T4 RNA ligase catalyzes the synthesis of dinucleoside polyphosphates

T4 RNA ligase has been shown to synthesize nucleoside and dinucleoside 5′‐polyphosphates by displacement of the AMP from the E‐AMP complex with polyphosphates and nucleoside diphosphates and triphosphates. Displacement of the AMP by tripolyphosphate (P3) was concentration dependent, as measured by SDS/PAGE. When the enzyme was incubated in the presence of 0.02 mm[α‐32P] ATP, synthesis of labeled Ap4A was observed: ATP was acting as both donor (Km, µm) and acceptor (Km, mm) of AMP from the enzyme. Whereas, as previously known, ATP or dATP (but not other nucleotides) were able to form the E‐AMP complex, the specificity of a compound to be acceptor of AMP from the E‐AMP complex was very broad, and with Km values between 1 and 2 mm. In the presence of a low concentration (0.02 mm) of [α‐32P] ATP (enough to form the E‐AMP complex, but only marginally enough to form Ap4A) and 4 mm of the indicated nucleotides or P3, the relative rate of synthesis of the following radioactive (di)nucleotides was observed: Ap4X (from XTP, 100); Ap4dG (from dGTP, 74); Ap4G (from GTP, 49); Ap4dC (from dCTP, 23); Ap4C (from CTP, 9); Ap3A (from ADP, 5); Ap4ddA, (from ddATP, 1); p4A (from P3, 200). The enzyme also synthesized efficiently Ap3A in the presence of 1 mm ATP and 2 mm ADP. The following T4 RNA ligase donors were inhibitors of the synthesis of Ap4G: pCp > pAp > pA2′p.