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  • The EMBO Journal (2001) 20, 2867 - 2877
  • doi:10.1093/emboj/20.11.2867

Human acrocentric chromosomes with transcriptionally silent nucleolar organizer regions associate with nucleoli

Gareth J. Sullivan1, Joanna M. Bridger2, Andrew P. Cuthbert2,3, Robert F. Newbold2, Wendy A. Bickmore4 and Brian McStay1

  1. Biomedical Research Centre, University of Dundee, Ninewells Hospital, Dundee DD1 9SY, UK
  2. Department of Biological Sciences, Brunel University, Uxbridge, Middlesex UB8 3PH, UK
  3. Present address: Division of Medical and Molecular Genetics, Guy's, King's and St Thomas' School of Medicine, London SE1 9RT, UK
  4. MRC Human Genetics Unit, Crewe Road, Edinburgh EH4 2XU, UK

Correspondence to:

Brian McStay, E-mail: mcstay@icrf.icnet.uk

Received 12 December 2000; Accepted 3 April 2001; Revised 30 March 2001

Human ribosomal gene repeats are distributed among five nucleolar organizer regions (NORs) on the p arms of acrocentric chromosomes. On exit from mitosis, nucleoli form around individual active NORs. As cells progress through the cycle, these mini-nucleoli fuse to form large nucleoli incorporating multiple NORs. It is generally assumed that nucleolar incorporation of individual NORs is dependent on ribosomal gene transcription. To test this assumption, we determined the nuclear location of individual human acrocentric chromosomes, and their associated NORs, in mouse> human cell hybrids. Human ribosomal genes are transcriptionally silent in this context. Combined immunofluorescence and in situ hybridization (immuno-FISH) on three-dimensional preserved nuclei showed that human acrocentric chromosomes associate with hybrid cell nucleoli. Analysis of purified nucleoli demonstrated that human and mouse NORs are equally likely to be within a hybrid cell nucleolus. This is supported further by the observation that murine upstream binding factor can associate with human NORs. Incorporation of silent NORs into mature nucleoli raises interesting issues concerning the maintenance of the activity status of individual NORs.

  • Keywords:

    • human acrocentric chromosomes,
    • nucleolar organizer regions,
    • nucleoli,
    • somatic cell hybrids,
    • UBF