOmM and 250mM immidazole. Bradford protein assays (Biorad) were carried out on 0.5ml fractions and DNA-binding activity assayed by gel mobility shift assay. Mobility shift assays have been described elsewhere (Lowndes et al . , 1991) . 6His-Skn7 protein was incubated with 0.5ng (1x105 cpm) 32P 5' -end labelled double-stranded oligonucleotides of the following sequence: HSE2-5' tcgaTTTTCCAGAACGTTCCATCGGC (SEQ ID NO: 16); MUT HSE2-5' tcgaTTTTCCAAAACGTTTCATCGGC (SEQ ID NO: 17). Binding reactions in 25mM Tris-HCl pH 7.5, lOOmM NaCl, ImM EDTA, 7mM MgCl2, 10% glycerol, protease mix as above and lμg poly(dl.dC) were incubated at room temperature for 15 minutes and on ice for a further 20 minutes. Protein-DNA complexes were resolved on a 4% non-denaturing polyacrylamide gel (37.5:1) by electrophoresis at 200V in 0.5X TBE buffer for 2 hours. Gels were dried on to Whatman 3MM paper and exposed to Kodak X-OMAT AR film over night at -20 * C. EXAMPLE 3 A further interaction of Skn7 has been found to be with the transcription factor Mbpl. Mbpl is a DNA binding protein which acts as a heterodimer in cόnjuction with the regulatory protein, Swi6 (Breeden and Nasmyth, 1987; Lowndes et al , 1992 Koch et al 1993) . Mbpl binds principally to MCB elements but also to SCB elements. Mbpl is involved in the regulation of transcription of DNA synthetic genes in the yeast S . cerevisiae as well as CLN1 and CLN2 which are expressed in late Gl . For more discussion see Morgan et al , 1995b. Preliminary genetic evidence suggests a role for Mbpl in regulating bud emergence in yeast . Identification of the direct physical interaction of Skn7 with Mpbl has been shown by a two-hybrid assay, an in vi tro pulldown experiment and an in vivo immunoprecipitation. A combination of these techniques were used to identify regions within Skn7 responsible for these interactions. Thus in another aspect of the invention, there is provided an assay for a putative inhibitor of fungal growth comprising: a) bringing into contact a Skn7 polypeptide, and a Mbpl polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding to the Mbpl polypeptide; and b) measuring the degree of inhibition of binding between the Skn7 and Mbpl polypeptides caused by said inhibitor compound. The term "Mbpl polypeptide" includes a polypeptide including the amino acid sequence for Saccharomyces cerevisiae wild-type Hsfl shown in Koch et al and variants thereof (which may be synthetic or naturally occurring) , in particular showing a characteristic of S. cerevisiae Mbpl polypeptide, such as binding to the Skn7 polypeptide and/or cooperating with it in the induction of Mbpl dependent transcription in S. cerevisiae in the absence of Swi6. Variants inlcude mutants, such as temperature sensitive mutants, alleles such as sequence variants of the proteins described above from S . cerevisiae which demonstrate a substantially similar phenotype, homologues and analogues, such as found in other species, or synthetic variants and derivatives, which retain the functions of the above described proteins to the extent necessary for the paricular assay format being utilised. Variants and fragment of Mbpl may be defined as above for those of Yapl and Hsfl, with the definitions above applicable mutatis mutandis to Mbpl. Thus variants may include those with at least 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95% amino acid identity to Mbpl of S . cerevisiae . Fragments of Mbpl and its variants, defined as above, may also be used. In assays of this aspect of the invention, particular fragments of Skn7 and its variants which may be used include those in which the RIM region (see below) is retained, together with sufficient flanking sequences such that the interaction with Mbpl is retained. Assays of this aspect of the invention may be in any of the formats described herein for other aspects of the invention, including a two-hybrid assay, a pull -down assay, an immunoprecipitation assay, of a DNA binding assay in the presence of DNA comprising MCB and SCB elements to which the Mbpl binds or a transcription assay. Assay conditions may be as described herein for the above assays of Skn7 with Yapl or Hsfl. The assays may be in vi tro or in vivo as appropriate. Cells which comprise a Skn7 polypeptide-encoding sequence operably linked to a heterologous promoter, together with an expression construct comprising a Mbpl polypeptide-encoding sequence operably linked to a heterologous promoter form a further embodiment of this further aspect of the invention. The two constructs may be present on separate vectors, or the same vector, as described above for Yapl or Hsfl. EXPERIMENTAL Two-hybrid interaction between Mbpl and Skn7 , The MBPI gene was cloned in-frame with the GAL4 DNA-binding domain and tested for interaction in a two-hybrid system with SKN7 in accordance with conventional methodology (Guarente, 1983 and Example 2) . As expected, MBPI interacts strongly with the control SWlδ . Significantly, MBPI also interacted strongly with SKN7. Since both proteins are transcriptional activators, the entire SW16 and SKN7 genes, rather than fusions to the GAL4 activation domain, were used in this study. In vi tro interaction between Mpbl and Skn7. A GST-pulldown experiment was used to determine whether the interaction between MBPI and SKN7 was direct, or whether it required ancillary proteins, Swi6 was used as a control and, as expected, in vi tro synthesised Mbpl was retained by a GST-Swi6 fusion protein but not by GST alone. More importantly, Mbpl also clearly bound to a GST-Skn7 fusion protein. The Mbpl/Skn7 association is a strong interaction, for it is stable in up to 1M salt. Jn vivo interaction between Skn7 and Mbpl . Either of Skn7 or Mbpl were overexpressed, each tagged with three histidines, in W303 cells to see if the other protein would be retained on a Ni ++ column. In both cases, an association of Mbpl and Skn7 was evident. However, because this interaction required overexpression of one of the two proteins, it was important to demonstrate the interaction at physiological levels of both proteins. For this purpose, 0.5 mg of crude protein extract was immunoprecipitated using a polyclonal serum directed against Skn7. We used a derivative of W303, where the MBPI gene is tagged with a triple HA-epitope at its C-terminus (W303 3HAMBP1) . The presence of Mbpl in the immunoprecipitates was then assayed by western blotting. A specific band was recognised by the 12CA5 monoclonal antibody in the immunoprecipitates from the W303 3HAMBP1 strain but not in isogenic strains carrying either an untagged MBPI gene or a deletion of SKN7, or both. This indicates that Skn7 and Mbpl form a complex in vivo . Identification of regions of Skn7 responsible for binding to Mbpl. Deletions were made in the GST-Skn7 fusion protein, to provide plasmids pAB61, pAB63, pAB64, pAB65 and pAB81 (see methods) . These deletions were then tested for their ability to bind Mbpl. A deletion starting from residue 473 at the C-terminus of the protein, removing the glutamine-rich region and a small part of the receiver domain, was still able to bind Mbpl to the same extent as the full protein, indicating that the glutamine region is not required for the interaction with Mbpl. On the other hand, when the receiver domain was completely deleted (truncated at residue 311) , the interaction was substantially reduced. In order to more fully asses the role of the received domain in the interaction, we created a fusion between GST and this region of Skn7 (residues 381-623) . This fusion did not bind Mbpl . Thus the received domain is involved in the interaction but other parts of the protein are involved as well . A short region in the Skn7 protein, designated RIM, has been shown to mediate interactions between Skn7 and certain other proteins. The RIM region lies between residues 238 and 261 at the beginning o the coiled-coil domain and deletion of the RIM region is expected to impair any protein-protein interaction involving the coiled-coil domain. To determine whether the interaction between SKN7 and MBPI required this domain as well, we used a construct in which the RIM sequences had been specifically deleted ( skn7ΔRIM) and then fused to the GAL4 activation domain. Although the entire SKN7 gene interacts strongly in the two-hybrid system with MBPI, this construct, lacking only the RIM region, was almost inert. Moreover, removing RIM sequences form the GST-Skn7 fusion protein almost abolished its ability to retain Mbpl. The RIM region is also required for at least part of the in vivo function of Skn7. We cloned skn7ΔRIM on a two-micron plasmid behind its own promoter and this construct was unable to suppress the swi4 ts swi βΔ double mutant at 37°C. Similarly, a C-terminal truncation, mission amino-acids 353-623 including the receiver domain and the glutamine-rich region (Morgan et al . , 1995b), did not suppress the temperature-sensitivity of a swi4 ts swi βΔ mutant, indicating that the receiver domain is also necessary for in vivo Skn7 function. Thus, the RIM region as well as the receiver domain are required for the in vivo interaction with Mbpl. Materials and Methods . Yeast strains and techniques were as described above in Examples 1 and 2. The W303 3HAMBP1 skn7A strain was constructed by disrupting SKN7 in W303 3HAMBP1. This was performed by transforming a restriction fragment carrying skn7 :HIS3 (Morgan et al , 1995b) into W303 3HAMBP1 and selecting His + transformants. Plasmid constructs. For expression of Skn7 , plasmid pAB52 was created by ligating the coding region of SKN7 , with BamHI (5') and Spel (3') linkers added by PCR, into pT7linktag, a plasmid which results in SKN7 being under the control of the T7 promoter. pAB53 was constructed by ligating the coding region of SKN7 , with BamHI (5') and Spel (3') linkers added by PCR, into PGEX-KG (Pharmacia) , so that Skn7 can be expressed in E coli as fusion with GST.pABδl, pAB63, pAB64 are deletions of pAB53, in which the fusion protein is truncated after residue 247, 473 and 311, respectively. pAB65 is a fusion between GST and the residues 381-623 of SKN7, including the receiver domain. The skn7ΔRIM allele, encoding a protein deleted from the residues 238-261, was amplified by PCR from the pGAD skn7ΔRIM plasmid, and inserted into the EcoRI site of pGEX-KG in frame with the GST, thus creating pAB81. The MPB1 open reading frame was amplified by PCR with BamHL sites -added at either end of the gene, and then cloned into pEMBLyex4 (Murray, 1987), to create pGAL-MBPl . For the two-hybrid experiments, an Ncol -Bglll fragment, corresponding to residues 215 to 833, was cloned in frame into pASl-CYH 2 (Harper et al . , 1983), thus creating- pAB75. The plasmid expressing SW16 under the control of the T7 promoter has already been " described (Primig et al . , 1992). GST-pulldown assays. GST-pulldown assays were performed essentially as described in Siegmund and Nasmyth (1996), except that, after binding the in vi tro synthesised protein to the GST fusion protein, 4 washes were performed at 500mM NaCl. Pellets were washed another two times in 20 mM Tris Hcl (Ph 8.0) 1 mM EDTA, before being boiled in SDS-PAGE sample buffer and loaded on a SDS-polyacrylamide gel. After migration, the gel was fixed, dried and subjected to autoradiography at -70°C. Immunoprecipitations . Protein extracts from log-phase cultures grown at 30°C in YEPD were prepared and 0.5 mg was immunoprecipitated with 1 μl of polyclonal antibody directed against Skn7 as previously described (Toyn and Johnston, 1994) . The presence of £HAMbpl in the immunoprecipitates was then assessed by Western blot with the 12CA5 monoclonal antibody (Morgan et al . , 1995). REFERENCES The following citations are incorporated herein by reference: Bourret, R.B., Borkovich, K.A. and Simon, M.I. (1991) Signal transduction pathways involving protein phosphorylation in prokaryotes. Annu. Rev. Biochem. 60, 401-441. Breeden, L and Nasmyth, K. (1987) Similarity between cell cycle genes of budding yeast and fission yeast and the Notch gene of Drosophila . Nature 329, 651-654. Brown, J.L., North, S. and Bussey, H. 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Claims: ' CLAIMS 1. An assay for a putative inhibitor of fungal growt comprising: a) bringing into contact Skn7 polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding to a Skn7-polypeptide-specific nucleotide sequence; b) providing a nucleic acid molecule which includes a Skn7- polypeptide-specific binding nucleotide sequence to which the polypeptide is capable of binding to activate transcription of a sequence operably linked to the Skn7- polypeptide-specific binding nucleotide sequence; and c) measuring the degree of inhibition of binding of Skn7 to the Skn7-polypeptide-specific binding nucleotide sequence or transcriptional activation caused by said inhibitor compound . 2. An assay according to claim 1 wherein said nucleotide sequence is found in the TRX2 or TRR1 promoter. 3. An assay according to claim 2 wherein said sequence is SEQ ID NO:2. 4. An assay according to claim 1 wherein said nucleotide sequence is found in a heat shock promoter. 5. An assay according to claim 4 wherein said sequence is SEQ ID NO: 3. 6. An assay for a putative inhibitor of fungal growth comprising: a) bringing into contact a Skn7 polypeptide and a Yapl polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding a Skn7-polypeptide- specific nucleotide sequence and cooperating with Yapl to activate transcription; b) providing a nucleic acid molecule which includes a Skn7- polypeptide-specific binding nucleotide -sequence to which the Skn7 polypeptide is capable of binding to activate transcription of a sequence operably linked to the Skn7- polypeptide-specific binding nucleotide sequence; and c) measuring the degree of inhibition of interaction of Skn7 polypeptide and Yapl polypeptide or of Skn7 polypeptide and the Skn7-polypeptide-specific binding nucleotide sequence or transcriptional activation caused by said inhibitor compound. 7. An assay for a putative inhibitor of fungal growth comprising : a) bringing into contact a Skn7 polypeptide, and a Hsfl polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding to the Hsfl polypeptide; and b) measuring the degree of inhibition of binding between the Skn7 and Hsfl polypeptides caused by said inhibitor compound . 8. An assay for a putative inhibitor of fungal growth comprising: a) bringing into contact a Skn7 polypeptide, and a Mbpl polypeptide and a putative inhibitor compound under conditions where the Skn7 polypeptide, in the absence of inhibitor, is capable of binding to the Mbpl polypeptide; and b) measuring the degree of inhibition of binding between the Skn7 and Mbpl polypeptides caused by said inhibitor compound . 9. An assay according to claim 7 or 8 in the form of a two- hybrid assay.

Inventors: JOHNSTON LELAND HERRIES, MORGAN BRIAN ALEXANDER, RAITT DESMOND COLUM, JOHNSTON, LELAND, HERRIES, MORGAN, BRIAN, ALEXANDER, RAITT, DESMOND, COLUM
Applicants: JOHNSTON LELAND HERRIES, MEDICAL RES COUNCIL, MORGAN BRIAN ALEXANDER, RAITT DESMOND COLUM, JOHNSTON, LELAND, HERRIES, MEDICAL RESEARCH COUNCIL, MORGAN, BRIAN, ALEXANDER, RAITT, DESMOND, COLUM
Application year: 1998
Main classification: C12Q1
IPC codes: C07K14, C12N15, C12Q1