Figure 7.

Evolution of RNase II enzymes and pseudoenzymes. (A) Bacterial RNase II has a domain structure that is conserved in evolution but lacks the eukaryote-specific insertions. RNA accesses the active site by funnelling into the core of the protein. (B) An ancestral Dis3/Dis3L2 enzyme may have acquired mobile regulatory elements that allow the enzyme to be finely regulated. The ‘ON’ state resembles that of the bacterial enzyme, while the ‘OFF’ state uses the CSD1 insertion and the autoinhibitory segment to block the funnel. (C) The autoinhibitory elements have been fixed in place in Ssd1 by the S1 insertion element and the active site residues have been lost. A new RNA binding site has been acquired.