Cell cycle-dependent localization and properties of a second mitochondrial DNA ligase in Crithidia fasciculata

The mitochondrial DNA in kinetoplastid protozoa is contained in a single highly condensed structure consisting of thousands of minicircles and approximately 25 maxicircles. The disk-shaped structure is termed kinetoplast DNA (kDNA) and is located in the mitochondrial matrix near the basal body. We have previously identified a mitochondrial DNA ligase (LIG kbeta) in the trypanosomatid Crithidia fasciculata that localizes to antipodal sites flanking the kDNA disk where several other replication proteins are localized. We describe here a second mitochondrial DNA ligase (LIG kalpha). LIG kalpha localizes to the kinetoplast primarily in cells that have completed mitosis and contain either a dividing kinetoplast or two newly divided kinetoplasts. Essentially all dividing or newly divided kinetoplasts show localization of LIG kalpha. The ligase is present on both faces of the kDNA disk and at a high level in the kinetoflagellar zone of the mitochondrial matrix. Cells containing a single nucleus show localization of the LIG kalpha to the kDNA but at a much lower frequency. The mRNA level of LIG kalpha varies during the cell cycle out of phase with that of LIG kbeta. LIG kalpha transcript levels are maximal during the phase when cells contain two nuclei, whereas LIG kbeta transcript levels are maximal during S phase. The LIG kalpha protein decays with a half-life of 100 min in the absence of protein synthesis. The periodic expression of the LIG kalpha transcript and the instability of the LIG kalpha protein suggest a possible role of the ligase in regulating minicircle replication.