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Structure and biological activity of the subgenomic Mtv-6 endogenous provirus
K Cho1, D A Ferrick, D W Morris
1Department of Medical Pathology, University of California at Davis 95616.
Abstract:
The Mtv-6 provirus has an incomplete genome, but retains a functional superantigen gene (sag) which directs the thymic deletion of CD4+ T cells expressing T cell receptors containing the V beta 3 or V beta 5 chains. To better understand the Mtv-6 superantigen, the structure and biological activity of the Mtv-6 provirus was analyzed. First, the complete nucleotide sequence was determined, and the mutation producing the subgenomic provirus was identified. Second, the nucleotide sequence of the 5' end of the sag gene transcript (including the splice junction) was determined by sequence analysis of a cDNA clone. Third, the superantigen activity of Mtv-6 was analyzed in mice carrying the Mtv-6 provirus isolated by selective breeding on a genetic background free of endogenous and exogenous mouse mammary tumor virus (MMTV). These studies demonstrate that (i) the Mtv-6 provirus contains a 6.2-kb deletion between two 12-bp direct repeats encompassing the central portion of the provirus but not affecting sag gene splicing or translation, (ii) the sag gene transcript has the structure predicted from previous S1 nuclease mapping studies, and (iii) the Mtv-6 superantigen can direct thymic deletion of target V beta 3+ and V beta 5+ T cells in the absence of gene products from full-length MMTV proviruses.
Insights
The Mtv-6 provirus, despite its incomplete genome, effectively directs thymic deletion of specific T cells via its superantigen gene (sag). This study clarifies its structure and confirms its biological activity in deleting V beta 3+ and V beta 5+ T cells.
Area of Science:
- Immunology
- Virology
- Molecular Biology
Background:
- The Mtv-6 provirus possesses an incomplete genome but retains a functional superantigen gene (sag).
- This superantigen directs the thymic deletion of CD4+ T cells expressing specific T cell receptors (V beta 3 or V beta 5 chains).
Purpose of the Study:
- To elucidate the structure and biological activity of the Mtv-6 provirus and its superantigen.
- To understand the mechanisms of Mtv-6 superantigen-mediated T cell deletion.
Main Methods:
- Determined the complete nucleotide sequence of the Mtv-6 provirus to identify mutations.
- Sequenced the 5' end of the sag gene transcript using cDNA analysis.
- Assessed Mtv-6 superantigen activity in mice lacking other mouse mammary tumor virus (MMTV) proviruses.
Main Results:
- Identified a 6.2-kb deletion in the Mtv-6 provirus, which does not affect sag gene splicing or translation.
- Confirmed the predicted structure of the sag gene transcript.
- Demonstrated that Mtv-6 superantigen mediates thymic deletion of V beta 3+ and V beta 5+ T cells independently of full-length MMTV products.
Conclusions:
- The Mtv-6 provirus's deletion does not impair its superantigen function.
- Mtv-6 superantigen activity is confirmed in a controlled genetic background.
- This provirus can independently induce specific T cell deletion in the thymus.