Self-Similarity of Friction Laws

The change of the friction law from a mesoscopic level to a macroscopic level is studied in the spring-block models introduced by Burridge-Knopoff. We find that the Coulomb law is always scale invariant. Other proposed scaling laws are only invariant under certain conditions.}Comment: Plain TEX. Figures not include

Trade-Offs of Escherichia coli Adaptation to an Intracellular Lifestyle in Macrophages

The bacterium Escherichia coli exhibits remarkable genomic and phenotypic variation, with some pathogenic strains having evolved to survive and even replicate in the harsh intra-macrophage environment. The rate and effects of mutations that can cause pathoadaptation are key determinants of the pace at which E. coli can colonize such niches and become pathogenic. We used experimental evolution to determine the speed and evolutionary paths undertaken by a commensal strain of E. coli when adapting to intracellular life. We estimated the acquisition of pathoadaptive mutations at a rate of 10-6 per genome per generation, resulting in the fixation of more virulent strains in less than a hundred generations. Whole genome sequencing of independently evolved clones showed that the main targets of intracellular adaptation involved loss of function mutations in genes implicated in the assembly of the lipopolysaccharide core, iron metabolism and di- and tri-peptide transport, namely rfaI, fhuA and tppB, respectively. We found a substantial amount of antagonistic pleiotropy in evolved populations, as well as metabolic trade-offs, commonly found in intracellular bacteria with reduced genome sizes. Overall, the low levels of clonal interference detected indicate that the first steps of the transition of a commensal E. coli into intracellular pathogens are dominated by a few pathoadaptive mutations with very strong effects.LAO/ITQB, FCT

Análise do sistema radicular para seleção de genótipos de milho mais eficientes na aquisição de fósforo.

O fósforo (P) é um dos macronutrientes mais limitante às culturas no mundo e apresenta uma baixa disponibilidade para as plantas. Para aumentar a capacidade de absorção de P, as plantas sofrem ajustes bioquímicos, fisiológicos e morfológicos. Este estudo utilizou materiais genéticos de milho contrastantes e previamente avaliados quanto à eficiência de utilização e absorção de P em campo para realizar uma caracterização detalhada de parâmetros morfológicos de raiz em solução nutritiva sob condições controladas. Foi realizada a padronização das condições de cultivo em solução nutritiva, assim como dos parâmetros a serem avaliados. Deste modo, foi possível determinar que há diferença significativa entre os genótipos contrastantes utilizando-se 2,5 ?M de P a partir de 12 dias de tratamento e foram definidas as principais características fenotípicas para distinguir os genótipos. As informações geradas são fundamentais para acelerar o processo de seleção de genótipos mais eficientes no uso e na aquisição de P.bitstream/item/26151/1/Bol-24.pd

Electrical activation and electron spin coherence of ultra low dose antimony implants in silicon

We implanted ultra low doses (2x10^11 cm-2) of 121Sb ions into isotopically enriched 28Si and find high degrees of electrical activation and low levels of dopant diffusion after rapid thermal annealing. Pulsed Electron Spin Resonance shows that spin echo decay is sensitive to the dopant depths, and the interface quality. At 5.2 K, a spin decoherence time, T2, of 0.3 ms is found for profiles peaking 50 nm below a Si/SiO2 interface, increasing to 0.75 ms when the surface is passivated with hydrogen. These measurements provide benchmark data for the development of devices in which quantum information is encoded in donor electron spins