Nucleic acid metabolism in yeast VI. Utilisation of exogenous dAMP

It is shown that mutants of Saccharomyces cerevisiae able to efficiently utilise exogenous dTMP can also utilise exogenous dAMP. Under extracellular conditions permissive for dTMP uptake label stemming from offered [8-3H]dAMP is incorporated preferentially into alkali-resistant, high molecular weight material (putative DNA); only about 30% of high molecular weight cell-bound dAMP label was found to be sensitive towards mild alkali hydrolysis. This putative RNA label can be minimised to practically zero when greater than or equal to mM Ade is employed in a dAMP labelling assay. Exogenous dAMP at much greater than 10 microM was found to be cytostatic similarly to much greater than microM dTMP and similarly to inhibit effectively import of exogenous Pi. We conclude from our results that there exists a yeast cytoplasmic membrane permease able to import dAMP. A model of this hypothetical permease system is presented.