Fig. 4. Single nucleotide RNA elongation by the various loop mutant 3Dpol enzymes after a 30-min incubation. The
polymerases were incubated with a short 8-bp hairpin RNA that included a 6-nt templating region coding for the sequential
addition of G, U, and C, as shown by the first three lanes for each mutant. The last lane shows that elongation does not
occur if the first nucleotide (G) is omitted from the reaction. The mutants are grouped into four classes based on these data
and processive elongation data from the poly(A)-templated reactions (see Table 1). Class I+ mutants are hyperactive,
Class I− mutants are hypoactive, Class II mutants bind RNA normally but only add a single nucleotide, and Class III
mutants show no activity and exhibit reduced RNA-binding affinity.