Cell cycle-dependent expression of a stable episomal human histone gene in a mouse cell.

We have constructed a recombinant plasmid that includes a cell cycle-dependent human H4 histone gene with 650 base pairs of 5' and 900 base pairs of 3' flanking sequences and the 69% transforming fragment of bovine papilloma virus. When transfected into C127 mouse cells, this plasmid is maintained as a stable episome with approximately 20 copies per cell. Micrococcal nuclease digestion indicates that the episomal human histone gene is packaged as chromatin. The human H4 histone transcript is initiated at the correct 5' start site and undergoes selective destabilization when DNA synthesis is inhibited. When C127 cells containing the episomal H4 histone gene are synchronized, the human H4 histone mRNA levels are regulated coordinately with DNA replication and parallel those of transcripts from the murine chromosomal H4 histone genes. Our results suggest that the regulatory sequences and/or regulatory molecules associated with murine and human histone genes are compatible. The human histone gene-bovine papillomavirus episome is therefore a viable system for studying cell cycle-regulated histone gene expression under conditions where control is not influenced at the site of chromosomal integration by cis-acting elements of genes normally not contiguous.