Utilization of guanine for nucleic acid biosynthesis by Tetrahymena geleii.

As a result of the investigations of Kidder and coworkers, it has recently become possible to grow the ciliated prot.oaoan Tetrahymena geleii in a culture medium which is of chemically known composition (1, 2), as well as free of other microorganisms. Interest was aroused by similarities in its requirements for amino acids (2, 3) and vitamins (4) to those of the mammal, although differences have been reported (5-8). Unlike the higher animals, however, this animal microorganism was found to have an absolute growth requirement for purines and pyrimidines (9), and, in view of the fact that guanine is not utilized by the rat for nucleic acid purines (lo), the observation in this organism that, of the naturally occurring purines, only guanine and its nucleoside and nucleotide would support growth was of interest from the point of view of nucleic acid biosynthesis. The question remained whether the requirement for guanine reflected an inability of the organism to synthesize nucleic acid purines from simpler molecules, or whether the guanine was utilized for some other purpose. The possibilities also existed that, even if the guanine were incorporated into nucleic acids, it might appear only in the guanine moiety, and not in the adenine moiety, or even that it might be utilized for one type of nucleic acid and not another, as has been suggested with regard to the incorporation of adenine into rat nucleic acids (11). It therefore seemed desirable to grow the organism in the presence of isotopically labeled guanine, and to det,ermine the incorporation of isotope into nucleic acid adenine and guanine.