Catalytic site components common to both splicing steps of a group II intron.

The splicing of group II introns occurs in two steps involving substrates with different chemical configurations. The question of whether these two steps are catalyzed by a single or two separate active sites is a matter of debate. Here, certain bases and phosphate oxygen atoms at conserved positions in domain V of a group II self-splicing intron are shown to be required for catalysis of both splicing steps. These results show that the active sites catalyzing the two steps must, at least, share common components, ruling out the existence of two completely distinct active sites in group II introns.