Comparison of the steady-state kinetic parameters for 18 lentiviral RT proteins. The K
(A) and k
(B) values of the 18 different RT enzymes (blue bars, HIV-1 RTs; purple bars, HIV-2 RTs; green bars, SIV RTs) were determined from the reactions described in Figure 1. dNTP concentrations found in macrophages (grey), activated CD4+ T cells (pink), and macrophages exposed to Vpx (blue) were marked in (A) . (C) The overall catalytic efficiency values (k
) were plotted with a 95% confidence interval and the efficiency difference between RTs of Vpx coding and noncoding viruses were compared. The V
values were determined by fitting the data to the Michaelis-Menten equation using nonlinear regression with Kaleidagraph (Synergy Software). k
was determined by dividing V
by molar enzyme concentration. Values reported represent means and standard deviations of HIV-1 and the group HIV-2/ SIV. Two-tailed Student’s t tests were used for the two group comparisons (Vpx + vs Vpx: p < 0.01; HIV-1 vs SIV: p < 0.01; HIV-1 vs HIV-2: p < 0.1; HIV-2 vs SIV: p = 0.12).