THE REC1 GENE OF USTILAGO-MAYDIS, WHICH ENCODES A 3'-]5'-EXONUCLEASE,COUPLES DNA-REPAIR AND COMPLETION OF DNA-SYNTHESIS TO A MITOTIC CHECKPOINT

Mutation in the REC1 gene of Ustilago maydis results in extreme sensit ivity to killing by ultraviolet light. The lethality of the rec1-1 mut ant was found to be partially suppressed if irradiated cells were held artificially in G2-phase by addition of a microtubule inhibitor. This mutant was also found to be sensitive to killing when DNA synthesis w as inhibited by external means through addition of hydroxyurea or by g enetic control in a temperature-sensitive mutant strain defective in D NA synthesis. Flow cytometric analysis of exponentially growing cultur es indicated that wild-type cells accumulated in G2 after UV irradiati on, while rec1-1 cells appeared to exit from G2 and accumulate in G1/S . Analysis of mRNA levels in synchronized cells indicated that the REC 1 gene is periodically expressed with the cell cycle and reaches maxim al level at G1/S. The results are interpreted to mean that a G2-M chec kpoint is disabled in the rec1-1 mutant. It is proposed that the REC1 gene product functions in a surveillance system operating during S-pha se and G2 to find and repair stretches of DNA with compromised integri ty and to communicate with the cell cycle apparatus.