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

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.