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Five of the laboratory's seven groups study mechanisms of gene regulation, chromosome condensation and segregation, and the transposition of retroelements in the unicellular eukaryotes Saccharomyces cerevisiae and Schizosaccharomyces pombe. The powerful genetic tools developed for these yeasts has made the organisms excellent model systems for studying basic processes common to all eukaryotic cells. The section headed by Alan Hinnebusch is studying a global regulatory mechanism called general amino acid control, which governs amino acid and nucleotide biosynthesis in Saccharomyces cerevisiae, as a paradigm for nutrient control of gene expression. Dissecting this regulatory pathway has led to an intensive study of the mechanism of protein synthesis and its control by protein phosphorylation, strategies for modulating protein kinase activity, and mechanisms of transcriptional activation. The group recently showed that four different eukaryotic translation initiation factors (eIFs) occur in a pre-assembled multifactor complex rather than binding to ribosomes individually in the step-wise manner envisioned in text-book accounts of protein synthesis. The group also made significant progress in determining how uncharged tRNAs stimulate the translation initiation factor 2 (eIF2) kinase, GCN2. The unit headed by Thomas Dever is characterizing the structure and function of several eIFs and multiple protein kinases that regulate eIF2 activity, focusing on the molecular principles of kinase-substrate recognition. The group recently determined the x-ray structure of the universally conserved translation initiation factor eIF5B/IF2 and showed that GTP hydrolysis by eIF5B results in significant domain rearrangements via an articulated lever mechanism. The unit headed by Rohinton Kamakaka studies the chromosomal proteins, DNA elements, and mechanisms involved in the establishment, maintenance, and propagation of gene silencing. The group recently demonstrated a role for a histone H2a variant in silencing and purified and characterized Sir2p-containing complexes from yeast. The unit headed by Alexander Strunnikov is investigating the factors that control chromosome condensation and accurate segregation of chromosomes during mitosis. The researchers recently discovered a crucial link between the dynamics of specialized chromatin domains (rDNA) and cell-cycle controls facilitating the specificity of targeting chromatin condensation molecules (condensins). The group also made important progress in dissecting the molecular basis of somatic chromosome pairing and sister chromatid cohesion. The section headed by Henry Levin is analyzing a retrotransposon with many structural features in common with mammalian retroviruses to define each step in the transposition process at the molecular level. An important goal is to identify the contributions of host factors involved in assembly of virus-like particles, migration of reverse transcripts to the nucleus, and integration of cDNAs into the host genome. The group recently showed that the retrotransposon Tf1 has strong preferences for specific sites of insertion throughout the genome. The anuran Xenopus laevis serves as a model system for the remaining two groups. The unit headed by Yun-Bo Shi studies gene regulatory mechanisms involving thyroid hormone receptor (TR) that establish the developmental program of metamorphosis and investigates the key molecules induced by TR, matrix metalloproteinases (MMPs), that govern postembryonic organ remodeling. The group recently demonstrated that TR uses histone deacetylases to repress target genes in development and that the MMP stromelysin-3 is required for thyroid hormone-induced apoptosis during metamorphosis. The section headed by Mary Dasso focuses on cell cycle-regulatory checkpoints that govern the onset and completion of mitosis, concentrating on two closely linked biochemical pathways involving the Ran GTPase and SUMO-1 conjugation system, which also have been implicated in nuclear trafficking. The group showed recently that the SUMO-1 pathway is regulated through the localization and differential expression of conjugation and deconjugation enzymes. It has also provided evidence of additional factor(s) that regulate the modification of RanGAP1. Because the cellular machines involved in transcription, translation, nuclear import, chromosome mechanics, and cell-cycle regulation are highly conserved among diverse eukaryotes, the findings from yeast and Xenopus are yielding important insights into these critical processes as they occur in mammalian cells. Likewise, the analysis of frog metamorphosis is providing valuable information about the role of thyroid hormone and metalloproteinases in postembryonic development in mammals.