| Abstrakt: |
The importance of respiratory chain activity in the induction of manganese superoxide dismutase biosynthesis was examined in the yeast Saccharomyces cerevisiae by immunological measurement of the level of manganese superoxide dismutase and comparison with copper/zinc superoxide dismutase and two subunits of respiratory chain proteins, cytochrome c1 and core 2, under conditions of growth in which respiratory chain activity was varied. Oxygen consumption by the yeast was also monitored during growth. These comparative studies indicated that under normoxic conditions, glucose repression of the respiratory chain subunits resulted in a parallel repression of the level of manganese superoxide dismutase protein. The increase in the protein levels of manganese superoxide dismutase and core 2 protein under derepressing growth conditions reflected an increase in the level of the mRNA for each protein; thus regulation is, at least in part, at the level of transcription. The following observations support the conclusion that under normoxic conditions manganese superoxide dismutase biosynthesis is primarily regulated by the same means as the respiratory chain components; that is, by glucose (catabolite) repression rather than by oxygen metabolites. 1) When yeast cells were transferred from repressing to derepressing growth conditions in normoxia, manganese superoxide dismutase biosynthesis increased at a rate parallel to that of core 2, and occurred approximately 5 h in advance of increased oxygen consumption by the yeast. 2) When an important site of mitochondrial superoxide radical generation, the cytochrome bc1 complex. was inactivated by deletion of the gene coding for one of its subunits, the level of manganese superoxide dismutase protein was not changed in the mutant compared with the parental strain. However, regulation of manganese superoxide dismutase can be separated from regulation of the respiratory chain proteins in certain instances. During the transition from the logarithmic growth phase to the stationary phase in non-fermentable carbon sources, the level of manganese superoxide dismutase decreased by approximately 50%, whereas the levels of cytochrome c1 and core 2 remained unchanged. Furthermore, yeast grown in hyperoxia of 70-80% oxygen utilizing either repressing or derepressing carbon sources, contained significantly higher levels of manganese superoxide dismutase and copper/zinc superoxide dismutase compared to yeast grown in normoxia, whereas the levels of respiratory chain proteins were not affected by hyperoxia. When a mutant of S. cerevisiae lacking manganese superoxide dismutase was grown in hyperoxia of 70-80% oxygen with carbon sources requiring a functional respiratory chain, a dramatic decrease in growth rate was observed compared to the parental cells grown under the same conditions. The growth rate of the mutant in 70- 80% oxygen did not differ from the parental cells, however, when 10% glucose was the carbon source. Under normoxic conditions the mutant cells grew as well as the parental cells on three and six-carbon sources. However, growth of the mutant was specifically inhibited on the two-carbon sources, ethanol and acetate. When the manganese-superoxide-dismutase-deficient mutant was transformed with a plasmid containing the gene coding for the manganese superoxide dismutase, the mutant regained the ability to grow on the two-carbon sources, indicating that the absence of manganese superoxide dismutase is directly responsible for the inability of this mutant to grow on two-carbon substrates. [ABSTRACT FROM AUTHOR] |
