| Abstrakt: |
cdc28-1Nmutants progress through the G1and S phases normally at the restrictive temperature but fail to undergo nuclear division. We have isolated a gene, NDD1, which at a high dosage suppresses the nuclear-division defect of cdc28-1N. NDD1(nuclear division defective) is an essential gene. Its expression during the cell cycle is tightly regulated such that NDD1RNA is most abundant during the S phase. Cells lacking the NDD1gene arrest with an elongated bud, a short mitotic spindle, 2N DNA content, and an undivided nucleus, suggesting that its function is required for some aspect of nuclear division. We show that overexpression of Ndd1 results in the upregulation of both CLB1and CLB2transcription, suggesting that the suppression of cdc28-1Nby NDD1may be due to an accumulation of these cyclins. Overproduction of Ndd1 also enhances the expression of SWI5, whose transcription, like that of CLB1and CLB2, is activated in the late S phase. Ndd1 is essential for the expression of CLB1,CLB2, and SWI5, since none of these genes are transcribed in its absence. Both CLB2expression and its upregulation by NDD1are mediated by a 240-bp promoter sequence that contains four MCM1-binding sites. However, Ndd1 does not appear to be a component of any of the protein complexes assembled on this DNA fragment, as indicated by gel mobility shift assays. Instead, overexpression of NDD1prevents the formation of one of the complexes whose appearance correlates with the termination of CLB2expression in G1. The inability of GAL1promoter-driven CLB2to suppress the lethality of NDD1null mutant suggests that, in addition to CLB1and CLB2, NDD1may also be required for the transcription of other genes whose functions are necessary for G2/M transition. |
