Two yeast forkhead genes regulate the cell cycle and pseudohyphal growth

Abstract

There are about 800 genes in Saccharomyces cerevisiae whose transcription is cell-cycle regulated. Some of these form clusters of co-regulated genes. The 'CLB2' cluster contains 33 genes whose transcription peaks early in mitosis, including CLB1, CLB2, SWI5, ACE2, CDC5, CDC20 and other genes important for mitosis. Here we find that the genes in this cluster lose their cell cycle regulation in a mutant that lacks two forkhead transcription factors, Fkh1 and Fkh2. Fkh2 protein is associated with the promoters of CLB2, SWI5 and other genes of the cluster. These results indicate that Fkh proteins are transcription factors for the CLB2 cluster. The fkh1 fkh2 mutant also displays aberrant regulation of the 'SIC1' cluster, whose member genes are expressed in the M-G1 interval and are involved in mitotic exit. This aberrant regulation may be due to aberrant expression of the transcription factors Swi5 and Ace2, which are members of the CLB2 cluster and controllers of the SIC1 cluster. Thus, a cascade of transcription factors operates late in the cell cycle. Finally, the fkh1 fkh2 mutant displays a constitutive pseudohyphal morphology, indicating that Fkh1 and Fkh2 may help control the switch to this mode of growth.

Disruption of Yeast Forkhead-associated Cell Cycle Transcription by Oxidative Stress

“…We were able to trace this difference to the activity of the yeast forkhead homologues by noting first that the forkhead transcription signature reported by Zhu et al (2000) corresponded with the residual periodic transcription pattern left after MD treatment. This suggests that forkhead-proteinassociated transcription is responsible for driving MDtreated cells through G2 and M. Supporting this hypothesis, we observed that fkh1fkh2 double mutants, when treated with MD, now altered the cell cycle similar to wild-type HP-treated cells.…”

“…A full depiction of subclusters, gene Table i. The residual cell cycle clusters that maintained periodic expression under MD (but not HP) treatment included many genes previously associated with regulation by the yeast forkhead homologues Fkh1p and Fkh2p (Zhu et al, 2000); also see Web Supplement Figure iv). The G2/M cluster corresponds to the Clb1 cluster, which is directly regulated by these proteins, whereas the M/G1 and G1 clusters correspond to the Sic1 cluster, which is regulated by a member of the Clb1 cluster, encoding the Swi5p transcription factor.…”

“…To exhaustively identify similarly regulated genes, we sorted all the genes by Pearson correlation with the values for either CLB1 or PIR1, genes which are at the heart of the G2/M and M/G1 clusters. This resulted in the identification of two additional genes in the G2/M cluster, not reported in Zhu et al (2000) encoding a putative integral membrane protein, Sur7p, and an RNA-binding and localizing protein, Hek2p, suggesting that they may have roles in cell cycle FKH-regulated processes. HEK2 was previously reported as FKH regulated (Pic et al, 2000).…”

Degradation of Ndd1 by APC/CCdh1 generates a feed forward loop that times mitotic protein accumulation

“…1b). Ndd1 is the activator of the Mcm1-Fkh2 transcription factor complex [21][22][23][24] . The protein levels of Ndd1 oscillate periodically and it has been previously speculated 28 that it might be degraded by the APC/C.…”

“…We thus set up an unbiased screen designed to probe the entire yeast proteome. During the pilot stage of this screen, we identified Ndd1, an essential protein, which activates the Mcm1-Fkh2 G2-specific transcription factor [21][22][23][24] . We show here that Ndd1 is an APC/C Cdh1 substrate and that the APC/C Cdh1 is a major regulator of the cell cycle-specific oscillations of Ndd1.…”