Cell-type-dependent gene activation by yeast transposon Ty1 involves multiple regulatory determinants

Role of Saccharomyces cerevisiae Rap1 protein in Ty1 and Ty1-mediated transcription

Binding sites for the transcription factor Rap1 are widespread in the yeast genome. With respect to many, but not all, genes, Rap1p has an apparent activation function. Whether Rap1 is itself a transcriptional activator, or whether it is in some way required for activation by additional factors, is not clear. We have identified a previously unrecognized Rap1p binding site in the internal regulatory region of Ty1 elements. We demonstrate that this site is capable of binding Rap1 in vitro and that, in vivo, Rap1p plays an important regulatory role in Ty1 and Ty1-mediated adjacent gene expression. Our data suggest that in Ty1 elements, maximal levels of RAP1-mediated activation depend on the formation of a complex with Mcm1, an independent DNA-binding protein that functions in transcription as well as in DNA replication, and with a third factor, IBF, previously identified as a binding activity with a site situated between the Rap1p and Mcm1p binding sites in this region of Ty1 elements.

The Ty1 LTR-retrotransposon of budding yeast, Saccharomyces cerevisiae

Long-terminal repeat (LTR)-retrotransposons generate a copy of their DNA (cDNA) by reverse transcription of their RNA genome in cytoplasmic nucleocapsids. They are widespread in the eukaryotic kingdom and are the evolutionary progenitors of retroviruses [1]. The Ty1 element of the budding yeast Saccharomyces cerevisiae was the first LTR-retrotransposon demonstrated to mobilize through an RNA intermediate, and not surprisingly, is the best studied. The depth of our knowledge of Ty1 biology stems not only from the predominance of active Ty1 elements in the S. cerevisiae genome but also the ease and breadth of genomic, biochemical and cell biology approaches available to study cellular processes in yeast. This review describes the basic structure of Ty1 and its gene products, the replication cycle, the rapidly expanding compendium of host co-factors known to influence retrotransposition and the nature of Ty1's elaborate symbiosis with its host. Our goal is to illuminate the value of Ty1 as a paradigm to explore the biology of LTR-retrotransposons in multicellular organisms, where the low frequency of retrotransposition events presents a formidable barrier to investigations of retrotransposon biology.

Spontaneous deletions and reciprocal translocations in Saccharomyces cerevisiae: influence of ploidy

Studying spontaneous chromosomal rearrangements throws light on the rules underlying the genome reshaping events occurring in eukaryotic cells, which are part of the evolutionary process. In Saccharomyces cerevisiae, translocation and deletion processes have been frequently described in haploids, but little is known so far about these processes at the diploid level. Here we investigated the nature and the frequency of the chromosomal rearrangements occurring at this ploidy level. Using a positive selection screen based on a particular mutated allele of the URA2 gene, spontaneous diploid revertants were selected and analysed. Surprisingly, the diploid state was found to be correlated with a decrease in chromosome rearrangement frequency, along with an increase in the complexity of the rearrangements occurring in the target gene. The presence of short DNA tandem repeat sequences seems to be a key requirement for deletion and reciprocal translocation processes to occur in diploids. After discussing the differences between the haploid and diploid levels, some mechanisms possibly involved in chromosome shortening and arm exchange are suggested.

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.

Enhanced Secretion of Human Nerve Growth Factor from Saccharomyces cerevisiae using an Advanced δ–Integration System

We have designed an advanced delta-integration system (integration of genes into the delta-sequence of yeast retrotransposon Ty) and used it for secretion of human nerve growth factor (hNGF) from Saccharomyces cerevisiae. The expression and secretion of hNGF was directed by the PGK promoter and MF alpha 1 prepro-signal. Using two selectable markers (URA3 and leu2-d), haploid yeast strains were constructed with approximately 20 copies of a delta-integrated hNGF expression cassette on four chromosomes. The strain secreted hNGF at levels 3-4 fold higher than a 2 micron-based plasmid. Northern and Western analyses revealed that the oversecretion was caused by an increased amount of mRNA. We also detected an unusual processing of the MF alpha 1 prepro-hNGF fusion protein that required the pep4 mutation. Application of this system for industrial purposes is discussed.

Four tandem defective P elements associated with positive regulation of the Drosophila melanogaster glucose-6-phosphate dehydrogenase gene

Three high-glucose-6-phosphate dehydrogenase (G6PD)-activity mutants (2512H, S44H, and 1FH) are characterized by two insertion sequences associated with the G6PD locus; one (Ins1; 3.5 kb long in 2512H and S44H and 2.9 kb long in 1FH) is present just 5' to exon I and consists of a KP' (the 32nd base of the KP was replaced by guanine), a core sequence and a KP, and the other is 4.2 kb long and resides within an intron. Southern blot analyses of revertants showing low G6PD activity suggested that the insertion sequence responsible for high G6PD activity may be the core sequence but not the flanking KP and KP' or the Ins2. DNA sequencing data of the clone carrying the core sequence of 2512H demonstrated that the core sequence is another type of defective P elements (core P). Interestingly, a protein(s) was found in the nuclear extract of Canton S embryos that specifically binds to the core P but not to the KP or various fragments of p pi 25.1. In addition, the mutant G6PD activity was found to be affected not only by the genotype, but also by cytoplasmic factors.

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.