Transfer ribonucleic acids from Escherichia coli treated with 5-fluorouracil

Moderate levels of 5-fluorocytosine cause the emergence of high frequency resistance in cryptococci

The antifungal agent 5-fluorocytosine (5-FC) is used for the treatment of several mycoses, but is unsuitable for monotherapy due to the rapid development of resistance. Here, we show that cryptococci develop resistance to 5-FC at a high frequency when exposed to concentrations several fold above the minimal inhibitory concentration. The genomes of resistant clones contain alterations in genes relevant as well as irrelevant for 5-FC resistance, suggesting that 5-FC may be mutagenic at moderate concentrations. Mutations in FCY2 (encoding a known permease for 5-FC uptake), FCY1, FUR1, UXS1 (encoding an enzyme that converts UDP-glucuronic acid to UDP-xylose) and URA6 contribute to 5-FC resistance. The uxs1 mutants accumulate UDP-glucuronic acid, which appears to down-regulate expression of permease FCY2 and reduce cellular uptake of the drug. Additional mutations in genes known to be required for UDP-glucuronic acid synthesis (UGD1) or a transcriptional factor NRG1 suppress UDP-glucuronic acid accumulation and 5-FC resistance in the uxs1 mutants.

Pathogenic fungi rapidly develop resistance to the antifungal agent 5-fluorocytosine (5-FC). Here, Chang et al. explore the mechanisms by which Cryptococcus develops 5-FC resistance at a high frequency, including mutations in several genes and altered levels of key metabolites.

Nucleoside analogue mutagenesis of a single-stranded DNA virus: evolution and resistance

It has been well established that chemical mutagenesis has adverse fitness effects in RNA viruses, often leading to population extinction. This is mainly a consequence of the high RNA virus spontaneous mutation rates, which situate them close to the extinction threshold. Single-stranded DNA viruses are the fastest-mutating DNA-based systems, with per-nucleotide mutation rates close to those of some RNA viruses, but chemical mutagenesis has been much less studied in this type of viruses. Here, we serially passaged bacteriophage X174 in the presence of the nucleoside analogue 5-fluorouracil (5-FU). We found that 5-FU was unable to trigger population extinction for the range of concentrations tested, but it negatively affected viral adaptability. The phage evolved partial drug resistance, and parallel nucleotide substitutions appearing in independently evolved lines were identified as candidate resistance mutations. Using site-directed mutagenesis, two single-nucleotide substitutions in the lysis protein E (T572C and A781G) were shown to be selectively advantageous in the presence of 5-FU. In RNA viruses, base analogue resistance is often mediated by changes in the viral polymerase, but this mechanism is not possible for X174 and other single-stranded DNA viruses because they do not encode their own polymerase. In addition to increasing mutation rates, 5-FU produces a wide variety of cytotoxic effects at the levels of replication, transcription, and translation. We found that substitutions T572C and A781G lost their ability to confer 5-FU resistance after cells were supplemented with deoxythymidine, suggesting that their mechanism of action is at the DNA level. We hypothesize that regulation of lysis time may allow the virus to optimize progeny size in cells showing defects in DNA synthesis.

Specific incorporation of 5-fluorocytidine into Escherichia coli RNA

RNAs isolated from Escherichia coli B grown in the presence of 5-fluorouracil have high levels of the analog replacing uridine and uridine-derived modified nucleosides. Cytidine has also been shown to be replaced in these RNAs by 5-fluorocytidine, a metabolic product of 5-fluorouracil, but to a considerably lesser extent. When 5-fluorocytidine is added to cultured of E. coli B little 5-fluorocytidine (0.20 mol%) is incorporated into cellular RNAs because of the active cytosine/cytidine deaminase activities. Addition of the cytidine deaminase inhibitor tetrahydrouridine (70 micrograms/ml) increases 5-fluorocytidine incorporation to about 3 mol% in tRNAs, but does not eliminate 5-fluorouridine incorporation. E. coli mutants lacking cytosine/cytidine deaminase activities are able to more than double the extent of 5-fluorocytidine incorporation into their transfer and ribosomal RNAs, replacing cytidine with no detectable 5-fluorouridine incorporation. Levels of 5-methyluridine, pseudouridine and dihydrouridine in tRNAs are not affected. These fluorocytidine-containing tRNAs show amino acid-accepting activities similar to control tRNAs. Fluorocytidine was found to be quite susceptible to deamination under alkaline conditions. Its conversion to primarily 5-fluorouridine follows pseudo-first-order reaction kinetics with a half-life of 10 h in 0.3 M KOH at 37 degrees C. This instability in alkali probably explains why 5-fluorocytidine was not found earlier in RNAs isolated from cells treated with 5-fluorouridine, since most early RNA hydrolyses were carried out in alkali. It may also explain the mild mutagenic properties observed in some systems following 5-fluorouridine treatment. Initial 19F-NMR measurements in fluorocytidine-containing tRNAs indicate that this modified tRNA may be useful in future structural studies of tRNAs and in probing tRNA-protein complexes.

Characterization of the fluorodihydrouracil substituent in 5-fluorouracil-containing Escherichia coli transfer RNA

The fluorodihydrouridine derivative previously detected in one of two isoaccepting forms of FUra-substituted Escherichia coli tRNAMetf has been further characterized. This substituent is responsible for the 19F resonance observed 15 ppm upfield from free FUra (= 0 ppm) in the high resolution 19F-NMR spectra of FUra-substituted tRNA purified by chromatography on DEAE-cellulose, at pH 8.9, to remove normal tRNA. Similar highfield 19F signals have now been observed in the spectra of two other purified fluorinated E. coli tRNAs, tRNAMetm and tRNAVal1, as well as in unfractionated tRNA, indicating the widespread occurrence of the constituent. Comparison with 19F spectrum of the model compound 5'-deoxy-5-fluoro-5,6-dihydrouridine (dH56FUrd) (delta FUra = -31.4 ppm; JHF = 48 Hz) indicates that the substituent does not contain an intact fluorodihydrouridine ring. dH56FUrd is considerably more alkali labile than 5,6-dihydrouridine (H56Urd). At pH 8.9, where H56Urd is stable, dH56FUrd is degraded to a derivative, presumably a fluoroureidopropionic acid, with a 19F resonance at - 15.7 ppm that nearly coincides with the upfield peak in the spectrum of pH 8.9-treated tRNA. The 19F-NMR spectrum of fluorinated tRNA, not exposed to pH 8.9, exhibits two peaks 31 and 32 ppm upfield of FUra, in place of the 19F signal at - 15 ppm. Hydrolysis of this tRNA with RNAase T2 produces a sharp doublet 33 ppm upfield (JHF = 45 Hz). Similarities of the 19F chemical shift and coupling constant to those of dH56FUrd, allows assignment of the peak at -33 ppm to an intact fluorodihydrouridine residue in the tRNA. Our results demonstrate that FUra residues incorporated into E. coli tRNA at sites normally occupied by dihydrouridine can be recognized by tRNA-modifying enzymes and reduced to fluorodihydrouridine. This substituent is labile at moderately alkaline pH values and undergoes ring-opening during purification of the tRNA.

Effects of 5-fluorouracil on the formation of modified nucleosides in yeast transfer RNA

Yeast cells grown in the presence of the drug FUra synthesize RNA in which Urd is partially replaced by FUrd. Transfer RNAs in which 1.5-50% of the Urd has been replaced by FUrd have been isolated and their base compositions measured to determine the effect of FUrd incorporation on posttranscriptional nucleoside modification. This replacement results in an extensive reduction in the amounts of Thd, H56Urd and psird found in mature tRNA. Quantitatively, the reduction of psird greater than or equal to Thd greater than H56Urd. The losses of psird, Thd and H56Urd are greater than can be accounted for by the stoichiometry of FUrd incorporation. The formation of 5-MeCyd is not affected by the drug, whereas the methylated purines show substoichiometric losses in FUrd-containing tRNAs. In Escherichia coli, we have not observed any effects of FUra on the methylated purine content, although the effects on psird, Thd and H56Urd are similar. These findings indicate that (a) in both pro- and eukaryotic systems the enzymes responsible for psird, Thd and H56Urd formation are affected by FUra treatment in a similar manner; (b) prokaryotic purine methylases may be more tolerant of structural aberrations resulting from FUrd incorporation than eukaryotic methylases and (c) different methylases within one system show different sensitivities as shown by those responsible for 1-MeAdo and 5-MeCyd formation.

Effects of 5-fluorouridine on modified nucleosides in mouse liver transfer RNA

Administration of the pyrimidine antimetabolite, 5-fluorouridine, to mice was found to cause a marked specific reduction of the amounts of 5-methyluridine, pseudouridine, and dihydrouridine but not of 3-(3-amino-3-carboxypropyl)uridine in tRNA from the livers of the treated animals. The data presented indicate that this effect is not simply due to the incorporation of 5-fluorouridine into tRNA; the drug appears to interfere directly with the enzymic reactions involved in the modification of the 5-position of uridine. 5-Fluorouridine was found to have no effect on the modification of adenosine, guanosine, and cytidine in mouse liver tRNA.

Effects of 5-azacytidine on nucleolar RNA and the preribosomal particles in Novikoff hepatoma cells

Examination of nucleolar RNA from cultured Novikoff hepatoma cells treated for 3 hr with 5 x 10-4 M 5-azacytidine shows that significant amounts of analog-substituted 45S RNA are processed to the 32S RNA species, but 28S RNA formation is completely inhibited. Under these conditions of analog treatment 37% of the cytidine residues in the 45S RNA is replaced by 5-azacytidine. During coelectrophoresis of nucleolar RNA from 5-azacytidine-treated and control cells, the analog-substituted 45S RNA and 32S RNA display reduced mobilities compared to the control 45S RNA and 32S RNA. Coelectrophoresis of analog-substituted and control RNA after formaldehyde denaturation shows no differences in electrophoretic mobility between the two RNA samples, suggesting that 5-azacytidine incorporation may alter the secondary structure of the 45S RNA and the 32S RNA. 5-Azacytidine at 5 x 10-4 M severely inhibits protein synthesis in Novikoff cells by 3 hr. After this length of treatment, however, CsCl buoyant density analysis reveals no difference in density of either the 80S or 55S preribosomal ribonucleoprotein particles when compared to normal particles. Also 5-azacytidine treatment does not appear to cause major changes in the polyacrylamide gel electrophoresis patterns of the proteins in the 80S and 55S preribosomal particles. These results together with previous findings suggest that 5-azacytidine's inhibition of rRNA processing is possibly related to its alteration of the structure of the ribosomal precursor RNAs and is not a consequence of a general inhibition of ribosomal protein formation.

Biosynthetic precursors of some modified nucleosides in the transfer ribonucleic acid of Mycoplasma mycoides var. capri

The ribosomal and transfer ribonucleic acid (tRNA) from Mycoplasma mycoides var. capri, grown in a medium containing uridine-((14)C)-5'-triphosphate and cytidine-(5-(3)H)-5'-triphosphate, were isolated and separated. The uridine in both species of RNA was shown to contain (14)C and the cytidine to contain both (3)H and (14)C. Comparison of the labeling of 4-thiouridine and pseudouridine, obtained from an enzymatic digest of the RNA, indicates that their biosynthetic precursor is uridine, not cytidine. It is probable that ribothymidine and dihydrouridine have the same derivation.

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

The effects of cycloheximide and 5-fluorouracil on the formation of low-molecular-weight ribonucleic acid (RNA) in yeast were investigated. Both compounds were found to affect the synthesis of low-molecular-weight RNA of ribosomal origin more than transfer RNA but less than the high-molecular-weight ribosomal RNA. 5-Fluorouracil-containing transfer RNA was separated from normal transfer RNA by chromatography on diethylaminoethyl cellulose at 80 C in the presence of 7 m urea.