SUMMARY: The nucleic acid requirements of several organisms were examined in a partially denned medium. The saprophytic strain A did not grow in the basal medium alone (Razin & Knight, 1960 ) but did when suitable concentrations of ribonucleic acid (RNA) and of deoxyribonucleic acid (DNA) were added. Too high a concentration of RNA inhibited growth; this inhibition was annulled by increasing the concentration of DNA. Similarly, too high a concentration of DNA inhibited growth, and this inhibition was annulled by RNA. Chemical and enzymic degradations of RNA showed that the growth-promoting effect could be brought about by a ribo-oligonucleotide but not by smaller fragments of the molecule. Similar degradations of DNA showed that the effective moiety was thymidine; thymine was less effective. The degradation of RNA abolished its growth-inhibitory activity. The growth-inhibitory activity of DNA was not affected by its degradation to oligonucleotides, and was only partially diminished by its degradation to nucleotides or nucleosides.

strain B grew in the basal medium when DNA alone was added. This nutritional requirement was also satisfied by thymidine, provided that some RNA was also present. The parasitic var. resembled strain B in responding to DNA alone, but differed from the saprophytic strains in its complete indifference to high DNA concentrations. Thymidine replaced DNA only to a certain extent when added together with RNA. A growth-promoting effect of DNA was also found with the L-phase of thymidine then replaced DNA completely. The RNA/DNA antagonism was found with all the organisms examined. Possible explanations for this phenomenon are discussed.


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