Characterization of transposable element-associated mutations that alter yeast alcohol dehydrogenase II expression

Host genes that affect the target-site distribution of the yeast retrotransposon Ty1

We report here a simple genetic system for investigating factors affecting Ty1 target-site preference within an RNAP II transcribed gene. The target in this system is a functional fusion of the regulatable MET3 promoter with the URA3 gene. We found that the simultaneous inactivation of Hir3 (a histone transcription regulator) and Cac3 (a subunit of the chromatin assembly factor I), which was previously shown by us to increase the Ty1 transposition rate, eliminated the normally observed bias for Ty1 elements to insert into the 5' vs. 3' regions of the MET3-URA3 and CAN1 genes. The double cac3 hir3 mutation also caused the production of a short transcript from the MET3-URA3 fusion under both repressed and derepressed conditions. In a hir3Delta single-mutant strain, the Ty1 target-site distribution into MET3-URA3 was altered only when transposition occurred while the MET3-URA3 fusion was actively transcribed. In contrast, transcription of the MET3-URA3 fusion did not alter the Ty1 target-site distribution in wild-type or other mutant strains. Deletion of RAD6 was shown to alter the Ty1 target-site preference in the MET3-URA3 fusion and the LYS2 gene. These data, together with previous studies of Ty1 integration positions at CAN1 and SUP4, indicate that the rad6 effect on Ty1 target-site selection is not gene specific.

Delta sequence of Ty1 transposon can initiate transcription of the distal part of the URA2 gene complex in Saccharomyces cerevisiae

Expression of a silent aspartate transcarbamylase (ATCase) domain can occur by insertion of a Tyl retrotransposon within the coding sequence of a mutated ura2 allele. This unusual type of Ty-mediated gene activation is possible as the URA2 gene product is a multifunctional protein containing the carbamoyl phosphate synthetase (CPSase), the ATCase and a cryptic dihydroorotase (DHOase) domain. The region in which transcription of the corresponding allele is initiated was determined by RT-PCR experiments. Expression is initiated by a sequence located in the delta element of the Tyl and not by a sequence of the URA2 gene itself. This situation differs with the Ty-mediated gene activation described thus far, in which the transposon substitutes only the 5' regulatory sequences and in which the normal transcription start point is used. The corresponding protein carries both the DHOase-like domain and the ATCase domain, suggesting that the DHOase-like domain is at least involved in the architecture of the protein and necessary to render the ATCase domain functional.

Structure and regulation of the Candida albicans ADH1 gene encoding an immunogenic alcohol dehydrogenase

The Candida albicans ADH1 gene encodes an alcohol dehydrogenase which is immunogenic during infections in humans. The ADH1 gene was isolated and sequenced, and the 5'- and 3'-ends of its mRNA were mapped. The gene encodes a 350 amino acid polypeptide with strong homology (70.5-85.2% identity) to alcohol dehydrogenases from Saccharomyces cerevisiae, Kluyveromyces lactis and Schizosaccharomyces pombe. The cloned C. albicans ADH1 gene was shown to be functional through complementation of adh mutations and efficient production of active alcohol dehydrogenase in S. cerevisiae. Northern analysis of C. albicans RNA revealed that ADH1 mRNA levels were regulated in response to carbon source and during batch growth. During growth on glucose, ADH1 mRNA levels rose to maximum levels during late exponential growth phase and declined to low levels in stationary phase. The ADH1 mRNA was relatively abundant during growth on galactose, glycerol, pyruvate, lactate or succinate, and less abundant during growth on glucose or ethanol. Alcohol dehydrogenase levels did not correlate closely with ADH1 mRNA levels under the growth conditions studied, suggesting either that this locus is controlled at both transcriptional and post-transcriptional levels, or that other differentially regulated ADH loci exist in C. albicans.

Sequence analysis of a 44 kb DNA fragment of yeast chromosome XV including the Tyl-H3 retrotransposon, the suf1(+) frameshift suppressor gene for tRNA-Gly, the yeast transfer RNA-Thr-1a and a delta element

We have sequenced on both strands a 44,019 bp fragment located on the left arm of Saccharomyces cerevisiae chromosome XV. The sequenced segment contains 22 open reading frames (ORFs) of at least 100 amino acids long, one of which probably contains an intron. Six of the 22 ORFs correspond to known proteins: the multicopy suppressor of Snf1 protein 1, the two Tyl-H3 transposon proteins TyA and TyB, the myo-inositol transporter 2, the transcription factor protein Ino4 and the 3,4-dihydroxy-5-hexaprenylbenzoate methyltransferase. Of the 16 remaining ORFs, two show highest homologies with the yeast serine/threonine protein kinase Ste20 and the human tryptophanyl-tRNA synthetase. Eight ORFs show slight similarities with protein sequences described in data banks. DNA sequence comparison reveals also the presence of three known sequences: the Tyl-H3 transposable element, the yeast suf1(+) frameshift suppressor gene for tRNA-Gly and the yeast transfer RNA-Thr-1a. A fourth DNA sequence shows striking identities with the yeast delta elements.

The TYE7 gene of Saccharomyces cerevisiae encodes a putative bHLH-LZ transcription factor required for Ty1-mediated gene expression

In Saccharomyces cerevisiae, expression of a gene adjacent to the retrotransposon Ty1 is often mediated by Ty-internal sequences. We have identified novel mutants, tye7, which are affected in Ty1-mediated expression of ADH2 through a Ty1 sequence distal to the 5' long terminal repeat sequence. The TYE7 gene has been isolated and characterized. It encodes a 33 kDa protein whose N-terminal third is extremely rich in serine residues (28%). Within its C-terminal sequence, a remarkable similarity to Myc and Max proteins can be found. Thus, TYE7 is a potential member of the basic region/helix-loop-helix/leucine-zipper protein family. TYE7 function is not essential for growth. It may primarily function as a transcriptional activator in Ty1-mediated gene expression, as has been confirmed by the activation of reporter gene expression by a LexA-TYE7 hybrid protein. ADH2 activation by defined Ty1 derivatives revealed that TYE7 acts positively through the more distal Ty1 enhancer element (region D), and negatively in a region between A (the 5' proximal enhancer element) and D.

The positive and negative cis-acting elements for methanol regulation in the Pichia pastoris AOX2 gene

The methylotrophic yeast Pichia pastoris has two alcohol oxidase genes, AOX1 and AOX2. The AOX2 gene is transcribed at a much lower level than the AOX1 gene. Apart from this difference in expression levels, the two genes are regulated similarly. To study the role of cis-acting elements in the promoter region of the AOX2 gene, we constructed expression plasmids in which the human serum albumin (HSA) gene was placed under the control of various deleted or mutated AOX2 promoter derivatives. By analyzing the expression of HSA in P. pastoris transformants, we have identified three cis-acting regulatory elements in the AOX2 promoter. The positive cis-acting element AOX2-UAS, located between positions -337 and -313 (relative to the transcription initiation codon), is required for response to transcriptional induction by methanol in an orientation-independent manner, and artificial amplification of the AOX2-UAS resulted in an increase in the transcriptional activity of the promoter. A sequence homologous to AOX2-UAS was also found in the AOX1 promoter, and in methanol-regulated promoters in other methylotrophic yeast. Two negative cis-acting elements, AOX2-URS1 and AOX2-URS2 play a role in repressing transcription from the AOX2 promoter. The function of AOX2-UAS is completely repressed by this unique repression system when both the AOX2-URS1 and AOX2-URS2 are functional.

The centromere and promoter factor, 1, CPF1, of Saccharomyces cerevisiae modulates gene activity through a family of factors including SPT21, RPD1 (SIN3), RPD3 and CCR4

In Saccharomyces cerevisiae, the CPF1 gene encodes a centromere binding protein that also plays a role in transcription; cpf1 strains are methionine auxotrophs. In this paper we describe four strains that are methionine prototrophs despite containing a defective CPF1 gene. These strains, which contain mutations at either the SPT21, RPD1 (SIN3), RPD3 or CCR4 loci, have defective centromere function and a chromatin structure around the CDEI elements in the MET25 promoter characteristic of strains lacking CPF1. This indicates that the roles of CPF1 in transcription, centromere function and chromatin modulation around CDEI sites are different. We propose that CPF1 functions to overcome the repressing action, mediated via inactive chromatin, of proteins such as SPT21 or RPD1 (SIN3) on gene expression. The absence of proteins such as SPT21 or RPD1 (SIN3) relieves this repression and explains how methionine prototrophy is restored in the absence of CPF1.

Identification of three genes required for the glucose-dependent transcription of the yeast transcriptional activator ADR1

Glucose repression of the ADH2 gene from Saccharomyces cerevisiae is mediated by the synthesis and activity of the transcriptional activator ADR1. In this study, we isolated mutations in three new genes (SAF1, SAF2 and SAF3) that suppressed the glucose-insensitive expression of ADH2 caused by the ADR1-5c allele. The mechanism by which the SAF genes maintain ADR1-5c function was investigated. Each of the mutated SAF genes was found to suppress ADR1-5c activity by lowering ADR1-5c steady state mRNA levels 5- to 8-fold under glucose growth conditions. ADR1 mRNA levels were similarly affected by the saf mutations. In contrast, mutations in the SAF genes had little or no effect on ADR1-5c or ADR1 mRNA levels under ethanol growth conditions. The stability of ADR1-5c mRNA was unaffected by mutations in each of the SAF genes, implying that the SAF genes are required for the transcription of ADR1 mRNA under glucose growth conditions. The possible function of the three SAF genes in ADR1 expression is discussed.

A ubiquitin-conjugating enzyme, RAD6, affects the distribution of Ty1 retrotransposon integration positions

A galactose-inducible Ty1 element was used to generate 59 independent Ty1 inserts that inactivate the CAN1 gene. As found in previous studies, the distribution of these elements shows a gradient of insertion frequency from highest to lowest between the 5' and 3' ends of the gene. However, 53 independent Ty1 and Ty2 insertions isolated by an identical procedure in an isogenic rad6 deletion strain do not show this bias. In this strain, the Ty elements insert randomly throughout CAN1. These results show that the ubiquitin-conjugating enzyme, RAD6, alters the integration site preferences of Ty1 retrotransposons.

Regulation of fitness in yeast overexpressing glycolytic enzymes: parameters of growth and viability

Current models predict that large increases over wild-type in the activity of one enzyme will not alter an organism's fitness. This prediction is tested in Saccharomyces cerevisiae through the use of a high copy plasmid that bears one of the following: hexokinase B (HEXB), phosphoglucose isomerase (PGI), phosphofructokinase (PFKA and PFKB), or pyruvate kinase (PYK). Transformants containing these plasmids demonstrate a four to ten-fold increase in enzyme specific activity over either the parent strain or transformants containing the plasmid alone. Haploid and diploid transformants derived from independent backgrounds were grown on both fermentable and non-fermentable carbon sources and evaluated for several components of fitness. These include growth rate under non-limiting conditions, maximum stationary phase density, and viability in extended batch culture. Cell viability is not affected by overproduction of these enzymes. Growth rate and stationary phase density do not differ significantly among strains that overexpress HEXB, PGI or contain the vector alone. PFKA, B transformants show reduced growth rate on glucose in one background only. For these loci the current model is confirmed. By contrast, when grown on glucose, yeast overexpressing PYK demonstrate reduced growth rate and increased stationary phase density in both backgrounds. These effects are abolished in cells containing plasmids with a Tn5 disrupted copy of the PYK gene. Our results are consistent with reports that the PYK locus may exert control over the yeast cell cycle and suggest that it will be challenging to model relations between fitness and activity for multifunctional proteins.

The CCR1 (SNF1) and SCH9 protein kinases act independently of cAMP-dependent protein kinase and the transcriptional activator ADR1 in controlling yeast ADH2 expression

cAMP-dependent protein kinase (cAPK) is implicated in the inactivation of the yeast transcriptional activator ADR1, which regulates glucose-repressible ADH2 gene expression. The interdependence of cAPK, SCH9 (a protein kinase that when overexpressed can functionally substitute for cAPK), and the CCR1 (SNF1) protein kinase that is required for ADH2 expression was studied. SCH9 was found to be required for ADH2 expression in contrast to the inhibitory role played by cAPK. CCR1 and SCH9 were observed to affect ADH2 expression independently of both ADR1 and cAPK. In contrast, cAPK was shown to exert its effects on ADH2 solely through ADR1. These results indicate that the SCH9 and CCR1 protein kinases are components of regulatory pathways separate from that utilized by cAPK to control ADR1 activity and ADH2 expression.

Sequence and localization of the gene encoding yeast phosphoglycerate mutase

In this study we report on the complete nucleotide sequence of the yeast phosphoglycerate mutase gene (GPM1) and its essential 5' and 3' non-coding regions. The transcriptional start points were determined by S1-mapping and sequencing of a cDNA clone. Several sequences identified as important for transcriptional regulation in yeast promoters are present upstream of the transcription start point. 3' to the coding region we sequenced a composite repetitive element which, apparently, originated from a recombination between a delta- and a tau-element. Finally, we mapped the GPM1 gene 13 cM distal to fas1 on chromosome XI.

Regulation of sugar and ethanol metabolism in Saccharomyces cerevisiae

This review briefly surveys the literature on the nature, regulation, genetics, and molecular biology of the major energy-yielding pathways in yeasts, with emphasis on Saccharomyces cerevisiae. While sugar metabolism has received the lion's share of attention from workers in this field because of its bearing on the production of ethanol and other metabolites, more attention is now being paid to ethanol metabolism and the regulation of aerobic metabolism by fermentable and nonfermentable substrates. The utility of yeast as a highly manipulable organism and the discovery that yeast metabolic pathways are subject to the same types of control as those of higher cells open up many opportunities in such diverse areas as molecular evolution and cancer research.

Analysis of yeast retrotransposon Ty insertions at the CAN1 locus

The target site distribution for 55 independent Ty insertions that inactivate the function of the Saccharomyces cerevisiae CAN1 gene is reported. Under some selection conditions Ty elements inserted preferentially into the promoter and exhibited an orientation bias. In contrast, under other conditions no insertions were detected in the promoter region and transposition appeared to occur randomly throughout the CAN1 coding sequence. These results show that the target site distribution for Ty insertions may be a function of the selection conditions.

DNA damage activates transcription and transposition of yeast Ty retrotransposons

A set of genes isolated from Saccharomyces cerevisiae showed increased transcript levels after yeast had been exposed to ultraviolet (UV) light or 4-nitroquinoline-1-oxide (4NQO). Included among these DNA damage responsive (DDR) genes were members of the Ty retrotransposon family of yeast. Northern hybridization analysis indicated that maximal levels of a 5.6 kb transcript encoded by the Ty elements accumulated in cells after 4 to 6 h of exposure to 4NQO. The induced levels of transcripts varied from two- to tenfold for different Ty probes although similar kinetics and dose responses were observed for transcripts hybridizing to the different Ty family members. Pulse labeling experiments suggested that the accumulation of Ty transcripts was due, in part, to an increased rate of Ty message synthesis. Transposition of Ty elements to two target loci encoding distinct alcohol dehydrogenase enzymes, ADH2 and ADH4, was examined in cells exposed to increasing doses of UV light or 4NQO. The frequency of Ty insertion into these genetic regions following DNA damaging treatments increased by as much as 17-fold compared with untreated cells. These results provide direct evidence that transposable elements can be activated by physical and chemical mutagens/carcinogens and that transpositional mutagenesis is induced by these agents in S. cerevisiae.

Cyclic AMP-dependent protein kinase phosphorylates and inactivates the yeast transcriptional activator ADR1

It has been proposed in several eukaryotic systems that the regulation of gene transcription involves phosphorylation of specific transcription factors. We report here that the yeast transcriptional activator ADR1 is phosphorylated in vitro by cyclic AMP-dependent protein kinase and that mutations which enhance the ability of ADR1 to activate ADH2 expression decrease ADR1 phosphorylation. We also show that increased kinase activity in vivo inhibits ADH2 expression in an ADR1 allele-specific manner. Our data suggest that glucose repression of ADH2 is in part mediated through a cAMP-dependent phosphorylation-inactivation of the ADR1 regulatory protein.

Isolation and analysis of a novel class of suppressor of Ty insertion mutations in Saccharomyces cerevisiae

Using a new scheme for the isolation of suppressor of Ty insertion mutations (spt mutations) in yeast, we have identified six new SPT genes. Mutations in two of these genes, SPT13 and SPT14, exhibit a novel suppression pattern: suppression of complete Ty insertion mutations, but not of solo delta insertion mutations. Transcriptional analysis shows that spt13- and spt14-mediated suppression of Ty insertion mutations is the result of an elevation in the levels of adjacent gene transcription. In spite of the failure of these mutations to suppress solo delta insertion mutations, they do cause changes in transcription of at least one solo delta insertion mutation. In addition, spt13 and spt14 mutations are epistatic to mutations in certain other SPT genes that do suppress solo delta insertion mutations. These results suggest that the SPT13 and SPT14 gene products may act via sequences in both the delta and epsilon regions of Ty elements. Finally, mutations in SPT13 cause sporulation and mating defects and SPT14 is essential for growth, suggesting that these two genes have important roles in general cellular functions.

The identification and characterization of ADR6, a gene required for sporulation and for expression of the alcohol dehydrogenase II isozyme from Saccharomyces cerevisiae

The alcohol dehydrogenase II isozyme (enzyme, ADHII; structural gene, ADH2) of the yeast, Saccharomyces cerevisiae, is under stringent carbon catabolite control. This cytoplasmic isozyme exhibits negligible activity during growth in media containing fermentable carbon sources such as glucose and is maximal during growth on nonfermentable carbon sources. A recessive mutation, adr6-1, and possibly two other alleles at this locus, were selected for their ability to decrease Ty-activated ADH2-6c expression. The adr6-1 mutation led to decreased ADHII activity in both ADH2-6c and ADH2+ strains, and to decreased levels of ADH2 mRNA. Ty transcription and the expression of two other carbon catabolite regulated enzymes, isocitrate lyase and malate dehydrogenase, were unaffected by the adr6-1 mutation. adr6-1/adr6-1 strains were defective for sporulation, indicating that adr6 mutations may have pleiotropic effects. The sporulation defect was not a consequence of decreased ADH activity. Since the ADH2-6c mutation is due to insertion of a 5.6-kb Ty element at the TATAA box, it appears that the ADR6+-dependent ADHII activity required ADH2 sequences 3' to or including the TATAA box. The ADH2 upstream activating sequence (UAS) was probably not required. The ADR6 locus was unlinked to the ADR1 gene which encodes another trans-acting element required for ADH2 expression.

The cloning and mapping of ADR6, a gene required for sporulation and for expression of the alcohol dehydrogenase II isozyme from Saccharomyces cerevisiae

The alcohol dehydrogenase II (ADH2) gene of the yeast, Saccharomyces cerevisiae, is not transcribed during growth on fermentable carbon sources such as glucose. Growth of yeast cells in a medium containing only nonfermentable carbon sources leads to a marked increase or derepression of ADH2 expression. The recessive mutation, adr6-1, leads to an inability to fully derepress ADH2 expression and to an inability to sporulate. The ADR6 gene product appears to act directly or indirectly on ADH2 sequences 3' to or including the presumptive TATAA box. The upstream activating sequence (UAS) located 5' to the TATAA box is not required for the Adr6- phenotype. Here, we describe the isolation of a recombinant plasmid containing the wild-type ADR6 gene. ADR6 codes for a 4.4-kb RNA which is present during growth both on glucose and on nonfermentable carbon sources. Disruption of the ADR6 transcription unit led to viable cells with decreased ADHII activity and an inability to sporulate. This indicates that both phenotypes result from mutations within a single gene and that the adr6-1 allele was representative of mutations at this locus. The ADR6 gene mapped to the left arm of chromosome XVI at a site 18 centimorgans from the centromere.

A TY1 element is inserted in the CYR1 control region of Saccharomyces cerevisiae strain AB320

Southern blotting using a 5'-proximal probe of the Saccharomyces cerevisiae CYR1 gene has revealed heterogeneity in laboratory strains. It is demonstrated that strain AB320 contains a Ty1 element inserted in the promoter region of CYR1. The Ty1 orientation suggests that transcription of CYR1 is initiated downstream from the insertion region.

Molecular characterization of transposable-element-associated mutations that lead to constitutive L-ornithine aminotransferase expression in Saccharomyces cerevisiae

The cargB or CAR2 gene, coding for ornithine aminotransferase, was isolated by functional complementation of a cargB- mutation in Saccharomyces cerevisiae. It was used as a hybridization probe to analyse RNA and chromosomal DNA from four strains bearing cis-dominant regulatory mutations leading to constitutive, mating-type-dependent, ornithine aminotransferase synthesis. The four mutations appear to be insertions. Their size and restriction pattern suggested that they were transposable elements, Ty1. All were inserted in the same orientation with respect to the cargB gene. We cloned the cargB gene with its associated insertion from two constitutive mutants (cargB+ Oh-1 and cargB+ Oh-2). We determined the sequence of the cargB 5' region from the wild-type gene and from the two mutated genes. The DNA sequences of the extremities of the two insertions were very homologous but not identical and were similar to previously reported Ty1 element direct repeats (delta). The same five-base-pair sequence, ATATA, was found at both ends of both Ty1 elements, indicating that both Ty1 were transposed to the same site. This site is located 115 base pairs upstream from the putative cargB coding region. The 5' end of cargB transcript as determined by S1 mapping was the same in the wild-type strain and in the four mutants. The cargB transcript was not detected in the wild-type strain grown under non-induced conditions, while under the same conditions it was present in all four mutants.

Resistance to antimycin A in yeast by amplification of ADH4 on a linear, 42 kb palindromic plasmid

A yeast strain lacking a functional copy of ADH1 has been isolated that is resistant to antimycin A because of the presence of multiple copies of a nuclear gene, ADH4. The amplified copies of ADH4 exist on linear molecules 42 kb in length, which can be separated from chromosomal DNA by orthogonal-field-alternation gel electrophoresis. These amplified molecules are palindromes that reanneal rapidly after denaturation to form linear, snap-back molecules 21 kb in length. The amplified ADH4 sequences are bounded by telomere-homologous sequences. The chromosomal copy of ADH4 is the most distal marker on the left arm of chromosome VII, and the amplified ADH4-containing molecules appear to contain two copies of the region extending from ADH4 to the telomere.

X-linked mutations that give rise to overproduction of glucose-6-phosphate dehydrogenase in Drosophila melanogaster

Two X-linked mutations that give rise to overproduction of glucose-6-phosphate dehydrogenase (G6PD) were found among the progenies of isogenic strains which had been subjected to selection for high G6PD activity. Mapping of the high-activity factor in these mutants was carried out using car ZwB sw males of low G6PD activity. As a result, the factor mapped 0.02-0.04 unit to the left of the Zw locus. The amount of the G6PD gene was also quantitated utilizing a cloned G6PD gene as a probe, but no significant difference was found between the mutants and low-G6PD activity flies which shared the same X, second, and third chromosomes with the mutants. These findings are consistent with our notion that the mutations might be regulatory mutations, possibly resulting from the insertion of a novel class of transposable genetic elements.

Effect on gluconeogenesis of mutants blocking two mitochondrial transport systems in the yeast Saccharomyces cerevisiae

Two mutants of Saccharomyces cerevisiae, ccr1 and tpy1, have been found to interfere with the transport of small molecules across the inner mitochondrial membrane. Both also have the effect of interfering with the synthesis of a number of cytoplasmically located enzymes involved in gluconeogenesis, even when the cells are released from glucose repression. The ccr1 mutant, defective in the transport of dicarboxylic acids across the inner membrane, represses the synthesis of gluconeogenic enzymes almost totally, but synthesis can be induced on complete medium without a carbon source. This mutant has low levels of intracellular malate under all growth conditions tested. The tpy1 mutant, defective in the transport of pyruvate across the inner membrane, shows repression of gluconeogenesis enzymes under some growth conditions, particularly high levels of ethanol in the medium. These conditions also lead to low levels of malate in the cells. Intracellular levels of malate in these mutants, and in the wild type, are correlated with the levels of gluconeogenic enzymes present. The ability of isolated mutant mitochondria to phosphorylate ADP is shown to be consistent with the interpretation that they are defective in inner membrane transport, although as yet no evidence is available that these defects are the primary lesions in the two mutants. The data are consistent with two general models. In one, the exhaustion of an extramitochondrial corepressor or introduction of a coinducer by mitochondrial activity triggers the induction of gluconeogenic enzyme synthesis. In the second, the mitochondria themselves trigger this induction, but only when the tricarboxylic acid cycle is able to operate at a high level.

Gene conversion and reciprocal exchange in a Ty-mediated translocation in yeast

A haploid yeast mutant carrying a reciprocal translocation was analyzed. Cloning and comparison of sequences involved in the translocation event in wildtype and mutant revealed that the crossover between nonhomologous chromosomes has occurred within Ty sequences. By DNA sequence analysis it could be demonstrated that the reciprocal recombination event is accompanied by a short segment of non-reciprocal exchange (gene conversion) in the immediate vicinity of the crossover. Analysis of the translocation mutant and revertant isolates also indicated that the regulatory effect of Ty elements on adjacent genes can be modified by discrete changes within a Ty element.

The SPT3 gene is required for normal transcription of Ty elements in S. cerevisiae

The transposable Ty elements consist of a central core, epsilon, flanked by direct repeats called deltas. In wild-type strains Ty transcripts initiate in one delta and terminate in the other. Insertion mutations caused by Ty elements have a wide variety of phenotypes, ranging from inhibition of gene expression to constitutive gene expression. Mutations in the SPT3 gene suppress these effects of Ty and delta insertion mutations on adjacent genes. In spt3 null mutants the Ty transcription pattern for the entire ensemble of Ty elements is changed. The delta-delta transcripts are absent and initiation begins at a position 800 bp into the epsilon region. In these spt3 strains, transcription that initiates in solo deltas and proceeds into adjacent structural genes is also abolished. The requirement of SPT3 for normal transcription from delta can explain the ability of spt3 mutations to suppress the mutations caused by Ty and delta insertions. In SPT3 strains transcription from the delta into adjacent sequences interferes with normal expression of those sequences, whereas in spt3 strains the aberrant transcript is not made. spt3 mutations also lead to defects in diploid formation and sporulation, suggesting that SPT3 is important for the expression of genes in addition to Ty elements.

Ty1-promoted expression of aspartate transcarbamylase in the yeast Saccharomyces cerevisiae

An entire copy of a Ty1 yeast transposon has been found inserted between two regions comprising the single transcriptional and translational URA2 units in yeast that code respectively for carbamylphosphate synthetase (CPSase) and aspartate transcarbamylase (ATCase). The mutant Rev 16 was obtained from an ATCase- strain blocked by multiple nonsense mutations in the proximal CPSase region and submitted to selective pressure for recovery of the enzyme activity coded by the distal part of the gene. The inserted Ty1 has one XhoI site in both delta elements, delimiting a 5.6 kb piece of DNA that shows a classical Ty1 restriction pattern. The orientation of this sequence in URA2 is the same as in the previously described examples in which Ty1 has positive effects on the expression of adjacent genes. In this case the Ty1 is situated more than 1 kb from the URA2 region in which ATCase structural mutants have been mapped. Nevertheless, transcription of the entire sequence distal to the Ty1 is restored and has become subject to mating-type control, leading to a weak enzyme activity. Our observations are in agreement with generally accepted ideas regarding the way in which Ty1 elements affect gene expression, and additionally, represent the first example of a Ty1 -promoted reinitiation occurring in the middle of a single transcription unit.

Preferential integration of yeast transposable element Ty into a promoter region

Mobile genetic elements have been identified in several eukaryotic organisms and some classes have been found to share common structural features with the proviral forms of animal retroviruses. The representatives of this class of mobile elements in the yeast Saccharomyces cerevisiae are called Ty elements, which could be a useful model system for studying the transposition of retrovirus-like elements. Here we have attempted to answer two questions often raised in discussions of the biological importance of transposition: what is the frequency of spontaneous Ty transposition, and are there certain chromosomal regions into which Ty elements preferentially integrate? We chose the LYS2 gene to investigate these questions because it allows direct selection of both mutants and revertants. We have found that 2% of spontaneous lys2 mutants are caused by Ty transposition with a preferential integration into the transcription initiation region.

DNA sequences of two yeast promoter-up mutants

The yeast Saccharomyces cerevisiae has three genetic loci encoding different alcohol dehydrogenase (ADH) isozymes: ADC1, which encodes the classical fermentative isozyme ADHI; ADR2, which encodes the glucose-repressed isozyme ADHII; and ADM, which encodes an ADH isozyme found associated with mitochondria. When yeast are grown on glucose, the ADC1 gene is expressed, and the ADR2 gene repressed. Conversely, growth on a non-fermentable carbon source such as ethanol or glycerol results in derepression of ADR2, and repression of ADC1. The ADC1 and ADR2 genes have been cloned and sequenced, and a number of cis-acting mutations identified that cause constitutive expression of ADR2, and seem to fall into two classes. The most abundant class consists of mutants that cannot be fully derepressed, and do not revert to wild type at a detectable level: these are caused by the insertion of a transposable element into the 5'-flanking region of the gene. The second class of mutants do revert to a glucose-repressed phenotype at a detectable frequency, and when grown on non-fermentable carbon sources derepress ADR2 to levels up to five times those found in wild-type cells. We report here the sequencing of the 5'-flanking regions of two such promoter-up, constitutive ADR2 mutants, in both of which the mutant phenotype is associated with an increase in length of a poly(A) X poly(T) tract 222 base pairs (bp) upstream of the gene.