LATE INDUCTION OF HUMAN DNA-LIGASE-I AFTER UV-C IRRADIATION

We have studied the regulation of DNA ligase I gene expression in UV-C irradiated human primary fibroblasts. An increase of similar to 6-fol d both in DNA ligase I messenger and activity levels was observed 24 h after UV treatment, when nucleotide excision repair (NER) is no longe r operating. DNA ligase I induction is serum-independent and is contro lled mainly by the steady-state level of its mRNA. The activation is a function of the UV dose and occurs at lower doses in cells showing UV hypersensitivity. No increase in replicative DNA polymerase alpha act ivity was found, indicating that UV induction of DNA ligase I occurs t hrough a pathway that differs from the one causing activation of the r eplication machinery. These data suggest that DNA ligase I induction c ould be linked to the repair of DNA damage not removed by NER.