PROGRAM IN CELLULAR REGULATION AND METABOLISM
Four of the seven groups in the Program in Cellular Regulation and Metabolism study transcriptional and translational control of gene regulation, chromosome condensation and segregation, and the transposition of retroelements in budding or fission yeast. Another group investigates the molecular basis of connection specificity in the Drosophila visual system; the remaining two groups use the aneuran Xenopus laevis as a model system.
Mary Dasso’s Section on Cell Cycle Regulation uses Xenopus laevis to study the Ran GTPase and the SUMO family of ubiquitin-like proteins as well as their role in the spindle assembly checkpoint. During the past year, the section continued its systematic analysis of SUMO proteases of the Ulp/SENP family, which catalyze both post-translational processing of SUMO proteins and SUMO deconjugation. Members of the section also investigated the assembly of inner centromere components and its regulation by the SAC kinase Bub1.
The Section on Protein Biosynthesis, headed by Thomas Dever, is characterizing the structure and function of several translation initiation factors and the molecular principles of kinase-substrate recognition. The section recently characterized a dimeric activation mechanism shared by the stress-responsive eIF2a kinases and revealed that the GTP hydrolysis by eIF5B regulates the domain movements required for release of both eIF5B and eIF1A from the ribosome following subunit joining.
The Section on Nutrient Control of Gene Expression, headed by Alan Hinnebusch, studies the transcriptional and translational control of amino acid biosynthetic genes. Recently, the section demonstrated that eIF1 is a gatekeeper that blocks initiation at non–AUG codons and is released from scanning ribosomes at AUGs under the control of eIF1A. The researchers discovered that the histone acetyltransferase GCN5 migrates into transcribed coding sequences and promotes histone eviction and processivity by elongating RNA Polymerase II. They further demonstrated that the nuclear cap binding complex (CBC) functions in transcription antitermination by blocking recruitment of cleavage/polyadenylation factor IA at weak terminators.
The Unit on Neuronal Connectivity, headed by Chi-Hon Lee, investigates the structure and development of color-vision circuitry in Drosophila. Recently, the unit discovered that TGF-beta/activin, via a novel autocrine mechanism, regulates axonal tiling of R7 photoreceptor neurons. The data indicate that activin signaling in R7s requires retrograde transport of the downstream transcription factor Smad2 from nerve growth cones to nuclei. The group also identified the target neurons that relay the R7 photoreceptor neurons to the higher optic ganglion lobula.
Henry Levin heads the Section on Eukaryotic Transposable Elements, which analyzes long-terminal repeat retrotransposons and their mechanisms of reverse transcription, import of particles into the nucleus, and integration of cDNA. Recently, the laboratory demonstrated that the preferential insertion of Tf1 into the 5¢ regions of genes results from the recognition of sequences within the promoter. In the case of the fbp1 promoter, the section found that integration occurred at the binding site for the transcription factor Atf1p and that the binding of Atf1p was required for integration. The results suggest that Atf1p binds to promoters and mediates Tf1 integration.
The Section on Molecular Morphogenesis, headed by Yun-Bo Shi, studies the gene-regulatory mechanisms controlled by thyroid hormone receptor (TR) that establish the developmental program of metamorphosis. The laboratory demonstrated dual functions of TR in development, i.e., corepressor repression of tadpole growth before metamorphosis and activation through co-activator recruitment for metamorphosis. The group also revealed different roles and mechanisms for several TR-regulated matrix metalloproteinases in tissue remodeling during metamorphosis.
Alexander Strunnikov’s Unit on Chromatin Structure and Function is studying SMC protein complexes, particularly the role of the condensin complex in mitotic chromosome condensation and segregation. Recently, the unit showed that condensin activity is monitored by the spindle-assembly checkpoint and characterized the role of condensin in the maintenance of centromere chromatin. The unit’s studies also established that condensin and RNA polymerase I play competing roles in the nucleolus, leading to a theory explaining the compartmentalization of transcription and chromosome condensation in chromosomal loci that are transcribed in mitosis.
Scientists in the Section on DNA Replication, Repair, and Mutagenesis, which is headed by Roger Woodgate, investiage the mechanisms by which mutations are introduced into damaged DNA. Many of the proteins long implicated in the mutagenic process are now known to be low-fidelity DNA polymerases that can replicate past damaged DNA in a process termed translesion DNA synthesis (TLS). In the past year, experiments aimed at understanding TLS have spanned the evolutionary spectrum and include characterization of E. coli polV(R391), the evolution of archaeal Taq-like polymerase for the PCR amplification of ancient DNA, and the in vivo and in vitro characterization of human DNA polymerase iota.
