Sites hypersensitive to, and protected from, nuclease digestion in the regulatory region of wild-type and mutant polyoma chromatin.

It has been shown that the untranscribed regulatory region of polyoma virus (Py) is hypersensitive (Hs) to DNase I treatment, and that this hypersensitivity is located in two areas which correspond to the A and B domains of the enhancer. We mapped the DNase I hypersensitive sites in the Py regulatory region of wild-type (PyA2) and of mutants, selected in neuroblastoma cells (PyNB), which are characterized by an extensive duplication involving the A domain, with or without deletion of the B domain. The experiments were performed in both a permissive host (3T6 mouse fibroblasts) and in a restrictive host (41A3 mouse neuroblasts). No significant differences were observed between the two hosts. Our results show that four sites, in addition to the ones already described, can be identified in the wild-type A2 strain. These newly identified sites coincide with the domains of the enhancer region as they have recently been established. In PyNB mutants duplications and deletions are generally correlated to the gain or loss of the corresponding hypersensitive sites. However, a new site is formed in one of the duplicated sequences, even if no corresponding hypersensitive site is present in the other identical sequence. A region protected from DNase I digestion occurs in the PyNB mutants which corresponds to the junction of the duplication which is absent in the wild-type strain. In this region, as a consequence of the rearrangement, a GGCGGG motif which is very similar to the one (GGGCGG) present at the binding sites of the cellular regulatory protein SP1, is found.(ABSTRACT TRUNCATED AT 250 WORDS)