A DNA polymerase β in the mitochondrion of the trypanosomatid Crithidia fasciculata

We previously purified a Crithidia fasciculata mitochondrial DNA polymerase that has unusual properties. Unlike a conventional mitochondrial DNA polymerase γ, this enzyme is small, non-processive, deficient in 3'- exonuclease activity, and error prone (Torri, A. F., Kunkel, T. A., and Englund, P. T. (1994) J. Biol. Chem. 269, 8165-8171). In all of these characteristics, the enzyme resembles DNA polymerase β, a nuclear enzyme thought to be involved in DNA repair. We have now cloned and sequenced the gene for this enzyme. The mitochondrial polymerase has significant homology, about 33% identity at the amino acid level, with human DNA polymerase β. However, sequence analysis of the clone revealed the presence of a cleaved N- terminal presequence, presumably a mitochondrial import signal, which resembles presequences on other C. fasciculata mitochondrial proteins. The polymerase's function may be to repair the many gaps in newly replicated kinetoplast (mitochondrial) DNA minicircles in this parasite. This enzyme is the first example of a mitochondrial DNA polymerase β.