Objective
The process of proviral integration has been intensively studied but relatively little attention has been paid to the localisation of the integrated provirus in the host's cell genome. The widespread and persistent opinion that viral integration is random was challenged 15 years ago by a new approach based on the compositional fractionation of mammalian genomes. These genomes are mosaics of isochores, namely of very long (>300 kb) DNA segments that are compositionally homogeneous, belong to a small number of families, and cover a very extended GC range (30-60% for the human genome).
The large DNA fragments, 50-100 kb in size, form routine DNA preparations and reflect the base composition of the isochores from which they derive. They can be fractionated by preparative equilibrium centrifugation in Cs2SO4 density gradients in the presence of sequence-specific DNA ligands. In the human genome, two GC-poor isochore families, L1 and L2, form 62% of the genome, and three GC-rich isochore families, H1, H2 and H3, make up 20%, 10% and 3-5% of the genome, respectively, the rest being represented by satellite and ribosomal DNA. Gene concentration is low and constant in GC-poor isochore families, and increasingly higher in GC-rich isochore. The latter correspond to reverse and telomeric bands, the former to Giemsa bands on metaphase chromosomes.
Hybridisation with appropriate probes of the integrated viral sequences in compositional fractions of host cell DNA has shown that, in all cases investigated so far, integration of expressed viral sequence is compartmentalised, i.e. it takes place in isochore families that show a compositional match with the viral sequences. This compartmentalised integration was found for bovine leukaemia virus BLV, hepatitis B virus HBV, and Rous sarcoma virus RSV, sequences which all integrate in GC-rich regions of the host genome, and for mouse mammary tumour virus MMTV, sequences which integrate in GC-rich regions. Recent results indicate that the GC-rich genome of HTLV-I is integrated in the GC-rich isochores of the human genome. Moreover, transcribed viral sequences appear to be located in compartments characterised by higher GC levels than untranscribed sequences.
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Programme(s)
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Multi-annual funding programmes that define the EU’s priorities for research and innovation.
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Coordinator
75005 Paris
France
The total costs incurred by this organisation to participate in the project, including direct and indirect costs. This amount is a subset of the overall project budget.