Characterization of nucleophosmin (B23) as a Myc target by scanning chromatin immunoprecipitation.

The genetic program through which a specific transcription factor regulates a biological response is fundamental to our understanding how instructions in the genome are implemented. The emergence of DNA microarray technology for gene expression analysis has generated vast numbers of target genes resulting from specific transcription factor activity. We use the oncogenic transcription factor c-Myc as proof-of-principle that human genome sequence analysis and scanning of a specific gene by chromatin immunoprecipitation can be coupled to identify target transcription factor binding sequences. We focused on nucleophosmin, also known as B23, which was identified as a candidate Myc-responsive gene from a subtractive hybridization screen, and we found that sequences in intron 1, and not 5' sequences in the proximal promoter, are bound by c-Myc in vivo. Hence, a scanning chromatin immunoprecipitation (SChIP) strategy is useful in analyzing functional transcription factor-binding sites.

Efficient plasmid DNA replication in Xenopus egg extracts does not depend on prior chromatin assembly.

Small plasmids replicate efficiently in unfertilized Xenopus eggs provided they are injected before rather than after activation of the cell cycle. Here we use Xenopus egg extracts to test the hypothesis that efficient replication results from chromatin assembly prior to activation giving preloaded plasmids a head start toward the formation of a replicating pseudonucleus (Sanchez, J.A., Marek, D., and Wangh, L.J. (1992) J. Cell Sci. 103, 907-918). As in ovum, plasmid DNA preincubated in unactivated egg cytoplasmcytostatic factor extracts) replicate more efficiently after extract activation than does the same DNA added to the same extract after activation. Unlike in ovum, however, plasmids that replicate efficiently in vitro do not assemble into chromatin during preincubation and become topologically knotted instead. But even DNA knotting does not explain subsequent efficient replication. Also, plasmids preassembled into chromatin in vitro do not replicate efficiently in activated egg cytoplasm unless first preincubated in a CSF extract. We conclude that unactivated eggs contain replication-enhancing activities that can act independently of plasmid chromatin assembly and DNA topology. These postulated "preloading" factor(s) may be related to licensing factor, an activity that controls initiation of DNA replication in eukaryotic cells. The experimental conditions described here will permit characterization of preloading/licensing factor(s) in the context of a small plasmid substrate.