Supercoiled loop organization of genomic DNA: a close relationship between loop domains, expression units, and replicon organization in rDNA from Xenopus laevis.

Analysis of the organization of a specific chromosomal gene, the gene for rRNA in Xenopus laevis, has evidenced a close relationship between loop organization, replication organization, and expression units. The nontranscribed spacer appears to be involved in all three levels of organization. Furthermore the replication origin region appears to be involved in nuclear matrix anchorage and is closely related to the 40 S transcription promoter. This organization suggests how expression domains may be regulated and how this functional organization may be transmitted to daughter cells after DNA replication, thus allowing selected expression patterns not to be lost during development.