Responses of sap flux and stomatal conductance of Pinus taeda L. trees to stepwise reductions in leaf area

Herbivory or artificial foliage removal has been shown to affect gas exchange and canopy water relations. In this study, canopy architecture and water relations in response to progressive defoliation were examined in a stand of 8-year-old loblolly pine (Pinus taeda L.) trees, a shade-intolerant, pioneer species common in the south-eastern USA. Sap flux was measured with constant heat sap flow gauges in order to estimate canopy stomatal conductance (Gs) while foliage in the 6 m high stand was harvested in 1 m increments from the bottom up. Leaf-level stomatal conductance and water potential data were also collected. Profiles of silhouette area ratio and specific leaf area showed no trends with crown height, reflecting an open canopy (leaf area index = 1.55). Therefore, short-term changes in Gs with foliage removal were attributed to hydraulic effects rather than influences of changes in mean microclimate conditions on Gs of remaining foliage. A large increase in Gs was observed during the 6 h pruning period which fully compensated for the reductions in foliage area down to 45%. Canopy stomatal conductance and whole plant liquid phase conductance as calculated from sap flux were both influenced by the rate of growth as indicated by the annual basal area increment.     

Identification of a minimal transforming domain of p53: negative dominance through abrogation of sequence-specific DNA binding.

Mutations in the p53 gene are most frequent in cancer. Many p53 mutants possess transforming activity in vitro. In cells transformed by such mutants, the mutant protein is oligomerized with endogenous cell p53. To determine the relevance of oligomerization for transformation, miniproteins containing C-terminal portions of p53 were generated. These miniproteins, although carrying no point mutation, transformed at least as efficiently as full-length mutant p53. Transforming activity was coupled with the ability to oligomerize with wild-type p53, as well as with the ability to abrogate sequence-specific DNA binding by coexpressed wild-type p53. These findings suggest that p53-mediated transformation may operate through a dominant negative mechanism, involving the generation of DNA binding-incompetent oligomers.     

Overproduction of protein p53 contributes to simian virus 40-mediated transformation.

The possible involvement of p53 overproduction in simian virus 40 (SV40)mediated transformation was studied by using the rat embryo fibroblast focus formation assay. Transformation by wild-type SV40 was enhanced two- to threefold by cotransfection of a plasmid overexpressing mouse p53. More significantly, such a plasmid could partially complement a transformation-defective deletion mutant of SV40. Hence, the ability of SV40 T antigen to induce high p53 levels may indeed be directly relevant to the viral transforming potential.     

Solution study of engineered quartz binding peptides using replica exchange molecular dynamics

We use replica-exchange molecular dynamics (REMD) to interrogate molecular structures and properties of four engineered dodecapeptides (in solution, in the absence of a surface) that have been shown to bind to quartz with different propensities. We find that all of the strong-binding peptides feature some polyproline type II secondary structure, have less conformational freedom, and feature fewer intrapeptide hydrogen bonds compared with the weak binder. The regions of contiguous proline content in a given sequence appear to play a role in fostering some of these properties of the strong binders. For preliminary insights into quartz binding, we perform lattice-matching studies between a grid corresponding with the quartz (100) surface and the strong-binding peptide REMD structures. Our findings indicate a commonality among the putative contact residues, even for peptide structures with very different backbone conformations. Furthermore, interpeptide interactions in solution are studied. Our preliminary findings indicate that the strong-binder interpeptide contacts are dominated by weak, nonspecific hydrophobic interactions, while the weak-binding peptide shows more variable behavior due to the distribution of charged residues. In summary, the solution structures of peptides appear to be significant. We propose that these differences in their intra- and interpeptide interactions can influence their propensity to bind onto a solid substrate.     

Living with p53, dying of p53

The p53 tumor suppressor protein acts as a major defense against cancer. Among its most distinctive features is the ability to elicit both apoptotic death and cell cycle arrest. In this issue of Cell, Das et al. (2007) and Tanaka et al. (2007) provide new insights into the mechanisms that dictate the life and death decisions of p53.