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 phase