The E47 transcription factor binds to the enhancer sequences of recombinant murine leukemia viruses and influences enhancer function

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. * Corresponding author. Phone: (904) 953-7126. Fax: (904) 953-7117. 4142

The potent enhancer activity of the polycythemic strain of spleen focus-forming virus in hematopoietic cells is governed by a binding site for Sp1 in the upstream control region and by a unique enhancer core motif, creating an exclusive target for PEBP/CBF

“…Sp1 may thus prevent de novo methylation as described for endogenous genes (11,35) and support the interaction with Ets factors and PEBP/CBF. Likewise, a higher-order complex with accessory factors targeting the enhancer might be stabilized; these include E-box family members (29,43) and the FV factors, which contribute to the tissue tropism of FMuLV-related retroviruses in vivo (32,36,37). In contrast, binding of ELP to the UCR of MoMuLV or FMuLV could block formation of the secondary structure in ES cells and thus could induce attenuation or even silencing of retroviral activity.…”

A promoter within the E6 ORF of human papillomavirus type 16 contributes to the expression of the E7 oncoprotein from a monocistronic mRNA

“…The so-called E box, which has been found in several promoters, is a sequence motif with the consensus CANNTG, where the two central nucleotides can vary (Ellenberger et al, 1994;Ou et al, 1994;Hu et al, 1990). The motif binds the basic helix-loop-helix (bHLH) class of proteins (Murre et al, 1989a, b;Ellenberger et al, 1994;Lawrenz-Smith & Thomas, 1995). As well as the bHLH proteins, the bHLH leucine zipper proteins can also bind the E box, including the ubiquitous factor AP-4.…”

Various modes of basic helix-loop-helix protein-mediated regulation of murine leukemia virus transcription in lymphoid cell lines

“…The MLV U3 regions represent an amalgamation of several cis transcriptional regulatory sequences. Recently, basic helixloop-helix (bHLH) transcription factors and their cognate DNA recognition sequence, CANNTG (E-box), have been characterized as putative mediators of MLV transcription regulation (9,21,28,31,32). The bHLH transcription factors are involved in cell growth control and transformation, sex determination, pancreas gene expression, neurogenesis, muscle differentiation, and hematopoietic gene expression and differentiation (reviewed in reference 26).…”

“…In transienttransfection assays, ALF1 can trans activate transcription from Akv, SL3-3, and the thymotropic Moloney MLV U3 regions, as well as the erythrotropic Friend spleen focus-forming virus, but not from the erythrotropic Friend MLV (32). Similarly, the bHLH transcription factor E47 can transactivate transcription of recombinant MLVs (21).…”