[Metabolism of 8-C14-guanine by yeast]

Studies on independent synthesis of cytoplasmic ribonucleic acids in Acetabularia mediterranea

1. The RNA content of anucleate and nucleate fragments of Acetabularia has been measured. It was found that there is a net synthesis of RNA in nucleate fragments. On the other hand, the RNA content of anucleate fragments did not change significantly after enucleation. 2. Anucleate fragments, however, can readily incorporate (14)C-labeled adenine, orotic acid, and carbon dioxide into their cytoplasmic RNA. 3. The results of experiments on (14)CO(2) incorporation into the RNA of anucleate and nucleate fragments suggest that there is a mechanism for de novo synthesis of RNA in anucleate cytoplasm. 4. In Acetabularia, 81 per cent of the cytoplasmic RNA is bound to a large granule fraction, consisting mainly of chloroplasts. Even after removal of the nucleus, RNA is synthesized in this "chloroplast" fraction. The chloroplasts are thus a major site of RNA synthesis in the cytoplasm of these algae. Synthesis of "chloroplastic" RNA, in anucleate fragments, possibly occurs at the expense of the RNA present in other fractions (microsomes and supernatant). 5. 8-Azaguanine stimulates regeneration and cap formation in anucleate fragments and does not inhibit RNA synthesis in these fragments.

Expulsion of uracil and thymine from the yeast Saccharomyces cerevisiae: contrasting responses to changes in the proton electrochemical gradient

The outflow of uracil from the yeast Saccharomyces cerevisiae is known to be relatively fast in certain circumstances, to be retarded by proton conductors and to occur in strains lacking a uracil proton symport. In the present work, it was shown that uracil exit from washed yeast cells is an active process, creating a uracil gradient of the order of -80 mV relative to the surrounding medium. Glucose accelerated uracil exit, while retarding its entry. DNP or sodium azide each lowered the gradient to about -30 mV, simultaneously increasing the rate of uracil entry. They also lowered cellular ATP content. Manipulation of the external ionic conditions governing delta mu H+ at the plasma membrane had no detectable effect on uracil transport in yeast preparations thoroughly depleted of ATP. It was concluded that uracil exit is probably not driven by the proton gradient but may utilize ATP directly. It is known that thymine is not normally absorbed by yeast. However, thymine expulsion was here observed during deamination of the substrate 5-methylcytosine in the presence of glucose. In the absence of glucose, or following ATP depletion, thymine uptake from the medium only occurred when delta mu H+ was dissipated, either by DNP or azide, or by manipulation of the external ionic environment. The yeast expelled absorbed thymine when delta mu H+ was restored to the physiological range. The properties of the system corresponded to those of an H+/thymine antiport that is distinct from the mechanism expelling uracil.

Purine metabolism in human lymphocytes

In peripheral human blood lymphocytes the uptake and metabolism of adenine, guanine, and hypoxanthine was investigated. This was achieved by incubation of purified lymphocytes with 14C-purine bases, separation of cells from the incubation medium by a rapid filtration technique, and subsequent separation of the acid soluble material by thin-layer chromatography. No perferential uptake for one of the purine bases was observed. In all cases only traces of 14C-purine bases not added originally and labeled nucleosides could be demonstrated. Approximately 2/3 of adenine and 1/2 of guanine or hypoxanthine were converted to nucleotides. Separation of formed nucleotides showed that adenine and guanine were metabolized mainly to their corresponding nucleotides; hypoxanthine was converted to a considerable amount to adenine nucleotides and only to a small proportion into its own nucleotides. These results demonstrate the predomonance of adenine nucleotide formation in normal human lymphocytes.