Precise and complex regulation is required for entering a developmental pathway at the correct time and in the appropriate cell type. Deviations from this regulation may lead to genome instability, causing either cell death or the formation of tumor cells 
. Inducing the correct set of genes in a coordinated manner is a key for developmental pathway regulation and is often achieved through a transcriptional regulatory cascade 
. The master activator initiating the cascade is usually controlled by multiple input signals, each with a small impact. It is the combinational nature of the induction of the master activator that ensures the correct spatial and temporal activity of the developmental pathway 
Transmission of a strong and short-lived signal by the master activator is assumed to be critical for the successful completion of a developmental pathway 
. Studies in mice 
and yeast meiosis 
have demonstrated the importance of a short-lived signal for efficient entry into a developmental pathway. However, whether a strong
signal is indeed essential for efficient entry into and completion of a developmental pathway and, if not, how cells cope with premature, delayed, reduced, or increased signals, remains a fundamental, unsolved question.
Our model system is meiosis in Saccharomyces cerevisiae
, in which a transcriptional cascade governs the initiation and progression through meiosis ( and 
). This cascade consists of a master activator, Ime1, which is essential for the transcription of the early meiosis-specific genes. The transcription of the middle genes depends on Ime2, a kinase that belongs to the early genes, and Ndt80, a transcriptional activator that belongs to the early middle genes. The transcription of the late genes is indirectly dependent on Ime1, Ime2 and Ndt80 
. The early genes encode proteins involved in DNA replication, synapsis of homologs and meiotic recombination, whereas the middle genes encode proteins required for nuclear divisions and spore formation 
A schematic model illustrating the transcriptional cascade that governs meiosis in S. cerevisiae.
We envision two modes by which a transcriptional cascade is regulated. (i
) Transcription of a network of genes (B) responds to a gradient (graded) effect produced by a master activator (A), ie, the quantity of A directly affects the quantity of B, and (ii
) transcription of the B genes requires a threshold level/activity of A. The first hypothesis is supported by a discrete computational model that faithfully describes the relations between a master transcriptional activator, (IME1
) in budding yeast, and the transcription of IME2
, an early meiosis-specific gene representing the B genes 
. The second hypothesis is supported by two observations. First, a study of the meiosis transcriptional cascade in S. cerevisiae
using time-lapse fluorescence microscopy of single cells demonstrated high variability in the duration of the first stage in the cascade (corresponding to the induction time of the early meiosis-specific genes) and in the level of Ime1 
. These results suggest that the induction of meiosis depends on a threshold level of Ime1. Second, pattern formation in Drosophila depends on a “buffered protein” that causes a gradient of a morphogen to regulate transcription in a threshold mode 
In this report we demonstrate that meiosis is not sensitive (robust) to increased or decreased transcript abundance of IME1, or to the level of expression of meiosis-specific genes. Transcription of the early meiosis-specific genes responds in a graded mode to the Ime1 levels. However, entry into premeiotic DNA replication is regulated in a threshold mode, namely, a decrease in the level of Ime1 and the early genes leads to a delayed entry into premeiotic S phase. The transcription of NDT80, the middle and late meiosis-specific genes are regulated in a threshold manner. We show that the time of transcription of NDT80 and the middle genes determines the time cells enter nuclear divisions, as premature transcription of NDT80 resulted in premature nuclear division and a reduction in asci formation. Correlation between premeiotic DNA replication and nuclear division is regulated by an early meiosis-specific gene whose activity is regulated in a threshold mode. We suggest that Ime2, which controls both initiation of premeiotic DNA replication and the transcription of NDT80, serves as the regulator that switches the graded mode of transcription of the early genes to a threshold mode for regulating entry into DNA replication and the transcription of the middle genes. We demonstrate that Cdk functions as a negative regulator of Ime2, by showing that point mutations of three serine/threonine residues in putative Cdk1 phosphorylation sites of Ime2 result in premature entry into premeiotic DNA replication. Accordingly, ectopic expression of the G1 cyclin CLN3 has the opposing effect, namely delaying entry into DNA replication and NDT80 transcription. Because Ime2 functions as a negative regulator of Cdk1/G1 cyclins, we suggest that a double negative feedback loop between Ime2 and Cdk1/Cln3 is responsible for the switch from a graded to a threshold mode of regulation. This switch provides the required coordination between DNA replication and nuclear division, ensuring robust meiosis.