FIGURE 3.

Model of tRNA processing and editing in monoblepharidalean mitochondria. A hypothetical unprocessed tRNA acceptor stem is shown. (A) 5′ and 3′ extensions are first removed by nuclease activities (the order of events is not known). (B) CCA tails are added by nucleotidyl transferase (CCAse) and a 5′-to-3′ exonuclease activity removes nucleotides from tRNA 5′ ends. The CCAse and exonuclease activities apparently function independently of one another. A presumed 3′-to-5′ nucleotidyltransferase (as proposed for A. castellanii based on in vitro results; Price and Gray 1999b) then fills in nucleotides removed by the 5′-to-3′ exonuclease. (C) If all three 5′ nucleotides have been removed by the exonuclease, three nucleotides are added that can form WC base pairs, resulting in a completely edited tRNA. (D) If only one or two 5′ nucleotides have been removed, a partially edited tRNA results. (E) If no nucleotides have been removed, an unedited tRNA results.