movement of residue 290 out of the hydrophobic
pocket and toward the active site that is always
accompanied by Ser288 in a down conformation and
has therefore been designated as out/down. When
the conformational change to out/down is modeled
into the poliovirus 3Dpol EC structure [8], there is a
clear steric clash between the loop and the
backbone of the RNA template strand (Fig. 5A).
This suggests that the out conformation cannot
coexist with RNA in a catalytically competent register
and that the observed structural transitions within the
loop may be involved in facilitating the movement of
RNA through the active site.