Increased messenger RNA from protein synthesis inhibited human fibroblasts

Evidence of transcription from the late region of the integrated simian virus 40 genome in transformed cells: location of the 5' ends of late transcripts in cells abortively infected and in cells transformed by simian virus 40

By means of S1 mapping, we observed that spliced 16S and 19S viral late mRNAs--in addition to early mRNAs--were present in cytoplasmic polyadenylated RNA preparations from simian virus 40-transformed cell lines of rat or mouse origin containing no detectable amount of free viral DNA. The amounts of early and late virus-specific mRNAs in these lines were quantified by hybridization of radioactive cytoplasmic polyadenylated RNA with cloned region-specific restriction fragments. The relative amount of late viral mRNA produced in these transformed cells was found to be of the same order as that produced in simian virus 40-infected, nonpermissive baby mouse kidney cells. Moreover, by using the S1 nuclease protection method, we compared the 5' ends of late mRNAs produced (i) in transformed cells, (ii) in abortively infected mouse cells, and (iii) in the late phase of the lytic cycle. The 5' ends of late mRNAs both in abortively infected and in transformed cells were less heterogeneous than the 5' ends of late mRNAs produced during the lytic cycle; however, they were a subset of the 5' ends of late transcripts produced in the lytic cycle.

Ability of nonpermissive mouse cells to express a simian virus 40 late function(s)

Mouse cells are fully nonpermissive for simian virus 40 (SV40). Infection does not lead to detectable virus replication. In this report, it was shown, first, that spliced 16S and 19S SV40 late mRNA were present in cytoplasmic and polysomal polyadenylated acid+ RNA preparations from SV40-infected baby mouse kidney cells. The 16S and 19S SV40 late mRNA's produced in infected baby mouse kidney cells were identical to or similar to the 16S and 19S SV40 late mRNA's produced in permissive monkey cells as judged by their S1 mapping patterns performed with the late strand of HpaII-BamHI fragment B and by their sedimentation patterns in a sucrose gradient. It was also shown that the 16S late mRNA from infected baby mouse kidney cells could be translated into a polypeptide which was identical to or similar to virion protein VP1 in every aspect examined, including the patter of peptide mapping by limited proteolysis. Second, we reported that mouse kidney cells produced detectable, although low, levels of SV40 virion protein VP1, as shown by the sodium dodecyl sulfate-polyacrylamide gel autoradiogram of [35S]methionine-labeled proteins immunoprecipitated by a rabbit antiserum directed against SV40 virion proteins. Third, it was reported that infected baby mouse kidney cells produced late mRNA's either (i) when the infection was done at a restrictive temperature with the nonleaky tsA58 mutant or (ii) in cells treated with 100 microgram of cycloheximide per ml, in which large T antigen synthesis was inhibited by more than 99.9%. This suggested that large T antigen was not required for the synthesis of late mRNA in mouse cells.

Paradoxical effects of protein synthesis inhibitors on uridine uptake in cultured cells: possible role of uncharged tRNA in regulating metabolism

Previous studies (J. Biol. Chem, 253: 99--105, 1978) showed that thyrotropin-releasing hormone (TRH) acutely stimulated uridine uptake in pituitary cell (GH 4Cl)) cultures. Studies on the role of protein synthesis in this response to TRH led to the finding that an inhibitor of ribosomal translation, cycloheximide, also stimulated uridine uptake acutely. Studies reported here attempt to determine the mechanism of cycloheximide action and whether cycloheximide and hormone stimulation of uridine uptake occurred by similar pathways. The experiments presented indicate that: (1) seven inhibitors of ribosomal translation stimulated uridine uptake; (2) in contrast, inhibition of protein synthesis at tRNA aminoacylation resulted in reduced rates of uridine uptake; (3) inhibition of tRNA aminoacylation blocked cycloheximide but not TRH stimulation of uptake; (4) cycloheximide stimulation of uptake was restricted to amino acid-depleted cultures; (5) amino acid supplementation stimulated uridine uptake with a time-course identical to that of cycloheximide; (6) cycloheximide and amino acid supplementation promoted reacylation of cellular tRNAs in amino acid-depleted cultures; and (7) cycloheximide stimulation of uridine uptake resulted from enhanced nucleoside phosphorylation rather than increased uridine transport. We conclude that cycloheximide and amino acid stimulation of uridine phosphorylation may be mediated through a common pathway involving the extent of amino-acylation of cellular tRNAs. Furthermore, cycloheximide and TRH stimulate uridine phosphorylation by pathways that are distinguishable. It is apparent that not all cellular effects of cycloheximide can be attributed solely to inhibition of the synthesis of proteins.

Interferon-mediated effect on ribosomal RNA metabolism

Ribosomal RNA (rRNA) has been shown to be involved in the binding of bacterial messenger RNA (mRNA) and an analogous 18 S rRNA.mRNA complex has been reported in eukaryotic systems. Thus, qualitative changes in host rRNA may be involved in the development of the interferon mediated antiviral state, a process thought to involve the inability of host ribosomes to bind and recognize viral mRNA. Data are reported which suggest that trisomy 21 human fibroblasts respond to human interferon with a marked reduction in cytoplasmic rRNA. [3H]Uridine was used to radioactively label the polysomal RNAs for 24 h beginning 12 h after interferon addition. Subsequent sucrose gradient analysis of the phenol or SDS-extracted RNA revealed that the reduction in radioactive rRNA was nearly complete for the 28 S rRNA. In contrast, considerable residual uridine incorporation was found in the 18 S rRNA species. Corollary data suggesting a net increase in mRNA synthesis and a net decrease in protein synthesis are reported.