Simian‐virus‐40 large‐T‐antigen‐catalyzed DNA and RNA unwinding reactions

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Abstract
Simian virus 40 large T antigen is a helicase separating the complementary strands of double‐stranded DNA in the presence of hydrolyzable ATP and of double‐stranded RNA in the presence of non‐ATP nucleotides (GTP, CTP or UTP). We have constructed partially single‐stranded nucleic acid substrates consisting of RNA or DNA strands hydrogen bonded to either RNA or DNA complements. We found that ATP is utilized as a cofactor for the T‐antigen‐catalyzed unwinding reaction when the substrates contain overhanging single‐stranded DNA, regardless of whether the double‐stranded region is DNA or hybrid DNA · RNA. Conversely, non‐ATP nucleotides are used when the overhanging single strand is RNA. Based on these and additional findings, we propose that the bound nucleic acid induces a conformational change in T antigen resulting in a proper orientation of both nucleic acid and nucleotide relative to the active center of the ATPase/helicase domain of the enzyme. The implications of our conclusion for the roles which T antigen may play in vivo are discussed.