SITES SENSITIVE TO S1 NUCLEASE and DISCONTINUITIES IN DNA NASCENT STRANDS OF ULTRAVIOLET IRRADIATED MOUSE CELLS

Abstract Mouse 3T3 cells irradiated with ultraviolet light synthesize DNA containing sites sensitive to the single-strand specific SI nuclease. The appearance of these sites correlates well with the presence of discontinuities in nascent strands, detected by the methodology of sedimentation in alkaline sucrose gradient. Thus, both the sites sensitive to SI nuclease and the discontinuities in nascent strands (i) are stabilized by caffeine; (ii) are no longer formed late after irradiation and (iii) disappear faster when a certain UV fluence is split into two fluences whose sum equals the single fluence. Moreover, the recovery in synthesizing DNA without SI sensitive sites is not dependent on excision repair of pyrimidine dimers or on continuous DNA synthesis. These SI sensitive sites are exclusive of replicative structures of irradiated cells and should correspond to stretches of single-strand DNA (gaps) formed during replication.