Splicing of SV40 late mRNA is a post-transcriptional process

Attenuation may regulate gene expression in animal viruses and cells

In eukaryotes, an abundant population of promoter-proximal RNA chains have been observed and studied, mainly in whole nuclear RNA, in denovirus type 2, and in SV40. On the basis of these results it has been suggested that a premature termination process resembling attenuation in prokaryotes occurs in eukaryotes. Moreover, these studies have shown that the adenosine analog 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) enhances premature termination, but its mode of action is not understood. The determination of the nucleotide sequences of SV40 and other viruses and cellular genes provide means for elucidating the nucleotide sequences involved in the attenuation mechanism. A model has recently been described in which attenuation and mRNA modulation in a feedback control system quantitatively regulate SV40 gene expression. The suggested mechanism described in this model opens up approaches to the investigation of attenuation and mRNA modulation as a possible mechanism whereby eukaryotes may regulate transcription in a variety of different circumstances.

Attenuation and modulation of mRNA secondary structure in a feedback control system regulating SV40 gene expression

Alternative secondary structures can be predicted for the initial 94 nucleotides synthesized from the major transcription initiation site of SV40 late RNA: a transcription-termination conformation results in the production of aborted RNA and a readthrough conformation leads to the synthesis of the primary SV40 late RNA. In the cytoplasm similar alternative conformations can be predicted for the initial nucleotides at the 5' ends o both the major 16S and 19S late mRNAs. In one of these alternative conformations the AUG initiation codon of the leader protein (agnoprotein) is sequestered and not available for ribosome binding. In the alternative conformation the same AUG is accessible for ribosome binding. We suggest that these mutually exclusive conformations are fundamental elements in a transcription and translation feedback control mechanism regulating the synthesis of 16S and 19S mRNA in the nucleus and the translation of their encoded proteins in the cytoplasm.

Site of premature termination of late transcription of simian virus 40 DNA: enhancement by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole

Sedimentation analysis of pulse-labeled RNA synthesized in nuclei isolated from simian virus 40-infected cells revealed an abundance of short cellular and viral RNAs. The relative amount of the short chains is increased in nuclei isolated from cells treated with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). The short viral RNAs were purified by hybridization to and elution from simian virus 40 DNA on filters, and their sizes were determined by gel electrophoresis. A major band of 93- to 95-nucleotide-long RNA was observed along with additional minor bands. Identical bands were revealed when the viral RNA was purified from nuclei of cells pretreated with DRB. The major band was identified as an aborted transcript of a RNA that initiated at the major initiation site (nucleotide 243). We have found that the DNA region where the RNA stops is A+T rich and is immediately preceded by a G+C-rich region that exhibits dyad symmetry, resembling the termination signal in prokaryotes. These observations show that RNA polymerase II responds to the same termination signal as the prokaryotic enzyme and suggest that a mechanism of attenuation regulates simian virus 40 late transcription.

Escherichia coli RNA polymerase in vitro mimics simian virus 40 in vivo transcription when the template is viral nucleoprotein

We have used a low-salt detergent-free extraction procedure on cells infected with simian virus 40 to obtain viral nucleoprotein late after infection. Addition of EScherichia coli RNA polymerase and ribonucleotide triphosphates to the viral minichromosomes permitted transcription of RNA from viral templates. This synthesis was initiated predominantly within a fragment of DNA spanning 0.67 to 0.76 map unit on the genome. The synthesis from this region proceeded primarily along the "late" strand in a clockwise direction. These results were in contrast to the synthesis obtained with naked viral DNA in which initiation occurred on other regions of the genome and from which transcription proceeded counterclockwise along the early strand. These findings indicate that the nucleoprotein template or factors tightly associated with it may be responsible for site(s) and strand selection in transcription of simian virus 40.

5,6-dichloro-1-beta-ribofuranosylbenzimidazole enhances premature termination of late transcription of simian virus 40 DNA

Short RNA chains initiating at the major promoter sites for simian virus 40 (SV40) late transcription are elongated to approximately 450 nucleotides in a molar ammount greater than that from any other region of the viral DNA. This conclusion is based on the following observations: (i) Transcriptional complexes isolated by Sarkosyl and by hypotonic leaching (minichromosomes) from nuclei of cells infected with SV40 as well as intact nuclei were pulse labeled in vitro with [alpha-32P]TUP and were observed to synthesize short RNA transcripts that hybridized predominantly to a SV40 DNA fragment spanning between 0.67 and 0.76 map units. (ii) In the presence of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), a drug known to accentuate premature transcriptional termination, accumulation of these short SV40 RNA chains was enhanced. When SV40-infected cells were pretreated with DRB and then labeled in vivo or in vitro, they synthesized short labeled viral RNAs that hydridized almost exclusively with the DNA fragment spanning between 0.67 and 0.76 map units. These observations suggest a mechanism in the regulation of SV40 late transcription.