Mitochondrial forms of
uracil DNA glycosylase and
UV endonuclease have been purified and characterized from the mouse
plasmacytoma cell line, MPC-11. As in other cell types, the mitochondrial
uracil DNA glycosylase has properties very similar to those of the nuclear
enzyme, although in this case the mitochondrial activity was also distinguishable by extreme sensitivity to dilution. Three mitochondrial
UV endonuclease activities are also similar to nuclear
enzymes; however, the relative amounts of these
enzyme activities in the mitochondria is significantly different from that in the nucleus. In particular, mitochondria contain a much higher proportion of an activity analogous to
UV endonuclease III. Nuclear
UV endonuclease III activity is absent from
XP group D fibroblasts and
XP group D lymphoblasts have reduced, but detectable levels of the mitochondrial form of this
enzyme. This residual activity differs in its properties from the normal mitochondrial form of
UV endonuclease III, however. The presence of these
enzyme activities which function in base excision repair suggests that such DNA repair occurs in mitochondria. Alternatively, these
enzymes might act to mark damaged mitochondrial genomes for subsequent degradation.