TRANSITION IN SPECIFICATION OF EMBRYONIC METAZOAN DNA-REPLICATION ORIGINS

In early Xenopus embryos, in which ribosomal RNA genes (rDNA) are not transcribed, rDNA replication initiates and terminates at 9- to 12-kil obase pair intervals, with no detectable dependence on specific DNA se quences. Resumption of ribosomal RNA (rRNA) synthesis at late blastula and early gastrula is accompanied by a specific repression of replica tion initiation within transcription units; the frequency of initiatio n within intergenic spacers remains as high as in early blastula, Thes e results demonstrate that for rRNA genes, circumscribed zones of repl ication initiation emerge in intergenic DNA during the time in metazoa n development when the chromatin is remodeled to allow gene transcript ion.