Nucleic Acids Research, Vol 25, Issue 22 4505-4512, Copyright © 1997 by Oxford University Press
RS Scott, KY Truong and JM Vos
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.
ARTICLES
Replication initiation and elongation fork rates within a differentially expressed human multicopy locus in early S phase
Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7295, USA.
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