A quantitative estimation of the global translational activity in logarithmically growing yeast cells.

BACKGROUND: Translation of messenger mRNAs makes significant contributions to the control of gene expression in all eukaryotes. Because translational control often involves fractional changes in translational activity, good quantitative descriptions of translational activity will be required to achieve a comprehensive understanding of this aspect of biology. Data on translational activity are difficult to generate experimentally under physiological conditions, however, translational activity as a parameter is in principle accessible through published genome-wide datasets. RESULTS: An examination of the accuracy of genome-wide expression datasets generated for Saccharomyces cerevisiae shows that the available datasets suffer from large random errors within studies as well as systematic shifts in reported values between studies, which make predictions of translational activity at the level of individual genes relatively inaccurate. In contrast, predictions of cell-wide translational activity are possible from such datasets with higher accuracy, and current datasets predict a production rate of about 13,000 proteins per haploid cell per second under fast growth conditions. This prediction is shown to be consistent with independently derived kinetic information on nucleotide exchange reactions that occur during translation, and on the ribosomal content of yeast cells. CONCLUSIONS: This study highlights some of the limitations in published genome-wide expression datasets, but also demonstrates a novel use for such datasets in examining global properties of cells. The global translational activity of yeast cells predicted in this study is a useful benchmark against which biochemical data on individual translation factor activities can be interpreted

Macromolecular organization of ATP synthase and complex I in whole mitochondria

We used electron cryotomography to study the molecular arrangement of large respiratory chain complexes in mitochondria from bovine heart, potato, and three types of fungi. Long rows of ATP synthase dimers were observed in intact mitochondria and cristae membrane fragments of all species that were examined. The dimer rows were found exclusively on tightly curved cristae edges. The distance between dimers along the rows varied, but within the dimer the distance between F1 heads was constant. The angle between monomers in the dimer was 70° or above. Complex I appeared as L-shaped densities in tomograms of reconstituted proteoliposomes. Similar densities were observed in flat membrane regions of mitochondrial membranes from all species except Saccharomyces cerevisiae and identified as complex I by quantum-dot labeling. The arrangement of respiratory chain proton pumps on flat cristae membranes and ATP synthase dimer rows along cristae edges was conserved in all species investigated. We propose that the supramolecular organization of respiratory chain complexes as proton sources and ATP synthase rows as proton sinks in the mitochondrial cristae ensures optimal conditions for efficient ATP synthesis