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The E47 transcription factor binds to the enhancer sequences of recombinant murine leukemia viruses and influences
S C Lawrenz-Smith1, C Y Thomas
1Department of Microbiology, University of Virginia, Charlottesville 22908, USA.
Abstract:
The genomes of most recombinant murine leukemia viruses (MuLVs) inherit pathogenic U3 region sequences from the endogenous xenotropic provirus Bxv-1. However, the U3 regions of about one-third of recombinant MuLVs from CWD mice, such as CWM-T15, have nonecotropic substitutions that are probably derived from an endogenous polytropic provirus. The CWM-T15 U3 region sequences contain five nucleotide substitutions compared with the less pathogenic sequences of the endogenous ecotropic virus parent, Emv-1. Three of these substitutions are located immediately 3' of the enhancer core, and two form part of an E-box motif that is also found in the Bxv-1 sequence. A series of electromobility shift assays revealed that nuclear extracts from S194 cells and the basic helix-loop-helix transcription factor E47 could distinguish between oligonucleotides that contained the core region sequences of CWM-T15 or Emv-1. The E47 homodimers appeared to bind to the CWM-T15 E-box motif and when expressed at high levels in cells transactivated the CWM-T15 but not the Emv-1 enhancer. Taken together, these results suggest that E47 or related basic helix-loop-helix proteins that are expressed in lymphoid cells bind to and transactivate the CWM-T15 enhancer in vivo. This transactivation may explain why the CWM-T15 and Bxv-1 U3 regions accelerate the onset of lymphoid neoplasms and why related enhancer core region sequences are preferentially incorporated into the genomes of recombinant MuLVs and are found in other leukemogenic mammalian retroviruses.
Insights
Murine leukemia viruses (MuLVs) from CWD mice utilize distinct U3 regions. The transcription factor E47 binds to and activates the CWM-T15 enhancer, potentially explaining accelerated lymphoid neoplasms.
Area of Science:
- Virology
- Molecular Biology
- Genetics
Background:
- Recombinant murine leukemia viruses (MuLVs) often acquire pathogenic U3 sequences from endogenous proviruses.
- Some MuLVs from CWD mice, like CWM-T15, possess nonecotropic U3 substitutions, possibly from a polytropic provirus.
Purpose of the Study:
- To investigate the molecular mechanisms underlying the distinct U3 region sequences in CWM-T15 MuLV.
- To determine the role of specific nucleotide substitutions in enhancer activity and pathogenicity.
Main Methods:
- Sequence analysis of U3 regions from CWM-T15 and Emv-1.
- Electromobility shift assays (EMSA) using nuclear extracts and E47.
- Reporter gene assays to assess enhancer transactivation in cells.
Main Results:
- CWM-T15 U3 region contains five nucleotide substitutions compared to Emv-1, including an E-box motif.
- Nuclear extracts and E47 protein distinguished between CWM-T15 and Emv-1 enhancer core sequences.
- E47 homodimers bound the CWM-T15 E-box and transactivated the CWM-T15 enhancer, but not Emv-1.
Conclusions:
- E47 or related helix-loop-helix proteins bind and transactivate the CWM-T15 enhancer in lymphoid cells.
- This transactivation may contribute to the accelerated onset of lymphoid neoplasms by CWM-T15 and Bxv-1 U3 regions.
- Specific enhancer core sequences are favored in recombinant MuLVs and other leukemogenic retroviruses.