Identification of the initiation region of DNA replication in the murine immunoglobulin heavy chain gene and possible function of the octamer motif as a putative DNA replication origin in mammalian cells.

An origin region of DNA replication in the murine immunoglobulin heavy chain (IgH) gene was identified by BrdU pulse labeling and PCR amplification methods. The origin region spans about 1000 base pairs and contains the region of transcriptional enhancer in which the octamer sequence is present. The octamer sequence, TNATTTGCAT, is a well-conserved promoter/enhancer element responsible for B cell-specific transcription and is also found in the regulatory sequences for histone genes and others. Its activity as an autonomously replicating sequence was further examined. The murine IgH enhancer region containing the octamer motif was cloned in pUC18 and transfected to HeLa cells. After 60-65 h, the low molecular weight DNA was extracted and the degree to which the plasmid DNA had been replicated in the cells was measured by back-transformation of competent bacteria. Five to ten copies of the plasmid were detected per cell. The replicated plasmid-form DNA could be detected by this assay for at least 7 days after transfection. Synthetic oligonucleotides corresponding to the octamer and the Ephrussi box in the IgH enhancer were also cloned into pUC18 and examined for replicating activity. These plasmids replicated provided that the octamer sequence remained intact, irrespective of the Ephrussi box sequence and of the sites of insertion. These results suggest that the octamer transcriptional element may also serve as a putative origin for cellular DNA replication.