Alternative interactions of the SV40 A protein with DNA.
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We have characterized the DNA binding properties of SV40 A protein (T antigen) from permissive cells. The viral protein protects 110- to 115-, 60- to 65-, 40- to 45-, and 30- to 35-bp fragments of SV40 DNA from an excess of DNase 1. The 60- to 65-bp fragment extends some 30-35 by to either side of the BglI site at the origin of replication. DNase footprinting confirms the location of this binding region and shows that it is the most completely protected DNA binding site under our conditions. The A protein selectively recognizes the 60- to 65-bp origin fragment in the complete absence of adjacent DNA and binds to it in three alternative ways that protect 60-65, 40-45, or 30-35 by from nuclease. Thus these three interactions with isolated origins recapitulate the binding of viral protein to intact SV40 DNA. We propose that the alternative interactions of SV40 A protein with the 60- to 65-bp origin fragment may represent three different conformational alignments of protein with DNA rather than a simple repetition of a single kind of binding interaction. Viral protein also protects pBR322 and phage DNA from nuclease to a limited extent. Similar interactions could possibly alter the function of DNA control regions of host cells. 1981.