Temporal order of replication of Xenopus laevis 5S ribosomal RNA genes in somatic cells.

The timing of replication of both the somatic and oocyte-type 5S ribosomal RNA genes of Xenopus laevis was determined in cultured cells by using 5-bromodeoxyuridine labeling of DNA coupled with a retroactive synchrony technique employing the fluorescence-activated cell sorter (FACS). The somatic genes replicate very early in S phase, while the oocyte genes replicate very late. These experiments provide direct evidence for a model [Gottesfeld, J. & Bloomer, L. S. (1982) Cell 28, 781-791] in which the transcription-activating factor TFIIIA is required at the time of Xenopus 5S rRNA gene replication to facilitate transcription of these genes.