Replication initiation and elongation fork rates within a differentially expressed human multicopy locus in early S phase.
Replication of the 400 copies of the 43 kb human ribosomal RNA (rDNA) locus spans most of the S phase. To examine the basis for the unusual pattern of rDNA replication, a sensitive strategy was developed to map origins of DNA replication and measure apparent rates of fork progression within a chromosomal locus. This technique, termed differential intragenomic replication timing, revealed that initiation within the actively transcribed rDNA occurred in early S within a 10.7 kb region spanning the promoter and 5' external transcribed spacer. Forks emanating from this early bidirectional origin progressed at an apparent slow rate with the sense and anti-sense forks moving at 0.32 and 0.23 kb/min. Using a photochemical-based technique, the chromatin status of the rDNA repeats was assayed throughout the S phase. Approximately 85% of the rDNA repeats were in a transcriptionally active chromatin structure at the start of S phase. A progressive decrease in the transcription state of the rDNA loci was observed, reaching a minimum between 3 and 6 h in mid S phase. Altogether, the data suggest a link between RNA polymerase I mediated transcription and site-specific initiation of DNA replication within the rDNA multicopy locus.
Scott
R S
RS
Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7295, USA.
Truong
K Y
KY
Vos
J M
JM
D002843:Chromatin; D002874:Chromosome Mapping; D004261:DNA Replication; D004275:DNA, Ribosomal / Q000096:biosynthesis*; D006801:Humans; D018741:Replication Origin; D016196:S Phase; D014407:Tumor Cells, Cultured
D002843:Chromatin; D004275:DNA, Ribosomal