Effects of sinefungin on rRNA production and methylation in the yeast Saccharomyces cerevisiae

Cell-based assays to detect inhibitors of fungal mRNA capping enzymes and characterization of sinefungin as a cap methyltransferase inhibitor

The m7GpppN cap at the 5' end of eukaryotic mRNAs is important for transcript stability and translation. Three enzymatic activities that generate the mRNA cap include an RNA 5'-triphosphatase, an RNA guanylyltransferase, and an RNA (guanine-7-) -methyltransferase. The physical organization of the genes encoding these enzymes differs between mammalian cells and yeast, fungi, or viruses. The catalytic mechanism used by the RNA triphosphatases of mammalian cells also differs from that used by the yeast, fungal, or viral enzymes. These structural and functional differences suggest that inhibitors of mRNA capping might be useful antifungal or antiviral agents. The authors describe several whole-cell yeast-based assays developed to identify and characterize inhibitors of fungal mRNA capping. They also report the identification and characterization of the natural product sinefungin in the assays. Their characterization of this S-adenosylmethionine analog suggests that it inhibits mRNA cap methyltransferases and exhibits approximately 5- to 10-fold specificity for the yeast ABD1 and fungal CCM1 enzymes over the human Hcm1 enzyme expressed in yeast cells.

Expression of lacZ gene fusions affects downstream transcription in yeast

Chimeric genes containing Escherichia coli lacZ sequences are often used to characterize gene expression in yeast cells. By Northern analysis, we found that such genes produce multiple transcripts due to inefficient 3'-end formation. The same transcript pattern was found for two related chimeric genes when these genes were cloned separately into the commonly used vector, YIp5, and integrated into the yeast genome at two different locations. Each chimeric gene was composed of promoter and N-terminal coding regions from the yeast SSA1 or SSA2 genes fused in-frame to the lac operon. Transcripts were shown to initiate within the yeast promoter fragment, but transcript size indicated that 3' ends were localized to three different regions: within the lac operon near the 3' end of the lacZ gene; near a terminator region previously identified upstream of the URA3 gene in YIp5; and at the URA3 terminator region. Readthrough transcription of the URA3 promoter from upstream lac sequences decreased the basal activity of the URA3 promoter, although induced URA3 transcription levels were unaffected. This readthrough transcription also resulted in a novel, longer URA3 transcript.

Molecular target of the antileishmanial action of sinefungin

Sinefungin, a natural nucleoside isolated from cultures of Streptomyces incarnatus and S. griseolus, is structurally related to S-adenosylhomocysteine and S-adenosylmethionine. Sinefungin has been shown to inhibit the development of various fungi and viruses, but its major attraction to date resides in its potent antiparasitic activity. This compound has been reported to display antiparasitic activity against malarial, trypanosomal, and leishmanial species. Very little is known about the antiparasitic mode of action of sinefungin. We found that S-adenosylmethionine was capable of reversing the inhibitory growth effects of sinefungin in Leishmania mexicana and that dATP was capable of reversing inhibitory effects of the drug on DNA polymerase activity when pyrophosphate release was measured. However, when incorporation of [3H]dTTP was used to measure DNA polymerase activity, no inhibition could be observed. Inhibition of DNA polymerase activity by sinefungin occurred only during the initial stages of purification of this enzyme, and inhibition by aphidicolin, a known DNA polymerase inhibitor, paralleled the inhibition by sinefungin. Neither sinefungin nor aphidicolin inhibited partially purified DNA polymerase. S-Adenosylmethionine synthetase was partially purified, and sinefungin, at levels active in vitro, had no significant effect. Sinefungin was significantly suppressive against both L. donovani and L. braziliensis panamensis infections in hamsters when compared with meglumine antimonate (Glucantime).

Early hypomethylation of 2'-O-ribose moieties in hepatocyte cytoplasmic ribosomal RNA underlies the protein synthetic defect produced by CCl4

Carbon tetrachloride (CCl4) treatment of rats produces an early defect in methylation of hepatocyte ribosomal RNA, which occurs concurrently with a defect in the protein synthetic capacity of isolated ribosomes. The CCl4-induced methylation defect is specific for the 2'-O-ribose position, and a corresponding proportional increase in m7G base methylation occurs in vivo. Undermethylated ribosomal subunits (rRNA) from CCl4-treated preparations can be methylated in vitro to a much greater extent than those from control preparations, and in vitro methylation restores their functional capacity. In vitro methylation of treated ribosomal subunits (which restores functional capacity) occurs at 2'-O-ribose positions (largely G residues). In contrast, in vitro methylation of control ribosomal subunits (which does not affect functional activity) represents base methylation as m7G, sites which are apparently methylated in treated preparations in vivo. Methylation/demethylation of 2'-O-ribose sites in rRNA exposed on the surface of cytoplasmic ribosomal subunits may represent an important cellular mechanism for controlling protein synthesis in quiescent hepatocytes, and it appears that CCl4 disrupts protein synthesis by inhibiting this 2'-O-ribose methylation.