The sum1-1 mutation affects silent mating-type gene transcription in Saccharomyces cerevisiae.

The silent mating-type genes (HML and HMR) of Saccharomyces cerevisiae are kept under negative transcriptional control by the trans-acting products of the four MAR/SIR loci. MAR/SIR gene mutations result in the simultaneous derepression of HML and HMR gene expression. The sum1-1 mutation was previously identified as an extragenic suppressor of mutations in MAR1 (SIR2) and MAR2 (SIR3). As assayed genetically, sum1-1 is capable of restoring repression of silent mating-type information in cells containing mar1 or mar2 null mutations. We show here that the mating-type phenotype associated with sum1-1 results from a dramatic reduction in the steady-state level of HML and HMR gene transcripts. At the same time, the sum1-1 mutation has no significant effect on the level of each of the four MAR/SIR mRNAs.

Cloning and expression of cDNA for a human low-Km, rolipram-sensitive cyclic AMP phosphodiesterase.

We have isolated cDNA clones representing cyclic AMP (cAMP)-specific phosphodiesterases (PDEases) from a human monocyte cDNA library. One cDNA clone (hPDE-1) defines a large open reading frame of ca. 2.1 kilobases, predicting a 686-amino-acid, ca. 77-kilodalton protein which contains significant homology to both rat brain and Drosophila cAMP PDEases, especially within an internal conserved domain of ca. 270 residues. Amino acid sequence divergence exists at the NH2 terminus and also within a 40- to 100-residue domain near the COOH-terminal end. hPDE-1 hybridizes to a major 4.8-kilobase mRNA transcript from both human monocytes and placenta. The coding region of hPDE-1 was engineered for expression in COS-1 cells, resulting in the overproduction of cAMP PDEase activity. The hPDE-1 recombinant gene product was identified as a low-Km cAMP phosphodiesterase on the basis of several biochemical properties including selective inhibition by the antidepressant drug rolipram. Known inhibitors of other PDEases (cGMP-specific PDEase, cGMP-inhibited PDEase) had little or no effect on the hPDE-1 recombinant gene product. Human genomic Southern blot analysis suggests that this enzyme is likely to be encoded by a single gene. The presence of the enzyme in monocytes may be important for cell function in inflammation. Rolipram sensitivity, coupled with homology to the Drosophila cAMP PDEase, which is required for learning and memory in flies, suggests an additional function for this enzyme in neurobiochemistry.

Rapamycin sensitivity in Saccharomyces cerevisiae is mediated by a peptidyl-prolyl cis-trans isomerase related to human FK506-binding protein.

Rapamycin is a macrolide antifungal agent with structural similarity to FK506. It exhibits potent immunosuppressive properties analogous to those of both FK506 and cyclosporin A (CsA). Unlike FK506 and CsA, however, rapamycin does not inhibit the transcription of early T-cell activation genes, including interleukin-2, but instead appears to block downstream events leading to T-cell activation. FK506 and CsA receptor proteins (FKBP and cyclophilin, respectively) have been identified and shown to be distinct members of a class of enzymes that possess peptidyl-prolyl cis-trans isomerase (PPIase) activity. Despite the apparent differences in their mode of action, rapamycin and FK506 act as reciprocal antagonists in vivo and compete for binding to FKBP. As a means of rapidly identifying a target protein for rapamycin in vivo, we selected and genetically characterized rapamycin-resistant mutants of Saccharomyces cerevisiae and isolated a yeast genomic fragment that confers drug sensitivity. We demonstrate that the resonse to rapamycin in yeast cells is mediated by a gene encoding a 114-amino-acid, approximately 13-kDa protein which has a high degree of sequence homology with human FKBP; we designated this gene RBP1 (for rapamycin-binding protein). The RBP1 protein (RBP) was expressed in Escherichia coli, purified to homogeneity, and shown to catalyze peptidyl-prolyl isomerization of a synthetic peptide substrate. PPIase activity was completely inhibited by rapamycin and FK506 but not by CsA, indicating that both macrolides bind to the recombinant protein. Expression of human FKBP in rapamycin-resistant mutants restored rapamycin sensitivity, indicating a functional equivalence between the yeast and human enzymes.

Dominant missense mutations in a novel yeast protein related to mammalian phosphatidylinositol 3-kinase and VPS34 abrogate rapamycin cytotoxicity.

Rapamycin is a macrolide antifungal agent that exhibits potent immunosuppressive properties. In Saccharomyces cerevisiae, rapamycin sensitivity is mediated by a specific cytoplasmic receptor which is a homolog of human FKBP12 (hFKBP12). Deletion of the gene for yeast FKBP12 (RBP1) results in recessive drug resistance, and expression of hFKBP12 restores rapamycin sensitivity. These data support the idea that FKBP12 and rapamycin form a toxic complex that corrupts the function of other cellular proteins. To identify such proteins, we isolated dominant rapamycin-resistant mutants both in wild-type haploid and diploid cells and in haploid rbp1::URA3 cells engineered to express hFKBP12. Genetic analysis indicated that the dominant mutations are nonallelic to mutations in RBP1 and define two genes, designated DRR1 and DRR2 (for dominant rapamycin resistance). Mutant copies of DRR1 and DRR2 were cloned from genomic YCp50 libraries by their ability to confer drug resistance in wild-type cells. DNA sequence analysis of a mutant drr1 allele revealed a long open reading frame predicting a novel 2470-amino-acid protein with several motifs suggesting an involvement in intracellular signal transduction, including a leucine zipper near the N terminus, two putative DNA-binding sequences, and a domain that exhibits significant sequence similarity to the 110-kDa catalytic subunit of both yeast (VPS34) and bovine phosphatidylinositol 3-kinases. Genomic disruption of DRR1 in a mutant haploid strain restored drug sensitivity and demonstrated that the gene encodes a nonessential function. DNA sequence comparison of seven independent drr1dom alleles identified single base pair substitutions in the same codon within the phosphatidylinositol 3-kinase domain, resulting in a change of Ser-1972 to Arg or Asn. We conclude either that DRR1 (alone or in combination with DRR2) acts as a target of FKBP12-rapamycin complexes or that a missense mutation in DRR1 allows it to compensate for the function of the normal drug target.

Mating-Type Regulation of Methyl Methanesulfonate Sensitivity in SACCHAROMYCES CEREVISIAE.

Heterozygosity at the mating-type locus (MAT) in Saccharomyces cerevisiae has been shown previously to enhance X-ray survival in diploid cells. We now show that a/alpha diploids are also more resistant to the radiomimetic agent methyl methanesulfonate (MMS) than are diploids that are homozygous at MAT (i.e., either a/a or alpha/alpha). Log-phase a/alpha cultures exhibit biphasic MMS survival curves, in which the more resistant fraction consists of budded cells (those cells in the S and G2 phases of the cell cycle). Survival curves for log-phase cultures of a/a or alpha/alpha diploids have little if any biphasic nature, suggesting that the enhanced S- and G2-phase repair capacity of a/alpha cells may be associated with heterozygosity at MAT. The survival of cells arrested at the beginning of the S phase with hydroxyurea indicates that MAT-dependent MMS repair is limited to S and G2, whereas MAT-independent repair can occur in G1.

Extragenic suppressors of mar2(sir3) mutations in Saccharomyces cerevisiae.

The silent mating-type genes (HML and HMR) of Saccharomyces cerevisiae are kept under negative transcriptional control by four trans-acting MAR (or SIR) loci. We have isolated extragenic suppressors of the mar2-1 mutation which, based on genetic complementation tests, define two additional loci involved in regulating the expression of HML and HMR. A strain with the genotype HMLa MAT alpha HMRa mar2-1 is sterile due to the simultaneous expression of a and alpha information. Two mutants exhibiting an alpha phenotype (which may result from the restoration of MAR/SIR repression) were isolated and genetically characterized. The mutations in these strains: (1) are recessive, (2) are capable of suppressing a mar2-deletion mutation, (3) are unlinked to MAT, (4) complement one another as well as the previously identified sum1-1 mutation, and (5) are not new alleles of the known MAR/SIR loci. We designate these new regulatory loci SUM2 and SUM3 (suppressor of mar). Unlike the sum1-1 mutation, suppression by sum2-1 and sum3-1 is mar2-locus specific. Both sum2-1 and sum3-1 affect the expression of a information at the HM loci. Transcript analysis shows a significant reduction in HMLa and HMRa gene transcription in mar2-1 sum2-1 and mar2-1 sum3-1 cells. Furthermore, we have found genetic evidence to suggest that mar2-1 sum2-1 cells exhibit only partial expression of silent alpha information. We conclude that the SUM2 and SUM3 gene products are required for expression of the HM loci and act downstream of the MAR2 (SIR3) gene function. Possible mechanisms for the action of the SUM gene products are discussed.

SUM1, an apparent positive regulator of the cryptic mating-type loci in Saccharomyces cerevisiae.

The mating-type information residing at the HML and HMR loci in Saccharomyces cerevisiae is kept unexpressed by the action of at least four MAR (or SIR) loci. To determine possible interactions between the MAR/SIR gene products and to find new regulatory loci, we sought extragenic suppressors of the mar1-1 mutation. A strain with the genotype HMLa MAT alpha HMRa mar1-1 is unable to mate because of the simultaneous expression of a and alpha information. A mutant of this strain was isolated that exhibits an alpha phenotype and, therefore, presumably fails to express the HML and HMR loci. We designate the new locus SUM1 (suppressor of mar). The mutation is recessive, centromere unlinked and does not correspond to the MAT, HML, HMR, SIR1, MAR1, MAR2 (SIR3) or SIR4 loci. The sum1 mutation affects expression of both a and alpha information at the HM loci. Suppression by sum1-1 is neither allele specific nor locus specific as it suppresses a deletion mutation of the MAR1 locus and mutations in SIR3 and SIR4. The sum1-1 mutation has no discernible phenotype in a Mar+ strain. We propose that the MAR/SIR gene products negatively regulate the SUM1 locus, the gene product of which is necessary for expression of the HM loci.

Missense mutations at the FKBP12-rapamycin-binding site of TOR1.

The TOR genes were first identified in Saccharomyces cerevisiae by the isolation of mutants which exhibit dominant resistance to the immunosuppressive and antifungal drug rapamycin (Rm). The originally characterized Rm-resistant (RmR) TOR1-1 and TOR2-1 alleles contain an Arg in place of a conserved Ser residue, which lies adjacent to the phosphatidylinositol (PI) kinase-related domain of TOR (Ser1972 in TOR1; Ser1975 in TOR2). Additional spontaneous RmR mutants containing Lys, Ile or Asn substitutions were subsequently isolated. As this Ser is a potential site for protein kinase C phosphorylation, we were interested in determining whether the observed RmR is due to steric hindrance of the FKBP12-Rm-TOR interaction or whether phosphorylation at this site is required to mediate the interaction. Using site-directed mutagenesis, we replaced the Ser1972 residue of TOR1 with either a conservative residue, Ala, an alternative potential phosphorylation site, Thr, or Asp to mimic phosphorylation. The TOR1 (S1972A) mutant protein retained Rm sensitivity (RmS), whereas both the Thr and Asp substitutions conferred RmR. RmS correlated with the ability to interact with FKBP12-Rm in a two-hybrid assay: both wild-type TOR1 and the S1972A mutant retained the ability to interact with FKBP12-Rm, whereas the S1972T, S1972D and S1972R mutants failed to interact. All mutant TOR1 proteins were able to complement the growth defect of tor1 null alleles, suggesting that the Ser1972 residue may not be required for TOR1 function in cycling cells. Since a TOR1(S1972A) mutant protein confers a RmS phenotype, interacts with FKBP12-Rm in a two-hybrid assay, and functions in vivo, we conclude that phosphorylation at Ser1972 is not necessary for the interaction between TOR1 and FKBP12-Rm.

Cloning and expression of the ARO3 gene encoding DAHP synthase from Candida albicans.

In Saccharomyces cerevisiae, the primary step in the aromatic (ARO) amino acid (aa) biosynthetic pathway is catalyzed by two isozymes of 3-deoxy-D-arabinoheptulosonate-7-phosphate synthase (DAHPS). The activity of one DAHPS isozyme (encoded by the ARO3 gene) is feedback inhibited by phenylalanine, whereas the other (encoded by the ARO4 gene) is inhibited by tyrosine. The expression of these genes is also regulated at the transcriptional level by the general control activator GCN4. We took advantage of the high degree of aa sequence homology between DAHPSs from several species to isolate ARO3 homologues from the pathogenic yeast Candida albicans. An ARO3/ARO4-specific sequence was generated from C. albicans genomic DNA by polymerase chain reaction amplification and used as a probe to screen a C. albicans cDNA library. A 1.3-kb cDNA clone was isolated and sequenced. The cDNA contains a long open reading frame predicting a 368-aa protein with significant homology to known DAHPSs, including both the S. cerevisiae ARO3 and ARO4 products (68.5% and 58.5% identity, respectively). Northern analysis of yeast and mycelial poly(A)+ RNA revealed equivalent expression of a 1.3-kb transcript in both cell types. A genomic clone was isolated by cross-hybridization, and analysis of the 5' untranslated region revealed the presence of a putative GCN4-binding site. This clone complemented an aro3 mutation in S. cerevisiae; functional complementation was inhibited by the presence of excess phenylalanine (but not tyrosine) in the growth medium, confirming that the cloned gene is the C. albicans homologue of ARO3.

Cloning and sequence analysis of a rapamycin-binding protein-encoding gene (RBP1) from Candida albicans.

Rapamycin (Rm) and FK506 are macrolide antifungal agents that exhibit potent immunosuppressive properties in higher eukaryotes which are mediated through interaction with specific receptor proteins (FKBPs or RBPs, for FK506- and Rm-binding proteins, respectively). These proteins possess peptidyl-prolyl cis-trans isomerase (PPIase) activity in vitro which is inhibited by the binding of Rm and FK506. We previously isolated a gene encoding an RBP from Saccharomyces cerevisiae, and demonstrated that null mutations in this gene (called RBP1) result in a recessive Rm-resistant (RmR) phenotype. We now have cloned the Candida albicans RBP1 gene via complementation of the RmR phenotype in S. cerevisiae. The predicted C. albicans RBP exhibits 61%, 52% and 49% amino acid (aa) sequence identity with RBPs (FKBPs) from S. cerevisiae, Neurospora crassa and human cells (FKBP-12), respectively. Furthermore, several of the aa residues identified as being important for drug binding in human FKBP-12 are conserved within the C. albicans RBP.

Yeast TOR (DRR) proteins: amino-acid sequence alignment and identification of structural motifs.

The yeast TOR1 (DRR1) and TOR2 (DRR2) proteins are putative targets of the immunosuppressive drug rapamycin (Rm), defined by dominant drug-resistance mutations. They share a large C-terminal domain that exhibits sequence similarity to the 110-kDa subunit of phosphatidylinositol (PI) 3-kinases. In this report, we present an amino acid (aa) sequence alignment of TOR1 (DRR1) and TOR2 (DRR2) and identify conserved and nonconserved motifs within the N-terminal domain that are indicative of possible nuclear localization. We also show that the mutations responsible for Rm resistance in four independent drr2dom alleles alter the identical aa (Ser1975-->Arg) previously identified in drr1dom mutants (Ser1972-->Arg or Asn). Models for TOR (DRR) protein function are discussed.

A Candida albicans homolog of a human cyclophilin gene encodes a peptidyl-prolyl cis-trans isomerase.

A Candida albicans cDNA and its genomic counterpart were isolated from lambda phage libraries using a human T-cell cyclophilin (Cyp) cDNA as a hybridization probe. The clones contain a 486-bp open reading frame predicting a 162-amino acid, approx. 18 kDa protein which is similar in size to, and which shares 68 and 81% homology with, human T-cell Cyp and cytosolic Saccharomyces cerevisiae Cyp, respectively. Northern blots show the presence of a single mRNA species of about 800 bp. However, genomic Southern blots suggest the presence of at least one other Cyp-related gene in C. albicans. The cDNA was engineered for expression in Escherichia coli, and the resulting recombinant protein, like mammalian Cyps, exhibited a peptidyl-prolyl cis-trans isomerase (PPIase) activity which was sensitive to inhibition by cyclosporin A in vitro. These results indicate that the gene which we have cloned encodes a C. albicans Cyp. We designate this gene CYP1 (cyclophilin). Interestingly, the predicted C. albicans protein contains only two cysteine residues which do not align with any of the four cysteines conserved among mammalian Cyps. This suggests that the PPIase catalytic mechanism may not involve an enzyme-bound hemithioorthoamide, as previously reported for porcine Cyp.

Secretion of N-glycosylated human recombinant interleukin-1 alpha in Saccharomyces cerevisiae.

We have expressed fragments of the cDNA coding for mature human interleukin-1 alpha (hIL-1 alpha) in Saccharomyces cerevisiae. Mature hIL-1 alpha contains one potential N-linked glycosylation site that is not recognized in mammalian cells. Translational fusions to either one of three yeast signal sequences resulted in secretion of bioactive, N-glycosylated hIL-1 alpha. The extent of glycosylation was significantly reduced using the alpha-factor signal sequence, which itself contains three N-linked glycosylation sites known to be core glycosylated. N-glycosylation has no effect on biological specific activity.

The yeast FKS1 gene encodes a novel membrane protein, mutations in which confer FK506 and cyclosporin A hypersensitivity and calcineurin-dependent growth.

FK506 and cyclosporin A (CsA) are potent immunosuppressive agents that display antifungal activity. They act by blocking a Ca(2+)-dependent signal transduction pathway leading to interleukin-2 transcription. Each drug forms a complex with its cognate cytosolic immunophilin receptor (i.e., FKBP12-FK506 and cyclophilin-CsA) which acts to inhibit the Ca2+/calmodulin-dependent protein phosphatase 2B, or calcineurin (CN). We and others have defined the Saccharomyces cerevisiae FKS1 gene by recessive mutations resulting in 100-1000-fold hypersensitivity to FK506 and CsA (as compared to wild type), but which do not affect sensitivity to a variety of other antifungal drugs. The fks1 mutant also exhibits a slow-growth phenotype that can be partially alleviated by exogenously added Ca2+ [Parent et al., J. Gen. Microbiol. 139 (1993) 2973-2984]. We have cloned FKS1 by complementation of the drug-hypersensitive phenotype. It contains a long open reading frame encoding a novel 1876-amino-acid (215 kDa) protein which shows no similarity to CN or to other protein phosphatases. The FKS1 protein is predicted to contain 10 to 12 transmembrane domains with a structure resembling integral membrane transporter proteins. Genomic disruption experiments indicate that FKS1 encodes a nonessential function; fks1::LEU2 cells exhibit the same growth and recessive drug-hypersensitive phenotypes observed in the original fks1 mutants. Furthermore, the fks1::LEU2 allele is synthetically lethal in combination with disruptions of both of the nonessential genes encoding the alternative forms of the catalytic A subunit of CN (CNA1 and CNA2). These data suggest that FKS1 provides a unique cellular function which, when absent, increases FK506 and CsA sensitivity by making the CNs (or a CN-dependent function) essential.

The yeast cyclophilin multigene family: purification, cloning and characterization of a new isoform.

Cyclophilins (Cyps) constitute a highly conserved family of proteins present in a wide variety of organisms. Historically, Cyps were first identified by their ability to bind the immunosuppressive agent cyclosporin A (CsA) with high affinity; they later were found to have peptidyl-prolyl cis-trans isomerase (PPIase) activity, which catalyzes the folding of oligopeptides at proline-peptide bonds in vitro and may be important for protein folding in vivo. Cells of Saccharomyces cerevisiae contain at least two distinct Cyp-related PPIases encoded by the genes CYP1 and CYP2. A yeast strain (GL81) containing genomic disruptions of three known yeast PPIase-encoding genes [CYP1, CYP2 and RBP1 (for rapamycin-binding protein); Koltin et al., Mol. Cell. Biol. 11 (1991) 1718-1723] was previously constructed and found to be viable. Soluble fractions of these cells possess residual CsA-sensitive PPIase activity (2-5% of that present in wild-type cells as assayed in vitro). We have purified an approx. 18-kDa protein exhibiting PPIase activity from a soluble fraction of GL81 cells and determined that its N-terminal amino acid (aa) sequence exhibits significant homology (but nonidentity) to the Cyp1 and Cyp2 proteins. We designate the gene for this new protein, CYP3. Using a degenerate oligodeoxyribonucleotide (oligo) based on the N-terminal aa sequence, plus an internal oligo homologous to a conserved region within the portion of CYP1 and CYP2 that had been deleted in the genome, a CYP3-specific DNA fragment was generated by the polymerase chain reaction (PCR) using GL81 genomic DNA as a substrate. This PCR fragment was used as a probe to isolate CYP3 genomic and cDNA clones.(ABSTRACT TRUNCATED AT 250 WORDS)

The tyrosine89 residue of yeast FKBP12 is required for rapamycin binding.

Rapamycin (Rm) is a macrolide antifungal agent related to FK506 that exhibits potent immunosuppressive properties which are mediated through interaction with specific cytoplasmic receptors (FKBPs or RBPs, for FK506- and Rm-binding proteins, respectively). These proteins possess peptidyl-prolyl cis-trans isomerase (PPIase) activity in vitro which is inhibited by the binding of Rm and FK506. In Saccharomyces cerevisiae, Rm sensitivity (Rms) is mediated by binding of the drug to RBP1, a homolog of the 12-kDa human FK506-binding protein (FKBP12); null mutations in the yeast RBP1 gene result in a recessive drug resistance phenotype. To identify missense mutations that define amino acid (aa) residues in RBP1 involved in drug sensitivity, we selected and genetically characterized over 250 independent RmR rbp1 mutants and screened them for both RBP1-specific mRNA and protein expression. Whereas all rbp1 mutants expressed abundant levels of RBP1 mRNA, stable RBP1 protein production was detected in only one mutant strain. The RBP1 gene was PCR-generated (in triplicate) from several rbp1 mutants and independent clones were sequenced. Most of the immunoblot-negative alleles were found to contain various types of null mutations; however, some alleles contained specific missense mutations that apparently affect protein stability in vivo. The single immunoblot-positive allele was found to contain a mutation altering a specific residue (Tyr89) which is conserved among the known FKBPs, and which, based on the solution and x-ray structures of human FKBP12, has been proposed to be part of a hydrophobic drug-binding pocket for FK506 and Rm.(ABSTRACT TRUNCATED AT 250 WORDS)

The CYP2 gene of Saccharomyces cerevisiae encodes a cyclosporin A-sensitive peptidyl-prolyl cis-trans isomerase with an N-terminal signal sequence.

Cells of Saccharomyces cerevisiae contain a major cytosolic cyclophilin (Cyp)-related peptidyl-prolyl cis-trans isomerase (PPIase) which is the target for cyclosporin A (CsA) cytotoxicity and which is encoded by the CYP1 gene [Haendler et al., Gene 83 (1989) 39-46]. We recently identified a second Cyp-related gene in yeast, CYP2 [Koser et al., Nucleic Acids Res. 18 (1990) 1643] which predicts a protein with a hydrophobic leader sequence. A sequence lacking 33 codons from the 5'-end of the CYP2 open reading frame was generated by the polymerase chain reaction and engineered for expression in Escherichia coli. The corresponding recombinant truncated protein was purified and found to exhibit PPIase activity which was inhibited by CsA. The CYP2 gene is genetically unlinked to CYP1. As with CYP1, genomic disruption of CYP2 had no effect on haploid cell viability. Disruption of all three of the known yeast PPIase-encoding genes [CYP1, CYP2, and RBP1 for rapamycin-binding protein; Koltin et al., Mol. Cell. Biol. 11 (1991) 1718-1723] in the same haploid cell also resulted in no apparent cellular phenotype, suggesting either that none of these enzymes have an essential function or that additional PPIases can compensate for their specific absence. Whereas cells containing a genomic disruption of CYP1 exhibited a CsA-resistant phenotype, genomic disruption of CYP2 had no effect on CsA sensitivity. This suggests that the CYP1 gene product is the primary cellular target for CsA toxicity in yeast. Since both purified Cyps display CsA sensitivity in vitro, our data suggest that Cyp1 and Cyp2 differ in terms of their cellular function and/or localization.

Secretory mutants in the cellular slime mold Dictyostelium discoideum.

Our initial studies have shown that the cellular slime mold Dictyostelium discoideum is a particularly suitable organism for the study of lysosomal enzyme secretion. During appropriate stages in the life cycle, secretion is prominent for a number of lysosomal enzymes. The methods described here have been developed to investigate various aspects of the secretion process. Moreover, our evidence that regulation of the secretory system is influenced by environmental changes and by cell differentiation indicates that this organism may be useful for studying the functional regulation of this organellar system. To a large degree these types of studies have been limited in the past due to the lack of an appropriate experimental system. The ability to isolate secretory mutants affecting the secretion of lysosomal enzymes adds another dimension to investigations using D. discoideum. In our initial attempts we have been successful in isolating a variety of different types of mutants that alter the secretion of one or more lysosomal enzymes. While the results are in agreement with the results of our physiological investigations, they also indicate much more heterogeneity in the lysosomal system than we had previously suspected. The indications that many of our secretory mutants may also affect modification of the enzymes is also intriguing. This observation may also help to explain the fact that many of these strains are defective in normal development. Together with the immunological methods available in this organism for studying posttranslational modification, the mutants may be valuable in deciphering the relationship between modification of lysosomal enzymes and their proper localization and secretion from the cell. Thus, Dictyostelium discoideum may become as useful for the study of some questions of cell biology as it has been for development.

Oligonucleotide-mediated, PCR-independent cloning by homologous recombination.

We have developed an oligonucleotide-mediated cloning technique based on homologous recombination in Saccharomyces cerevisiae that allows precise DNA sequences to be transferred independent of restriction enzymes and PCR. In this procedure, linear DNA sequences are targeted to a chosen site in a yeast vector by DNA linkers, which consist of two partially overlapping oligonucleotides. The linkers contain relatively short regions of both yeast vector sequences and insert sequences, which stimulate homologous recombination between the vector and the insert. The linkers can also contain sequences not found in either the vector or the insert (e.g., sequences that encode ribosome binding sites, epitope tags, preferred codons, etc.), thus allowing modification of the transferred DNA. Linkers can be designed such that DNA sequences can be transferred with just two reusable universal oligonucleotides and two gene-specific oligonucleotides. This cloning method, which is performed by co-transforming yeast with linear vector, substrate DNA, and unannealed oligonucleotides, has been termed the yeast-based, oligonucleotide-mediated gap repair technique (YOGRT).