Conserved homeodomain proteins interact with MADS box protein Mcm1 to restrict ECB-dependent transcription to the M/G1 phase of the cell cycle

Abstract

Cyclin-dependent kinases (Cdks) drive the cell cycle in all eukaryotic cells. In budding yeast, Cdk1 (Cdc28) expression is constant, but cyclin transcription, stability, and activity are regulated across the cell cycle (Miller and Cross 2001). These multiple levels of regulation result in the ordered appearance of different G1 (Cln)-and B-type (Clb) cyclins, which direct the phase-specific localization and/or substrate specificity of the kinase. There is a critical distinction between G1 phase and the rest of the cell cycle, in that G1 is expandable in response to the environment (Rupe 2002). The length of G1 is influenced by age, growth conditions, and the size of the cell (Hartwell and Unger 1977;Johnston et al. 1979). In contrast, once the cells exit G1, the length of the rest of the cycle is fairly constant (Jagadish and Carter 1977), even after severe nutrient limitation (Johnston et al. 1977). Accumulation of G1 cyclins (Clns) is rate-limiting for the G1 to S transition, and Clns are regulated at virtually every level (Wittenberg et al. 1990;Gallego et al. 1997;Polymenis and Schmidt 1997;MacKay et al. 2001;Newcomb et al. 2002). However, one of the great remaining mysteries is what triggers the rapid accumulation of Clns and causes the irreversible transition into S phase in the normal mitotic cycle.Entry into G1 requires that Clb kinase activity be eliminated (Zachariae and Nasmyth 1999). Clb kinase activity decays due to cessation of CLB transcription, targeted proteolysis of the Clbs by the anaphase-promoting complex (APC), and the M/G1-specific expression of an inhibitory subunit, Sic1, which inactivates Clb/Cdk complexes. Low Clb kinase activity allows the nuclear localization and assembly of Cdc6 and Mcm2-7 onto origin DNA to form the prereplication complexes (PRCs; Tye 1999). These PRC components are transcribed coordinately at the M/G1 boundary, and the assembly of this highly conserved complex sets the stage for DNA replication. Once the PRCs are formed, Clb kinases are required to initiate replication. This is brought about by the accumulation of Cln/Cdk complexes, which phosphorylate and promote the degradation of Sic1 (Schneider et al. 1996;Tyers 1996;Nash et al. 2001) and restore Clb kinase activity.Accumulation of the G1 cyclins requires the activation of Cln3/Cdk. This kinase is uniquely capable of activating two late G1-specific transcription complexes (SBF and MBF;Dirick et al. 1995;Stuart and Wittenberg 1996). Once activated, SBF and MBF cause a burst of transcription of the late G1 cyclins CLN1 and CLN2, and many other genes required for S phase. The burst of CLN1 and CLN2 transcription is delayed under conditions that prolong G1 (Sillje et al. 1997). This indicates that Cln3/Cdk and/or the transcription factors (SBF and MBF) are the likely targets of G1 regulation. Cold Spring Harbor Laboratory Press on May 9, 2018 -Published by genesdev.cshlp.org Downloaded from

Connecting protein structure with predictions of regulatory sites

“…YOX1 and YHP1 are the transcriptional repressors that, together with MCM1, bind early cell cycle boxes found in the promoters of genes expressed during the M/G 1 phase of the cell cycle (43). We used the MAT␣2-MCM1-MAT␣2 complex as a prior for both YOX1 and YHP1 bound sequences and found matches in more than half the genes.…”

Symbiotic adaptations in the fungal cultivar of leaf-cutting ants

“…The contig05937 transcript was weakly homologous to the homeodomain transcriptional repressor Yox1 (syst. YML027w, bit-score ¼ 45.2, alignment length ¼ 245 aa, e-value ¼ 2.6 Â 10 À 4 ), which in yeast restricts transcription during the cell cycle to the early G1 phase and, when overexpressed, decreases the rate of vegetative growth and delays further progression of the cell cycle, while prolonging the S phase (see Supplementary Discussion) 32 . The most similar basidiomycete sequence in the uniprot database (B0DSF2, bitscore ¼ 192, alignment length ¼ 492, e-value ¼ 3.0 Â 10 À 46 ) came from L. bicolor and also contained a homeodomain, which further corroborates the putative regulatory DNA-binding function of contig05937.…”

Budding YeastSSD1-VRegulates Transcript Levels of Many Longevity Genes and Extends Chronological Life Span in Purified Quiescent Cells

“…Total RNA, 30 g, was extracted from both ssd1-d and SSD1-V cells and used for cDNA synthesis. Labeling was performed using the amino-allyl reverse transcription labeling protocol, the labeled cDNAs were hybridized to yeast cDNA arrays (Pramila et al, 2002). Array analysis was performed using the GenePix Prosoftware from Axon Instruments (Foster City, CA).…”

“…The budding percentage was calculated as the number of budded cells divided by the number of total cells at each time point. To monitor the DNA synthesis through the cell cycle, cells stained with Sytox-Green (Invitrogen, Carlsbad, CA) were analyzed on a FACScan cytometer (BD Biosciences, San Jose, CA) as described in Pramila et al (2002). …”