title: Developmental control of late replication and S phase length.
abstract: BACKGROUND
  Fast, early embryonic cell cycles have correspondingly fast S phases. In early Drosophila embryos, forks starting from closely spaced origins replicate the whole genome in 3.4 min, ten times faster than in embryonic cycle 14 and a hundred times faster than in a wing disc. It is not known how S phase duration is regulated. Here we examined prolongation of embryonic S phases, its coupling to development, and its relationship to the appearance of heterochromatin.
  
  
  RESULTS
  Imaging of fluorescent nucleotide incorporation and GFP-PCNA gave exquisite time resolution of S phase events. In the early S phases, satellite sequences replicated rapidly despite a compact chromatin structure. In S phases 11-13, a delay in satellite replication emerged in sync with modest and progressive prolongation of S phase. In S phase 14, major and distinct delays ordered the replication of satellites into a sequence that occupied much of S phase. This onset of late replication required transcription. Satellites only accumulated abundant heterochromatin protein 1 (HP1) after replicating in S phase 14. By cycle 15, satellites clustered in a compact HP1-positive mass, but replication occurred at decondensed foci at the surface of this mass.
  
  
  CONCLUSIONS
  The slowing of S phase is an active process, not a titration of maternal replication machinery. Most sequences continue to replicate rapidly in successive cycles, but increasing delays in the replication of satellite sequences extend S phase. Although called constitutively heterochromatic, satellites acquire the distinctive features of heterochromatin, compaction, late replication, HP1 binding, and aggregation at the chromocenter, in successive steps coordinated with developmental progress.
authors.affiliation: Department of Biochemistry & Biophysics, University of California, San Francisco, San Francisco, CA 94143-2200, USA.
authors.forename: Antony W
authors.initials: AW
authors.lastname: Shermoen
authors.forename: Mark L
authors.initials: ML
authors.lastname: McCleland
authors.forename: Patrick H
authors.initials: PH
authors.lastname: O'Farrell
mesh_terms: D000818:Animals; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics / Q000378:metabolism; D004261:DNA Replication; D004276:DNA, Satellite / Q000235:genetics / Q000378:metabolism; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism; D004331:Drosophila melanogaster / Q000196:embryology* / Q000235:genetics*; D006570:Heterochromatin / Q000378:metabolism; D018809:Proliferating Cell Nuclear Antigen / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D016196:S Phase
chemical_list: D002868:Chromosomal Proteins, Non-Histone; D004276:DNA, Satellite; D029721:Drosophila Proteins; D006570:Heterochromatin; D018809:Proliferating Cell Nuclear Antigen; D011993:Recombinant Fusion Proteins; C452193:heterochromatin protein 1, Drosophila