Cytochrome oxidase is the primary oxygen sensor in the cat carotid body

Isolation and molecular characterization of KlCOX14, a gene of Kluyveromyces lactis encoding a protein necessary for the assembly of the cytochrome oxidase complex

The yeast Kluyveromyces lactis was mutagenized with ethyl methane sulphonate and mutants unable to grow on respiratory carbon sources were isolated. Functional complementation of one of these mutants led to the isolation of KlCOX14, a gene encoding a 64 amino acid protein which is the functional homologue of Saccharomyces cerevisiae Cox14p, a protein necessary for the assembly of the cytochrome oxidase holoenzyme (Glerum et al., 1995). The disruption of KlCOX14 resulted in the absence of the absorption bands relative to cytochromes a and a(3) and in the complete loss of respiratory activity. Klcox14 mutants display the typical phenotype of pet mutants and have a reduced growth rate. In addition, unlike the wild-type, Klcox14 mutants are able to grow by fermentation also in the presence of low glucose. The nucleotide sequence of KlCOX14 has been deposited in the EMBL databank with Accession No. AJ238801.

Lack of oxygen sensing by mitochondria in platelets

The range over which cells are sensitive to changes in oxygen concentration remains uncertain. Wilson and colleagues [Wilson, D.F. (1994) Med. Sci. Sports Exerc. 26, 37-43] have suggested that cytochrome oxidase is sensitive to oxygen concentrations below about 40 microM, but proposed that this sensitivity is obscured in intact cells because an increase in reduction state of cytochrome c acts to maintain oxygen consumption. We have tested this hypothesis in platelets, which are small cells (2-4 micrometer diameter, < 0.5 micrometer thick) that do not decrease their rate of oxygen consumption until oxygen concentrations fall below 2.5 microM. Contrary to the expectations of the hypothesis, the reduction state of cytochrome c, the concentration of NADH and the rate of glycolytic output are not changed as oxygen concentration declines from 40 microM down to 5 microM. Therefore, we conclude that at least some cell types contain mitochondria that are not capable of sensing oxygen above 5 microM by the mechanism proposed by Wilson and colleagues.

Oxygen sensing in yeast: evidence for the involvement of the respiratory chain in regulating the transcription of a subset of hypoxic genes

Oxygen availability affects the transcription of a number of genes in nearly all organisms. Although the molecular mechanisms for sensing oxygen are not precisely known, heme is thought to play a pivotal role. Here, we address the possibility that oxygen sensing in yeast, as in mammals, involves a redox-sensitive hemoprotein. We have found that carbon monoxide (CO) completely blocks the anoxia-induced expression of two hypoxic genes, OLE1 and CYC7, partially blocks the induction of a third gene, COX5b, and has no effect on the expression of other hypoxic or aerobic genes. In addition, transition metals (Co and Ni) induce the expression of OLE1 and CYC7 in a concentration-dependent manner under aerobic conditions. These findings suggest that the redox state of an oxygen-binding hemoprotein is involved in controlling the expression of at least two hypoxic yeast genes. By using mutants deficient in each of the two major yeast CO-binding hemoproteins (cytochrome c oxidase and flavohemoglobin), respiratory inhibitors, and cob1 and rho0 mutants, we have found that the respiratory chain is involved in the anoxic induction of these two genes and that cytochrome c oxidase is likely the hemoprotein "sensor." Our findings also indicate that there are at least two classes of hypoxic genes in yeast (CO sensitive and CO insensitive) and imply that multiple pathways/mechanisms are involved in modulating the expression of hypoxic yeast genes.

Contributions of mitochondria to animal physiology: from homeostatic sensor to calcium signalling and cell death

Over recent years, it has become clear that mitochondria play a central role in many key aspects of animal physiology and pathophysiology. Their central and ubiquitous task is clearly the production of ATP. Nevertheless, they also play subtle roles in glucose homeostasis, acting as the sensor for substrate supply in the transduction pathway that promotes insulin secretion by the pancreatic -cell and that modulates the excitability of the hypothalamic glucose-sensitive neurons involved in appetite control. Mitochondria may also act as sensors of availability of oxygen, the other major mitochondrial substrate, in the regulation of respiration. Mitochondria take up calcium, and the high opacity mitochondrial calcium uptake pathway provides a mechanism that couples energy demand to increased ATP production through the calcium-dependent upregulation of mitochondrial enzyme activity. Mitochondrial calcium accumulation may also have a substantial impact on the spatiotemporal dynamics of cellular calcium signals, with subtle differences of detail in different cell types. Recent work has also revealed the centrality of mitochondrial dysfunction as an irreversible step in the pathway to both necrotic and apoptotic cell death. This review looks at recent developments in these rapidly evolving areas of cell physiology in an attempt to draw together disparate areas of research into a common theme.

Models for oxygen sensing in yeast: implications for oxygen-regulated gene expression in higher eucaryotes

Adaptation to changes in oxygen tension in cells, tissues, and organisms depends on changes in the level of expression of a large and diverse set of proteins. It is likely that most cells and tissues possess an oxygen sensing apparatus and signal transduction pathways for regulating expression of oxygen-responsive genes. Although progress has been made in understanding the transcriptional machinery involved in oxygen-regulated gene expression of eucaryotic genes the underlying mechanism(s) of oxygen sensing and the signaling pathways that connect oxygen sensor(s) to the transcription machinery of eucaryotes are still poorly understood. The yeast Saccharomyces cerevisiae is ideal for addressing these problems. Indeed, it is well-suited for broadly based studies on oxygen sensing at the cellular level because it lends itself well to genetic and biochemical studies and because its genome has been completely sequenced. This review focuses on oxygen-regulated gene expression and current models for oxygen sensing in this yeast and then considers their applicability for understanding oxygen sensing in mammals.