Polyadenylate polymerase (PAP) catalyzes the synthesis of 3'-polyadenylate tails 
onto mRNA. A comprehensive steady-state kinetic analysis of PAP was conducted 
which included initial velocity studies of the forward and reverse reactions, 
inhibition studies, and the use of alternative substrates. The reaction (A(n) + 
ATP <--> A(n+1) + PP(i)) is adequately described by a rapid equilibrium random 
mechanism. Several thermodynamic parameters for the reaction were determined or 
calculated, including the overall equilibrium constant (K(eq) = 84) and the 
apparent equilibrium constant of the internal step (K(int) = 4) which involves 
the rate-determining interconversion of central complexes. A large (100-fold) 
difference in Vmax accounts for nucleotide specificity (ATP vs CTP), despite an 
only 3-fold difference in Km. Comparison of the sulfur elemental effect on Vmax 
for ATP and CTP suggests that the chemical step is rate-determining for both 
reactions. Comparison of the sulfur elemental effect on Vmax/Km revealed 
differences in the mechanism by which either nucleotide is incorporated. 
Consistent with these data, an induced fit mechanism for nucleotide specificity 
is proposed whereby PAP couples a uniform binding mechanism, which selects for 
ATP, with a ground-state destabilization mechanism, which serves to accelerate 
the velocity for the correct substrate.