A mitochondrial DNA primase from the trypanosomatid Crithidia fasciculata

We have purified to near homogeneity a DNA primase from a mitochondrial fraction of the trypanosomatid Crithidia fasciculata. The enzyme is a single polypeptide chain of 28 kDa. Using a poly(dT) template and ATP as a substrate, the enzyme makes oligonucleotides of which the vast majority are about 10 nucleotides in size or smaller. With a single-stranded M13 DNA template and the four rNTPs as substrates, the enzyme makes heterogeneous oligonucleotides in the same size range. These oligonucleotides efficiently prime the synthesis of DNA by the Klenow DNA polymerase. Immunolocalization with antibodies against the purified enzyme confirms that the primase is mitochondrial. Furthermore, the enzyme localizes to specific regions of the cell's single mitochondrion, above and below the condensed kinetoplast DNA. The primase does not co-localize with the mitochondrial topoisomerase II and DNA polymerase beta, both of which are associated with two protein complexes positioned on opposite sides of the kinetoplast disc. These localization studies have significant implications for the mechanism of kinetoplast DNA replication.