The early region of the SV40 genome (hereafter referred to as the A gene), can reactivate silent ribosomal RNA (rRNA) genes in human-mouse hybrid cells which contain the rRNA genes of both species but express only those of the dominant species. The present experiments were carried out to determine whether the necessary information for reactivation of rRNA genes could be localized to a specific region of the A gene. Human > mouse hybrid cells (#55-54) were infected with five different nondefective adenovirus-SV40 hybrid viruses, each containing a different size fragment of the SV40 A gene. Viruses containing SV40 DNA sequences mapping from 0.59 to 0.11 (Ad2+ND4), from 0.44 to 0.11 (Ad2+ND2), and from 0.39 to 0.11 (Ad2+ND,5) reactivated silent rRNA genes. Hybrid viruses mapping from 0.28 to 0.11 (Ad2+ND1) and from 0.18 to 0.11 (Ad2+ND1) and the nonhybrid adenovirus itself, failed to reactivate the silent rRNA genes. Silent gene reactivation also occurred when 55-54 cells were microinjected with fragments of cloned SV40 DNA mapping from 0.99 to 0.14 (94G2), and from 0.73 to 0.27 (HpaIIIPstI A fragment). A fragment mapping from 0.75 to 0.375 (HpaI B fragment) failed to reactivate the silent rRNA genes when microinjected into 55-54 cells, while the PvuII A fragment (0.70-0.32 map units) gave a very weak, positive result. These results indicate that the sequences from 0.67 to 0.39 and from 0.27 to 0.17 of the SV40 A gene are not necessary for the reactivation of silent rRNA genes. It is of interest that the same 600 base pairs involved in this particular function of the SV40 A gene are those which are most highly conserved and have the maximum homology with the DNA of polyoma virus.
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