Rhodotorula gracilis : adaptation of the mithocondrial apparatus to shortage of oxygen

Oxygen is known to induce the synthesis, in fermenting yeasts, of oxidating enzymes and of the whole mitochondrial apparatus in general. The mitochondrial apparatus is, on the other hand, reduced under conditions of shortage of oxygen. Previous experiments had shown that Rhodotorula gracilis maintains an extraordinary metabolic activity even at very low (1-3 mm Hg in the culture medium) partial pressures of oxygen, thus indicating the existence of systems capable of utilizing oxygen which have a great affinity for it. EM observations of cells grown at low oxygen concentrations (about 1 mm Hg) show a well defined increase of the system of mitochondrial membranes as against control cells grown under aerobic conditions. At slightly higher oxygen concentrations (2 mm Hg) there is no indication of significant changes in the mitochondrial system. Spectrophotometric analysis of whole yeast cells shows that at 1 mm Hg of oxygen partial pressure alterations occur in the peaks of absorption specific for cytochromes as measured both as absolute and as differential (ox-red)absorbancy. As cellular synthetic activities are strongly inhibited under these conditions, the results so far obtained seem to suggest that cytoplasmic growth is likely to be much more sensitive to oxygen shortage than the mitochondrial one. Similarly, the observed discrepancy between mitochondrial and cytoplasmic volumes seems to be related to a stronger repression of cytoplasmic synthetic processes rather than to a stimulation of mitochondrial syntheses