Suzanne Sandmeyer

Mobile DNA III

This new edition of the bestselling series on movable genetic elements highlights the many exciting advances in the field over the last decade, including conservative site-specific recombination, programmed rearrangements, DNA-only transposons, and LTR, and non-LTR retrotransposons. Virtually all organisms contain multiple mobile DNAs that can move from place to place, and in some organisms, mobile DNA elements make up a significant portion of the genome. Mobile DNA III provides a comprehensive review of recent research, revealing the many important roles that mobile DNAs play in genome structure, function, and evolution. This book is part three of a series on mobile DNA. This title is published by the American Society of Microbiology Press and distributed by Taylor and Francis in rest of world territories.

Requirement of RNA polymerase III transcription factors for in vitro position-specific integration of a retroviruslike element

The yeast retroviruslike element Ty3 inserts at the transcription initiation sites of genes transcribed by RNA polymerase III (Pol III). An in vitro integration assay was developed with the use of Ty3 viruslike particles and a modified SUP2 tyrosine transfer RNA (tRNA(Tyr)) gene target. Integration was position-specific and required Ty3 integrase, Pol III transcription factor (TF) IIIB-, TFIIIC-, and Pol III-containing fractions showed that TFIIIB and TFIIIC, together, were sufficient for position-specific Ty3 integration, but not for transcription. This report demonstrates that in vitro integration of a retroelement can be targeted by cellular proteins.

Integration by design

As goes history, so goes research: this year, activity in areas of retrovirus research related only indirectly have provoked events that are notable when considered together. Last summer it was reported that a patient in one X-linked severe combined immunodeficiency retroviral vector gene therapy trial had developed leukemia. Now disquietingly, there has been a second such event, and a third patient is reported to have a vector insertion near the same gene (LMO2) as observed in the other two individuals (1). Meanwhile, in a basic research laboratory, experiments have moved us another step closer to understanding the mechanics of insertion specificity for retrovirus-type integrases (IN). As reported in this issue of PNAS, investigators have produced active retroviruslike elements with synthetic insertion specificities (2). Dan Voytas and colleagues at Iowa State University (Ames) study the Saccharomyces long terminal repeat (LTR)-retrotransposon Ty5, which targets heterochromatic regions (3). Now, in an elegant adaptation of the two-hybrid system, the 6-aa Ty5 targeting domain (TD) was exchanged for two heterologous domains shown to mediate interaction of their respective proteins with protein partners. When domains from those partners were produced fused to the LexA DNA-binding domain, targeting to LexA-binding sites was observed. Although integration specificity in the system was by no means absolute, these results are of interest to genetic engineers and future gene therapists.

Tailoring the genome: the power of genetic approaches

In the last century, genetics has developed into one of the most powerful tools for addressing basic questions concerning inheritance, development, individual and social operations and death. Here we summarize the current approaches to these questions in four of the most advanced models organisms: Saccharomyces cerevisiae (yeast), Caenorhabditis elegans (worm), Drosophila melanogaster (fly) and Mus musculus (mouse). The genomes of each of these four models have been sequenced, and all have well developed methods of efficient genetic manipulations.

The yeast Ty3 retrotransposon contains a 5′–3′ bipartite primer‐binding site and encodes nucleocapsid protein NCp9 functionally homologous to HIV‐1 NCp7

Retroviruses, including HIV‐1 and the distantly related yeast retroelement Ty3, all encode a nucleoprotein required for virion structure and replication. During an in vitro comparison of HIV‐1 and Ty3 nucleoprotein function in RNA dimerization and cDNA synthesis, we discovered a bipartite primer‐binding site (PBS) for Ty3 composed of sequences located at opposite ends of the genome. Ty3 cDNA synthesis requires the 3′ PBS for primer tRNAiMet annealing to the genomic RNA, and the 5′ PBS, in cis or in trans, as the reverse transcription start site. Ty3 RNA alone is unable to dimerize, but formation of dimeric tRNAiMet bound to the PBS was found to direct dimerization of Ty3 RNA–tRNAiMet. Interestingly, HIV‐1 nucleocapsid protein NCp7 and Ty3 NCp9 were interchangeable using HIV‐1 and Ty3 RNA template–primer systems. Our findings impact on the understanding of non‐canonical reverse transcription as well as on the use of Ty3 systems to screen for anti‐NCp7 drugs.

A vector set for systematic metabolic engineering in Saccharomyces cerevisiae

A set of shuttle vectors was constructed to facilitate expression of genes for metabolic engineering in Saccharomyces cerevisiae. Selectable markers include the URA3, TRP1, MET15, LEU2‐d8, HIS3 and CAN1 genes. Differential expression of genes can be achieved as each marker is available on both CEN/ARS‐ and 2 µ‐containing plasmids. Unique restriction sites downstream of TEF1, PGK1 or HXT7‐391 promoters and upstream of the CYC1 terminator allow insertion of open‐reading frame cassettes for expression. Furthermore, a fragment appropriate for integration into the genome via homologous recombination can be readily generated in a polymerase chain reaction. Vector marker genes are flanked by loxP recognition sites for the CreA recombinase to allow efficient site‐specific marker deletion and recycling. Expression and copy number were characterized for representative high‐ and low‐copy vectors carrying the different marker and promoter sequences. Metabolic engineering typically requires the stable introduction of multiple genes and genomic integration is often preferred. This requires an expanded number of stable expression sites relative to standard gene expression studies. This study demonstrated the practicality of polymerase chain reaction amplification of an expression cassette and genetic marker, and subsequent replacement of endogenous retrotransposons by homologous recombination with flanking sequences. Such reporters were expressed comparably to those inserted at standard integration loci. This expands the number of available characterized integration sites and demonstrates that such sites provide a virtually inexhaustible pool of integration targets for stable expression of multiple genes. Together these vectors and expression loci will facilitate combinatorial gene expression for metabolic engineering. Copyright

Transposition of the yeast retroviruslike element Ty3 is dependent on the cell cycle.

Host cell cycle genes provide important functions to retroviruses and retroviruslike elements. To define some of these functions, the cell cycle dependence of transposition of the yeast retroviruslike element Ty3 was examined. Ty3 is unique among retroviruslike elements because of the specificity of its integration, which occurs upstream of genes transcribed by RNA polymerase III. A physical assay for Ty3 transposition which takes advantage of this position-specific integration was developed. The assay uses PCR to amplify a product of Ty3 integration into a target plasmid that carries a modified tRNA gene. By using the GAL1 upstream activating sequence to regulate expression of Ty3, transposition was detected within one generation of cell growth after Ty3 transcription was initiated. This physical assay was used to show that Ty3 did not transpose when yeast cells were arrested in G1 during treatment with the mating pheromone alpha-factor. The restriction of transposition was not due to changes in transcription of either Ty3 or tRNA genes or to aspects of the mating pheromone response unrelated to cell cycle arrest. The block of the Ty3 life cycle was reversed when cells were released from G1 arrest. Examination of Ty3 intermediates during G1 arrest indicated that Ty3 viruslike particles were present but that reverse transcription of the Ty3 genomic RNA into double-stranded DNA had not occurred. In G1, the Ty3 life cycle is blocked after particle assembly but before the completion of reverse transcription.

Ty3 Integrase Is Required for Initiation of Reverse Transcription

ABSTRACT The integrase (IN) encoded by the Saccharomyces cerevisiae retroviruslike element Ty3 has features found in retrovirus IN proteins including the catalytic triad, an amino-terminal zinc-binding motif, and a nuclear localization sequence. Mutations in the amino- and carboxyl-terminal domains of Ty3 IN cause reduced accumulation of full-length cDNA in the viruslike particles. We show that the reduction in cDNA is accompanied by reduced amounts of early intermediates such as minus-strand, strong-stop DNA. Expression of a capsid (CA)-IN fusion protein (CA-IN) complemented catalytic site and nuclear localization mutants, but not DNA mutants. However, expression of a fusion of CA, reverse transcriptase (RT), and IN (CA-RT-IN) complemented transposition of catalytic site and nuclear localization signal mutants, increased the amount of cDNA in some of the mutants, and complemented transposition of several mutants to low frequencies. Expression of a CA-RT-IN protein with a Ty3 IN catalytic site mutation did not complement transposition of either a Ty3 catalytic site mutant or a nuclear localization mutant but did increase the amount of cDNA in several mutants and complement at least one of the cDNA mutants for transposition. These in vivo data support a model in which independent IN domains can contribute to reverse transcription and integration. We conclude that during reverse transcription, the Ty3 IN domain interacts closely with the polymerase domain and may even constitute a domain within a heterodimeric RT. These studies also suggest that during integration the IN catalytic site and at least portions of the IN carboxyl-terminal domain act in cis.

Small Heat Shock Protein Suppression of Vpr-Induced Cytoskeletal Defects in Budding Yeast

Expression of the auxiliary human immunodeficiency virus type 1 (HIV-1) protein Vpr causes arrest of primate host cells in G2. Expression of this protein in budding yeast has been previously reported to cause growth arrest and a large-cell phenotype. Investigation of the effect of Vpr expression in budding yeast, reported here, showed that it causes disruption of the actin cytoskeleton. Expression of HSP42, the gene for a small heat shock protein (sHSP), from a high-copy-number plasmid reversed this effect. The sHSPs are induced by exposure of cells to thermal, osmotic, and oxidative stresses and to mitogens. In animal cells, overexpression of sHSPs causes increased resistance to stress and stabilization of actin stress fibers. Yeast cells subjected to mild stress, such as shifting from 23 to 39 degrees C, arrest growth and then resume cell division. Growth arrest is accompanied by transient disorganization of the cytoskeleton. Yeast in which the HSP42 gene was disrupted and which was subjected to moderate thermal stress reorganized the actin cytoskeleton more slowly than did wild-type control cells. These results demonstrate that in yeast, as in metazoan cells, sHSPs promote maintenance of the actin cytoskeleton.

A Truncation Mutant of the 95-Kilodalton Subunit of Transcription Factor IIIC Reveals Asymmetry in Ty3 Integration

ABSTRACT Position-specific integration of the retroviruslike element Ty3 near the transcription initiation sites of tRNA genes requires transcription factors IIIB and IIIC (TFIIIB and TFIIIC). Using a genetic screen, we isolated a mutant with a truncated 95-kDa subunit of TFIIIC (TFIIIC95) that reduced the apparent retrotransposition of Ty3 into a plasmid-borne target site between two divergently transcribed tRNA genes. Although TFIIIC95 is conserved and essential, no defect in growth or transcription of tRNAs was detected in the mutant. Steps of the Ty3 life cycle, such as protein expression, proteolytic processing, viruslike particle formation, and reverse transcription, were not affected by the mutation. However, Ty3 integration into a divergent tDNA target occurred exclusively in one orientation in the mutant strain. Investigation of this orientation bias showed that TFIIIC95 and Ty3 integrase interacted in two-hybrid and glutathioneS-transferase pulldown assays and that interaction with the mutant TFIIIC95 protein was attenuated. The orientation bias observed here suggests that even for wild-type Ty3, the protein complexes associated with the long terminal repeats are not equivalent in vivo.