Tad3 is part of the functional heterodimeric deaminase Tad2/3, where it contributes to catalysis. Tad3 is not simply a structural component of the dimeric enzyme. In T. brucei only one zinc ion out of two play a role in catalysis. Tad2/3 tRNA A:I editing enzyme plays a role in both A:I editing in position 34 and C:U editing in position 32 in vivo. In vitro, this A-to-I tRNA deaminase catalyzes also C-to-U deamination of DNA. The A34-to-I34 deamination reaction was tested with T. brucei tRNA-Thr as substrate. However, as other eukaryotic heterodimeric Tad2/3 it also works on other naturally occurring A34-containing tRNA (Alfonzo J. , unpublished data).