Juan Carlos Abanades García

A genomic approach for the identification and classification of genes involved in cell wall formation and its regulation in Saccharomyces cerevisiae

Using a hierarchical approach, 620 non-essential single-gene yeast deletants generated by EUROFAN I were systematically screened for cell-wall-related phenotypes. By analyzing for altered sensitivity to the presence of Calcofluor white or SDS in the growth medium, altered sensitivity to sonication, or abnormal morphology, 145 (23%) mutants showing at least one cell wall-related phenotype were selected. These were screened further to identify genes potentially involved in either the biosynthesis, remodeling or coupling of cell wall macromolecules or genes involved in the overall regulation of cell wall construction and to eliminate those genes with a more general, pleiotropic effect. Ninety percent of the mutants selected from the primary tests showed additional cell wall-related phenotypes. When extrapolated to the entire yeast genome, these data indicate that over 1200 genes may directly or indirectly affect cell wall formation and its regulation. Twenty-one mutants with altered levels of β1,3-glucan synthase activity and five Calcofluor white-resistant mutants with altered levels of chitin synthase activities were found, indicating that the corresponding genes affect β1,3-glucan or chitin synthesis. By selecting for increased levels of specific cell wall components in the growth medium, we identified 13 genes that are possibly implicated in different steps of cell wall assembly. Furthermore, 14 mutants showed a constitutive activation of the cell wall integrity pathway, suggesting that they participate in the modulation of the pathway either directly acting as signaling components or by triggering the Slt2-dependent compensatory mechanism. In conclusion, our screening approach represents a comprehensive functional analysis on a genomic scale of gene products involved in various aspects of fungal cell wall formation.

Hydrogen-related effects in GaInP/GaAs HBTs: incorporation, removal and influence on device reliability

Abstract We report the result of investigation on hydrogen effects on GaInP/GaAs HBT structures originating from different MOCVD and CBE suppliers. It is demonstrated that hydrogen gives rise to initial unstable electrical behaviour by cross-examination of samples with and without hydrogen either intrinsically or by thermal-assisted removal. Annealing conditions to remove hydrogen have been optimized on the basis of SIMS analyses and Gummel plot characteristics to control eventual degradation of the junctions. It has been found that under particular doping and growth conditions, C 2 H complexes can be formed. These defects appear more stable than CH complexes which may explain the difficulty to remove hydrogen from some epitaxial layers.