Effects of cycloheximide and 5-fluorouracil on formation of low-molecular-weight ribonucleic acid in yeast

Combined in silico and 19F NMR analysis of 5-fluorouracil metabolism in yeast at low ATP conditions

The cytotoxic effect of 5-fluorouracil (5-FU) on yeast cells is thought to be mainly via a misincorporation of fluoropyrimidines into both RNA and DNA, not only DNA damage via inhibition of thymidylate synthase (TYMS) by fluorodeoxyuridine monophosphate (FdUMP). However, some studies on Saccharomyces cerevisiae show a drastic decrease in ATP concentration under oxidative stress, together with a decrease in concentration of other tri- and diphosphates. This raises a question if hydrolysis of 5-fluoro-2-deoxyuridine diphosphate (FdUDP) under oxidative stress could not lead to the presence of FdUMP and the activation of so-called 'thymine-less death' route. We attempted to answer this question with in silico modeling of 5-FU metabolic pathways, based on new experimental results, where the stages of intracellular metabolism of 5-FU in Saccharomyces cerevisiae were tracked by a combination of 19F and 31P NMR spectroscopic study. We have identified 5-FU, its nucleosides and nucleotides, and subsequent di- and/or triphosphates. Additionally, another wide 19F signal, assigned to fluorinated unstructured short RNA, has been also identified in the spectra. The concentration of individual metabolites was found to vary substantially within hours, however, the initial steady-state was preserved only for an hour, until the ATP concentration dropped by a half, which was monitored independently via 31P NMR spectra. After that, the catabolic process leading from triphosphates through monophosphates and nucleosides back to 5-FU was observed. These results imply careful design and interpretation of studies in 5-FU metabolism in yeast.

Linking uracil base excision repair and 5-fluorouracil toxicity in yeast

5-fluorouracil (5-FU) is a widely used anticancer drug that disrupts pyrimidine nucleotide pool balances and leads to uracil incorporation in DNA, which is then recognized and removed by the uracil base excision repair (BER) pathway. Using complementary biochemical and genetic approaches we have examined the role of uracil BER in the cell killing mechanism of 5-FU. A yeast strain lacking the enzyme uracil DNA glycosylase (Ung1), which excises uracil from the DNA backbone leaving an abasic site, showed significant protection against the toxic effects of 5-FU, a G1/S cell cycle arrest phenotype, and accumulated massive amounts of U/A base pairs in its genome (approximately 4% of T/A pairs were now U/A). A strain lacking the major abasic site endonuclease of Saccharomyces cerevisiae (Apn1) showed significantly increased sensitivity to 5-FU with G2/M arrest. Thus, efficient processing of abasic sites by this enzyme is protective against the toxic effects of 5-FU. However, contrary to expectations, the Apn1 deficient strain did not accumulate intact abasic sites, indicating that another repair pathway attempts to process these sites in the absence Apn1, but that this process has catastrophic effects on genome integrity. These findings suggest that new strategies for chemical intervention targeting BER could enhance the effectiveness of this widely used anticancer drug.

Methylated nucleosides in polyadenylate-containing yeast messenger ribonucleic acid

Yeast messenger RNA was found to be methylated. A calculation of the specific methylation showed that the average yeast messenger RNA molecule contains only two methylated nucleosides which occur in one alkali stable oligonucleotide. Similar to other eukaryotic messengers, the 5' terminus of yeast messenger RNA is blocked by 7-methylguanosine, linked through a di- or triphosphate bridge to a ribosemethylated nucleoside. Contrary to the messengers of high eucaryotic organisms, no additional base methylated constituents were found.

5-Bromouridylyl-(3-5)-adenosine isolated from 5-bromouracil-induced filaments of Escherichia coli

A dinucleoside monophosphate was isolated from 5-bromouracil-induced filaments of a thymine auxotroph of Escherichia coli K-12. The dinucleoside monophosphate was fractioned from a [(14)C]5-bromouracil-labeled perchloric acid extract using Dowex-1-formate ion-exchange chromatography. Sephadex chromatography revealed its molecular weight to be 710. Snake venom phosphodiesterase digest of the dinucleoside monophosphate yielded [(14)C]5-bromouridine and adenosine 5'-monophosphate. The presence of [(14)C]5-bromouracil in bacterial ribonucleic acid indicates that ribonucleic acid, which had incorporated 5-bromouracil, was the probable source of this dinucleoside monophosphate, 5-bromouridylyl-(3' --> 5')-adenosine.