Biochemical and genetic characterization of the DNA ligase encoded by Saccharomyces cerevisiae open reading frame YOR005c, a homolog of mammalian DNA ligase IV

Here we demonstrate that the Saccharomyces cerevisiae DNA ligase activity, which we previously designated DNA ligase II, is encoded by the genomic DNA sequence YOR005c, Based on its homology with mammalian LIG4, this yeast gene has been named DNL4 and the enzyme activity renamed DnI4. In agreement with others, we find that DNL4 is not required for vegetative growth but is involved in the repair of DNA double-strand breaks by non-homologous end joining. In contrast to a previous report, we find that a dnI4 null mutation has no effect on sporulation efficiency, indicating that DnI4 is not required for proper meiotic chromosome behavior or subsequent ascosporogenesis in yeast. Disruption of the DNL4 gene in one strain, M1-2B, results in temperature-sensitive vegetative growth. At the restrictive temperature, mutant cells progressively lose viability and accumulate small, nucleated and non-dividing daughter cells which remain attached to the mother cell. This novel temperature-sensitive phenotype is complemented by retransformation with a plasmid-borne DNL4 gene. Thus, we conclude that the abnormal growth of the dnI4 mutant strain is a synthetic phenotype resulting from DnI4 deficiency in combination with undetermined genetic factors in the M1-2B strain background.