Deletion in the 3' pol sequence correlates with aberration of RNA expression in certain replication-defective avian sarcoma viruses

Abstract

The RNA expression of a series of replication-defective recovered avian sarcoma viruses (rASVs) were studied. Abnormal-sized viral RNAs, both larger and smaller than the genome, were observed in the nonproducer cells infected with rASVs containing env and pol deletions. Each nonproducer clone contained a single provirus integrated at a unique site and expressed a unique RNA pattern. Upon rescuing of the sarcoma virus with a helper virus and subsequent cloning, the RNA pattern of individual nonproducer clones again displayed variation according to the integration sites. This was not seen in nondefective rASV or in rASVs containing only an env deletion. The aberrant RNA expression did not result from the lack of reverse transcriptase activity per se, since neither nonconditional nor temperature-sensitive mutants of RSV expressed abnormal viral RNAs in the absence of a functional reverse transcriptase. The abnormal RNA patterns could not be corrected in trans by helper virus functions. The unusual-sized RNAs in env pol-rASV-infected cells are not due to splicing to alternative acceptor sites for src mRNA because there are no extra viral sequences between the 5' leader and the src sequences; instead, they are due to the presence of extra sequences, most likely of cellular origin, at the 3' ends of the viral RNAs. Based upon the extent of deletions in the viral genomes, the data suggest that deletion in the 3' pol region of those rASVs results in a cis effect on the transcription and processing of the 3' ends of viral RNAs. The unusual-sized viral RNAs are most likely due to read-through transcription from the right-hand terminus of provirus into downstream cellular sequences, followed by cleavage and polyadenylation at multiple sites of the 3' region of the RNA transcripts. The extent of read-through transcription appears to depend on the chromosomal location of the provirus.

Two distant upstream regions containing cis-acting signals regulating splicing facilitate 3'-end processing of avian sarcoma virus RNA

“…ASV RNA was previously suggested by the experiments of Wang (59), who showed that the presence of aberrantly large RNA species in CEF cell lines transformed by defective ASV correlated with deletion of this region. This result was shown to be a cis effect and could not be complemented by expression of a helper virus genome.…”

Differential transcription from the long terminal repeats of integrated avian leukosis virus DNA

“…In addition, the level of readthrough RNA was approximately threefold greater in lymphoma 4328 than in lymphoma NB1. The amounts of readthrough and normal RNAs derived from individual proviruses are likely to vary depending on the downstream cellular sequences (31). The presence of a polyadenylation signal in the cellular sequences could affect the stability of readthrough RNAs or the efficiency at which the viral polyadenylation site is used to generate normal viral RNAs.…”

Transduction of c-src coding and intron sequences by a transformation-defective deletion mutant of Rous sarcoma virus

“…tdlO9 retains 296 nucleotides of the 3' src sequence but lacks all of the 5' src and 316 nucleotides of its upstream region, including the src mRNA splice acceptor site. In two other studies, the approximate extent of deletions and junctions of recombination in a series of tdlO9-derived rASVs, including the three studied here, were mapped (47,48). To further pursue the mechanisms for recombination between tdlO9 and c-src, we set out to determine the tdlO9-derived rASV and c-src sequences at the recombination junctions.…”

“…The src mRNA splice acceptor site of SR-A RSV was deleted in tdlO9 and was not regained from c-src in the three rASV genomes. Previous analyses of the viral RNAs in tdlO9derived rASV-infected cells showed that the spliced src mRNAs of rASV3812 and rASV374 had a size indistinguishable from that of SR-A and that the src mRNA of rASV382 was larger than that of SR-A (47,48). It was suggested previously that the env gene splice acceptor site could be used for the formation of src message in rASV382-infected cells (47, 48).…”