DNA-REPLICATION OF HISTONE GENE REPEATS IN DROSOPHILA-MELANOGASTER TISSUE-CULTURE CELLS - MULTIPLE INITIATION SITES AND REPLICATION PAUSE SITES

We showed previously that DNA replication initiates at multiple sites in the 5-kb histone gene repeating unit in early embryos of Drosophila melanogaster. The present report shows evidence that replication in t he same chromosomal region initiates at multiple sites in tissue cultu re cells as well. First, we analyzed replication intermediates by the two-dimensional gel electrophoretic replicon mapping method and detect ed bubble-form replication intermediates for all fragments restricted at different sites in the repeating unit. Second, we analyzed bromodeo xyuridine-labeled nascent strands amplified by the polymerase chain re action method and detected little differences in the size distribution of nascent strands specific to six short segments located at differen t sites in the repeating unit. These results strongly suggest that DNA replication initiates at multiple sites located within the repeating unit. We also found several replication pause sites located at 5' upst ream regions of some histone genes.