Figure 2.

Pyrophosphorolytic activity of purified DHBV cores. (a) The ability of 1 mM PP_i and various PP_i analogs (5 mM rNTPs) to act as substrates for pyrophosphorolysis was assayed by monitoring the removal [α-³²P]dCMP from viral DNA. For these and subsequent data, background radioactivity was subtracted, and values are percentages relative to the extension reaction (ext). (b) The primer termini were labeled by the incorporation of chain-terminating nucleotide analogs, and their ability to serve as substrates for DHBV-RT pyrophosphorolysis was compared by the addition of 1 mM PP_i. The results are depicted for the end-point of all reactions: ddNTPs were completely removed in less than 1 min, whereas the data of a 1-h incubation with PP_i are shown for 3TC. (c) The pyrophosphorolytic activity of various DHBV-RT mutants was determined as described for wild type. The results are for a 1-h incubation with 1 mM PP_i. (d) The substrate specificity of the LMMV variant was tested as for wild-type DHBV-RT.