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Retrovirus activation in embryonal carcinoma cells by cellular promoters
1Vollum Institute for Advanced Biomedical Research, Oregon Health Sciences University, Portland 97201.
Abstract:
Retrovirus expression in embryonal carcinoma (EC) cells is blocked at a postintegration stage of the viral life cycle, because of the inadequate function of the viral long terminal repeat (LTR) promoter in this cell type. However, rare sites in the EC cell genome permit provirus expression by undefined mechanisms. Our analysis of three expressed proviruses indicates that they have inserted into actively transcribed regions. Two of the three, examined in detail, integrated into the first introns of cellular transcription units in close proximity to active cellular promoters. One of these cellular genes is the probable murine homolog of the yeast ribosomal protein L3, responsible for trichodermin resistance. In all cases, virus activation appears to involve production of viral transcripts that are initiated in the 5'-flanking region, transcribed through the viral LTR, and subsequently spliced from a cellular donor to a viral acceptor. Our results suggest a general procedure for the isolation of active genes and promoters in different tissues.
Insights
Retrovirus expression in embryonal carcinoma cells is typically blocked. However, integration into active cellular gene regions can activate provirus expression through novel transcriptional mechanisms.
Area of Science:
- Molecular Biology
- Virology
- Genomics
Background:
- Retrovirus expression is usually inhibited in embryonal carcinoma (EC) cells due to poor function of the viral long terminal repeat (LTR) promoter.
- Certain genomic locations within EC cells allow for provirus expression via unknown mechanisms.
Purpose of the Study:
- To investigate the mechanisms by which retroviruses are expressed in embryonal carcinoma cells.
- To identify the genomic features associated with active provirus expression in EC cells.
Main Methods:
- Analysis of three expressed proviruses in EC cells.
- Examination of the integration sites and surrounding cellular genomic regions.
- Transcriptional analysis to identify viral and cellular RNA interactions.
Main Results:
- Expressed proviruses were found to have integrated into actively transcribed cellular regions.
- Two proviruses integrated into the first introns of cellular genes, near active promoters.
- Virus activation involved transcripts initiated in flanking regions, passing through the LTR, and spliced from cellular to viral sequences.
Conclusions:
- Provirus expression in EC cells is facilitated by integration into active cellular transcription units.
- Activation involves a splicing mechanism utilizing cellular transcriptional machinery.
- This study proposes a method for isolating active genes and promoters in various tissues.