http://hdl.handle.net/2027.42/78360 2013-05-25T01:44:57Z http://hdl.handle.net/2027.42/75694 Pedomorphosis revisited: thyroid hormone receptors are functional in Necturus maculosus Safi, Rachid; Vlaeminck-Guillem, Virginie; Duffraisse, Marilyne; Seugnet, Isabelle; Plateroti, Michelina; Margotat, Alain; Duterque-Coquillaud, Martine; Crespi, Erica J.; Denver, Robert J.; Demeneix, Barbara; Laudet, Vincent Heterochrony, a difference in developmental timing, is a central concept in modern evolutionary biology. An example is pedomorphosis, retention of juvenile characteristics in sexually mature adults, a phenomenon largely represented in salamanders. The mudpuppy ( Necturus maculosus ) is an obligate pedomorphic amphibian, never undergoing metamorphosis. Thyroid hormone induces tissue transformation in metamorphosing species and this action is mediated by nuclear thyroid hormone (TH) receptors (TRs). The absence of metamorphosis in Necturus has been attributed to a resistance to TH action as treatment with exogenous TH fails to induce transformation. The failure to metamorphose could be due to the lack of TR expression in target tissues, or to a loss of TR function. Toward understanding the molecular basis for the failure of Necturus tissues to respond to TH, and the ultimate cause for the expression of the obligate pedomorphic life history, we characterized the structure, function, and expression of TR genes in Necturus . Strikingly, we found that Necturus TRΑ and TRΒ genes encode fully functional TR proteins. These TRs bind both DNA and TH and can transactivate target genes in response to TH. Both TRΑ and TRΒ are expressed in various tissues. TH treatment in vivo induced expression in the gill of some but not all genes known to be activated by TH in anuran larvae, caused whole organism metabolic effects, but induced no external morphological changes in adults or larvae. Thus, Necturus possesses fully functional TRs and its tissues are not generally resistant to the actions of TH. Rather, the absence of metamorphosis may be due to the loss of TH-dependent control of key genes required for tissue transformation. 2006-05-01T00:00:00Z http://hdl.handle.net/2027.42/75652 The Caulobacter crescentus GTPase CgtA C is required for progression through the cell cycle and for maintaining 50S ribosomal subunit levels Datta, Kaustuv; Skidmore, Jennifer M.; Pu, Kun; Maddock, Janine R. The Obg subfamily of bacterial GTP-binding proteins are biochemically distinct from Ras-like proteins raising the possibility that they are not controlled by conventional guanine nucleotide exchange factors (GEFs) and/or guanine nucleotide activating proteins (GAPs). To test this hypothesis, we generated mutations in the Caulobacter crescentus obg gene ( cgtA C ) which, in Ras-like proteins, would result in either activating or dominant negative phenotypes. In C. crescentus , a P168V mutant is not activating in vivo , although in vitro , the P168V protein showed a modest reduction in the affinity for GDP. Neither the S173N nor N280Y mutations resulted in a dominant negative phenotype. Furthermore, the S173N was significantly impaired for GTP binding, consistent with a critical role of this residue in GTP binding. In general, conserved amino acids in the GTP-binding pocket were, however, important for function. To examine the in vivo consequences of depleting CgtA C , we generated a temperature-sensitive mutant, G80E. At the permissive temperature, G80E cells grow slowly and have reduced levels of 50S ribosomal subunits, indicating that CgtA C is important for 50S assembly and/or stability. Surprisingly, at the non-permissive temperature, G80E  cells  rapidly  lose  viability  and  yet  do not display an additional ribosome defect. Thus, the essential nature of the cgtA C gene does not appear to result from its ribosome function. G80E cells arrest as predivisional cells and stalkless cells. Flow cytometry on synchronized cells reveals a G1-S arrest. Therefore, CgtA C is necessary for DNA replication and progression through the cell cycle. 2004-12-01T00:00:00Z http://hdl.handle.net/2027.42/75549 Polymerizing the fibre between bacteria and host cells: the biogenesis of functional amyloid fibres Epstein, Elisabeth Ashman; Chapman, Matthew R. Amyloid fibres are proteinaceous aggregates associated with several human diseases, including Alzheimer's, Huntington's and Creutzfeldt Jakob's. Disease-associated amyloid formation is the result of proteins that misfold and aggregate into β sheet-rich fibre polymers. Cellular toxicity is readily associated with amyloidogenesis, although the molecular mechanism of toxicity remains unknown. Recently, a new class of ‘functional’ amyloid fibres was discovered that demonstrates that amyloids can be utilized as a productive part of cellular biology. These functional amyloids will provide unique insights into how amyloid formation can be controlled and made less cytotoxic. Bacteria produce some of the best-characterized functional amyloids, including a surface amyloid fibre called curli. Assembled by enteric bacteria, curli fibres mediate attachment to surfaces and host tissues. Some bacterial amyloids, like harpins and microcinE492, have exploited amyloid toxicity in a directed and functional manner. Here, we review and discuss the functional amyloids assembled by bacteria. Special emphasis will be paid to the biology of functional amyloid synthesis and the connections between bacterial physiology and pathology. 2008-07-01T00:00:00Z http://hdl.handle.net/2027.42/75526 REVOLUTA regulates meristem initiation at lateral positions Otsuga, Denichiro; DeGuzman, Bernadette; Prigge, Michael J.; Drews, Gary N.; Clark, Steven E. While the shoot apical meristem (SAM) is indirectly responsible for the initiation of all above-ground postembryonic organs, in most plants the vast majority of these organs are directly initiated by lateral meristems. In Arabidopsis thaliana , the lateral meristems include flower meristems (FMs), which form on the flanks of the SAM, and lateral shoot meristems (LSMs), which develop in leaf axils. While significant progress has been made on the molecular genetic basis of SAM initiation during embryo development, relatively little is known about the initiation of meristems at lateral positions. Here we have characterized the phenotypic consequences and genetic interactions of mutations in the REVOLUTA ( REV ) gene, with an emphasis on the role of REV in lateral meristem initiation. Our observations indicate that REV is required for initiation of both LSMs and FMs, and likely acts in the same pathway as, and upstream of, known meristem regulators. We identified the REV gene and found it encodes a predicted homeodomain leucine zipper transcription factor that also contains a START sterol-lipid binding domain. REV is the same as the IFL gene. REV was expressed at the earliest stages of LSM and FM formation. Within the inflorescence shoot meristem, REV expression appeared to predict 3 5 incipient flower primordia on the flanks of the SAM, and REV expression at stage 1 and stage 2 matched that of WUS and STM , respectively. We propose that REV acts at lateral positions to activate the expression of known meristem regulators. 2001-01-01T00:00:00Z