Three biochemically distinct
DNA ligase activities have been identified in mammalian
cell extracts. We have recently purified
DNA ligase II and
DNA ligase III to near homogeneity from bovine liver and testis tissue, respectively.
Amino acid sequencing studies indicated that these
enzymes are encoded by the same gene. In the present study, human and murine
cDNA clones encoding
DNA ligase III were isolated with probes based on the
peptide sequences. The human
DNA ligase III cDNA encodes a
polypeptide of 862
amino acids, whose sequence is more closely related to those of the
DNA ligases encoded by poxviruses than to replicative
DNA ligases, such as human
DNA ligase I. In vitro transcription and translation of the
cDNA produced a catalytically active
DNA ligase similar in size and substrate specificity to the purified bovine
enzyme. The
DNA ligase III gene was localized to human chromosome 17, which eliminated this gene as a candidate for the
cancer-prone disease
Bloom syndrome that is associated with
DNA joining abnormalities.
DNA ligase III is ubiquitously expressed at low levels, except in the testes, in which the steady-state levels of
DNA ligase III mRNA are at least 10-fold higher than those detected in other tissues and cells. Since
DNA ligase I mRNA is also present at high levels in the testes, we examined the expression of the
DNA ligase genes during spermatogenesis.
DNA ligase I mRNA expression correlated with the contribution of proliferating spermatogonia cells to the testes, in agreement with the previously defined role of this
enzyme in DNA replication. In contrast, elevated levels of
DNA ligase III mRNA were observed in primary spermatocytes undergoing recombination prior to the first meiotic division. Therefore, we suggest that
DNA ligase III seals
DNA strand breaks that arise during the process of meiotic recombination in germ cells and as a consequence of DNA damage in somatic cells.