Correction of the defect in initiation of DNA replication in a temperature-sensitive mutant hamster cell line by in vitro addition of extracts from normal cells.

ts BN-2 is a temperature-sensitive hamster cell line that is defective in DNA synthesis at the restrictive temperature. The mutant expresses its defect during in vitro replication in whole-cell lysates. Addition of a high-salt-concentration extract from wild-type BHK-21, revertant RBN-2, or CHO cells to mutant cells lysed with 0.01% Brij 58 increased the activity in the mutant three- to fourfold, so that it reached 85% of the control value, and restored replicative synthesis. The presence of extract had an insignificant effect on wild-type and revertant replication and on mutant replication at the permissive temperature. Extract prepared from mutant cells was less effective than the wild-type cell extract was. Also, the stimulatory activity was more heat labile in the mutant than in the wild-type extract. Nuclear extract was as active as whole-cell extract.