The c‐Myc protein is involved in cell proliferation, differentiation and apoptosis though heterodimerization with Max to form a transcriptionally active sequence‐specific DNA binding complex. By means of sequential immunoprecipitation of chromatin using anti‐Max and anti‐Myc antibodies, we have identified a Myc‐regulated gene and genomic sites occupied by Myc‐Max in vivo. Four of 27 sites recovered by this procedure corresponded to the highest affinity ‘canonical’ CACGTG sequence. However, the most common in vivo binding sites belonged to the group of ‘non‐canonical’ E box‐related binding sites previously identified by in vitro selection. Several of the genomic fragments isolated contained transcribed sequences, including one, MrDb, encoding an evolutionarily conserved RNA helicase of the DEAD box family. The corresponding mRNA was induced following activation of a Myc‐estrogen receptor fusion protein (Myc‐ER) in the presence of a protein synthesis inhibitor, consistent with this helicase gene being a direct target of Myc‐Max. In addition, as for c‐Myc, the expression of MrDb is induced upon proliferative stimulation of primary human fibroblasts as well as B cells and down‐regulated during terminal differentiation of HL60 leukemia cells. Our results indicate that Myc‐Max heterodimers interact in vivo with a specific set of E box‐related DNA sequences and that Myc is likely to activate multiple target genes including a highly conserved DEAD box protein. Therefore, Myc may exert its effects on cell behavior through proteins that affect RNA structure and metabolism.

Foamy virus (FV) vectors show promise for gene therapy applications. However, existing FV vectors either retain a significant portion of the wild-type virus genome or are produced at low titers. We describe a transient cotransfection system that produces high-titer FV vectors with minimal cis-acting regions. These vector genomes have deletions in the gag, pol, env, and bel1-3 accessory genes, as well as the LTR U3 region, but retain an essential 2.5-kb cis-acting region. In addition, stop codons were introduced into the remaining gag sequences to prevent expression of viral peptides and to eliminate dominant-negative effects of a Gag-Pol fusion protein. Although these deleted foamy (deltaphi) vectors were produced at relatively low titers with our prior packaging construct, we designed separate helper plasmids for Gag, Pol, and Env expression that allowed us to routinely produce helper-free, unconcentrated vector stocks with titers of over 10(5) transducing units/ml by four-plasmid transient transfection. The deltaphi vector stocks were then concentrated by ultracentrifugation to titers over 10(7) transducing units/ml. A deltaphi vector containing a 9.2-kb transgene cassette was produced at unconcentrated titers of over 10(5) transducing units/ml, demonstrating the utility of these deleted vectors for large therapeutic genes.