A NEWLY IDENTIFIED DNA-LIGASE OF SACCHAROMYCES-CEREVISIAE INVOLVED INRAD52-INDEPENDENT REPAIR OF DNA DOUBLE-STRAND BREAKS

Eukaryotic DNA ligases are ATP-dependent DNA strand-joining enzymes th at participate in DNA replication, repair, and recombination. Whereas mammalian cells contain several different DNA ligases, encoded by at l east three distinct genes, only one DNA ligase has been detected previ ously in either budding yeast or fission yeast. Here, we describe a ne wly identified nonessential Saccharomyces cerevisiae gene that encodes a DNA ligase distinct from the CDC9 gene product. This DNA ligase sha res significant amino acid sequence homology with human DNA ligase IV; accordingly, we designate the yeast gene LIG4. Recombinant LIG4 prote in forms a covalent enzyme-AMP complex and can join a DNA single-stran d break In a DNA/RNA hybrid duplex, the preferred substrate in vitro. Disruption of the LIG4 gene causes only marginally increased cellular sensitivity to several DNA damaging agents, and does not further sensi tize cdc9 or rad52 mutant cells. In contrast, lig4 mutant cells have a 1000-fold reduced capacity for correct recircularization of linearize d plasmids by illegitimate end-joining after transformation. Moreover, homozygous lig4 mutant diploids sporulate less efficiently than isoge nic wild-type cells, and show retarded progression through meiotic pro phase I. Spore viability is normal, but lig4 mutants appear to produce a higher proportion of tetrads with only three viable spores. The mut ant phenotypes are consistent with functions of LIG4 in an illegitimat e DNA end-joining pathway and ensuring efficient meiosis.