Genetics, Vol 143, 165-174, Copyright © 1996

INVESTIGATIONS

The REC1 Gene of Ustilago maydis, Which Encodes a 3' -> 5' Exonuclease, Couples DNA Repair and Completion of DNA Synthesis to a Mitotic Checkpoint

K. Onel, A. Koff, R. L. Bennett, P. Unrau and N. K. Holloman
Hearst Microbiology Research Center, Cornell University Medical College, New York, New York 10021

Mutation in the REC1 gene of Ustilago maydis results in extreme sensitivity to killing by ultraviolet light. The lethality of the rec1-1 mutant 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 was inhibited by external means through addition of hydroxyurea or by genetic control in a temperature-sensitive mutant strain defective in DNA synthesis. Flow cytometric analysis of exponentially growing cultures indicated that wild-type cells accumulated in G2 after UV irradiation, while rec1-1 cells appeared to exit from G2 and accumulate in G1/S. Analysis of mRNA levels in synchronized cells indicated that the REC1 gene is periodically expressed with the cell cycle and reaches maximal levels at G1/S. The results are interpreted to mean that a G2-M checkpoint is disabled in the rec1-1 mutant. It is proposed that the REC1 gene product functions in a surveillance system operating during S-phase and G2 to find and repair stretches of DNA with compromised integrity and to communicate with the cell cycle apparatus.

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