DNA LIGASE-III IS THE MAJOR HIGH-MOLECULAR-WEIGHT DNA JOINING ACTIVITY IN SV40-TRANSFORMED HUMAN FIBROBLASTS - NORMAL LEVELS OF DNA LIGASE-III ACTIVITY IN BLOOM-SYNDROME CELLS

The phenotypes of cultured cell lines established from individuals wit h Bloom syndrome (BLM), including an elevated spontaneous frequency of sister chromatid exchanges (SCEs), are consistent with a defect in DN A joining. We have investigated the levels of DNA ligase I and DNA lig ase III in an SV40-transformed control and BLM fibroblast cell line, a s well as clonal derivatives of the BLM cell line complemented or not for the elevated SCE phenotype. No differences in either DNA ligase I or DNA ligase III were detected in extracts from these cell lines. Fur thermore, the data indicate that in dividing cultures of SV40-transfor med fibroblasts, DNA ligase III contributes > 85% of high molecular we ight DNA joining activity. This observation contrasts with previous st udies in which DNA ligase I was reported to be the major DNA joining a ctivity in extracts from proliferating mammalian cells.