Two Forms of Mitochondrial DNA Ligase III Are Produced in Xenopus laevis Oocytes

Abstract

Full-length cDNAs for DNA ligase IV and the α and β isoforms of DNA ligase III were cloned from Xenopus laevis to permit study of the genes encoding mitochondrial DNA ligase. DNA ligase IIIα and IIIβ share a common NH2 terminus that encodes a mitochondrial localization signal capable of targeting green fluorescent protein to mitochondria while the NH 2 terminus of DNA ligase IV does not. Reverse transcriptase-polymerase chain reaction analyses with adult frog tissues demonstrate that while DNA ligase IIIα and DNA ligase IV are ubiquitously expressed, DNA ligase IIIβ expression is restricted to testis and ovary. Mitochondrial lysates from X. laevis oocytes contain both DNA ligase IIIα and IIIβ but no detectable DNA ligase IV. Gel filtration, sedimentation, native gel electrophoresis, and in vitro cross-linking experiments demonstrate that mtDNA ligase IIIα exists as a high molecular weight complex. We discuss the possibility that DNA ligase IIIα exists in mitochondria in association with novel mitochondrial protein partners or as a homodimer.