The effects of oxygen availability on the rate of growth and on RNA and protein metabolism in Rhodotorula gracilis

The present work deals with the effects of low oxygen availability on the rate of growth and of RNA and protein synthesis in Rhodotorula gracilis cells growing in a liquid mineralglucose medium. In these experiments O2 partial pressure in tl.!e medium has been measured and maintained satisfactorily constant by means of a Clark oxygen electrode. It has been found that: a) the optimal rate of growth requires O2 partial pressure above 3 mm of Hg; b) lowering O2 pressure below this value induces a progressive inhibition of growth; c) under conditions of suboptimal O2 availability (ca. 1 \ub10.2 mm Hg) the accumulation of RNA and DNA is inhibited in an earlier period and to larger extent than protein synthesis. Experiments of incorporation of 32p orthophosphate into RNA show that the \uab low O2 \ubb condition is characterized by a marked inhibition of RNA synthesis, even in the case of relatively short 'pulses' (5 minutes), while no important changes of the rate RNA breakdown is observed. In fact, the rate of 32p incorporation into RNA remains practically linear in time for at least 30 minutes. This behaviour is in contrast with the one observed in this yeast as a consequence of nitrogen or of glucose starvation; in these cases the block of the net synthesis of RNA appears to correspond rather to an increased rate of breakdown of the newly formed RNA than to an inhibition at transcription level

RNA changes in Rhodotorula gracilis in different growth phases

Rhodotorula gracilis is an obligatory aerobic yeast, easily grown in a mineral medium added with glucose. In this communication some observations are reported on the behaviour of the yeast when the stationary phase is reached for the exhaustion of glucose and the progressive drop of O2 availability in the medium. It has been found that through the exponential phase of growth DNA, RNA and proteins are synthesized at the same ra cell duplication. When the yeast enteres into the stationary phase the net synthesis of DNA and protein decrease, while RNA level stops to increase or begins to decrease slowy. One hour pulse experiments with phosphate-P32, orotic acid-Cl4 and adenine-CH show that the block of RNA accumulation does not correspond to a block of RNA synthesis. Labelling experiments are against any preferential turnover of the RNA synthesized in the log phase. The stationary phase is accompanied by changes in the sedimentation profiles of ribosomal preparations; the ribosomal peak is shifted towards the heavier region of the gradients; labelling experiments show that in this regard both the new and old ribosomes behave in the same way. The base ratios of total RNA change when the cells enter in the stationary phase, the main change being an increase of the purine/pyrimidine ratio. No changes are found for the base ratios of the RNA purified from the ribosomal fraction. Experiments in which the shift from logari thmic to stationary phase has been induced by removing either O2 (in a glucose rich medium) or glucose (with excess O2 present) show that in both cases the increase of the purine/pyrimidine ratio is observed only for total RNA, and not for ribosomal RNA. Moreover there is some evidence that the base ratio changes depend on the nature of the small amount of RNA synthesized after the shift from the logarithmic to the stationary phas

Changes in the potato (Solanum tuberosum L.) tuber at the onset of dormancy and during storage at 23 degrees C and 3 degrees C. II. Evaluation of protein patterns

The changes in polypeptide profiles (2D-PAGE) occurring in the soluble and microsomal fractions of parenchymatic tissue of potato (Solanum tuberosum L.) tubers were studied during the last 30 d of maturation and during storage at 23 \ub0C and 3 \ub0C. The major changes were observed in the last period of tuber maturation, when several polypeptides disappeared and new ones appeared. At both 23 \ub0C and 3 \ub0C specific polypeptides disappeared in dormant tubers and new polypeptides appeared during storage. At 3 \ub0C specific changes in protein composition occurred, particularly in the microsomal fraction. The changes in polypeptide profiles are discussed in relation to the transition from 'sink' to 'source' of the tuber, the onset of dormancy and of sprouting ability and the activation of cold acclimation responses. The results are also discussed on the basis of the physiological and biochemical changes that occur in the parenchymatic tissue