Novel functional requirements for non-homologous DNA end joining in Schizosaccharomyces pombe

DNA double strand break (DSB) repair by non-homologous end joining (NHEJ) i n mammalian cells requires the Ku70-Ku80 heterodimer, the DNA-PK catalytic subunit DNA-PKcs, as well as DNA ligase IV and Xrcc4, NHEJ of plasmid DSBs in Saccharomyces cerevisiae requires Ku, Xrcc4 and DNA ligase IV, as well a s Mre11, Rad50, Xrs2 and DNA damage checkpoint proteins. Saccharomyces cere visiae Ku is also required for telomere length maintenance and transcriptio nal silencing, We have characterized NHEJ in Schizosaccharomyces pombe usin g an extrachromosomal assay and find that, as anticipated, it is Ku70 and D NA ligase IV dependent. Unexpectedly, we find that Rad32, Rad50 (the S.pomb e homologues of Mre11 and Rad50, respectively) and checkpoint proteins are not required for NHEJ. Furthermore, although S.pombe Ku70 is required for m aintenance of telomere length, it is dispensable for transcriptional silenc ing at telomeres and is located throughout the nucleus rather than concentr ated at the telomeres, Together, these results provide insight into the mec hanism of NHEJ and contrast significantly with recent studies in S. cerevis iae.