Specification of a DNA replication origin by a transcription complex.

In early Xenopus development, transcription is repressed and DNA replication initiates at non-specific sites. Here, we show that a site-specific DNA replication origin can be induced in this context by the assembly of a transcription domain. Deletion of the promoter element abolishes site-specific initiation, and its relocalization to an ectopic site induces a new origin of replication. This process does not require active transcription, and specification of the origin occurs mainly through a decrease in non-specific initiation at sites distant from the promoter. Finally, chromatin immunoprecipitation experiments suggest that site-specific acetylation of histones favours the selection of the active DNA replication origin. We propose that the specification of active DNA replication origins occurs by secondary epigenetic events and that the programming of chromatin for transcription during development contributes to this selection in higher eukaryotes.

DNA replication initiates at domains overlapping with nuclear matrix attachment regions in the xenopus and mouse c-myc promoter.

Only a very few origins have been mapped in different multicellular organisms, and they do not share detectable consensus sequence elements. Moreover, it is not clear if origins are localized at similar positions in the corresponding locus in genomes of different organisms. Here, we have mapped DNA replication origins in the c-myc locus both in Xenopus and mouse, allowing a comparison of the corresponding sites in three different animal species (Xenopus, mouse, human). An origin of DNA replication is present in the three homologous c-myc loci. In Xenopus, a main DNA replication origin was located 3 kilobases (kb) upstream of the active c-myc promoter, whereas, in mouse, we detected an origin 1 kb upstream of the promoter, as previously mapped in human c-myc. We also identified a nuclear matrix attachment region in both Xenopus and mouse, which is localized to two different regions of the c-myc promoter region. However, in both cases, the nuclear matrix attachment sites are close to the DNA replication origin mapped in the locus. These data suggest that global features of chromatin organization in different organisms may contribute to DNA replication origin localization.