Transcriptional regulation of meiosis in budding yeast

Figure legends
Fig. 1. Developmental choices of MATa/MATα diploid cells of Saccharomyces cerevisiae.
In the presence of carbon and nitrogen sources cells adopt the yeast form morphology; upon
nitrogen limitation and in the presence of high levels of glucose, a dimorphic transition to a
filamentous growth reminiscent of hyphae takes places. In the absence of nitrogen and glucose
and the presence of acetate as the sole carbon source cell enter the meiotic cycle, forming four
haploid spores engulfed in a sac, the ascus.
Fig. 2. A transcriptional cascade governs initiation of meiosis.
The meiotic signals, i.e. the presence of Mata1 and Matα2, the absence of glucose and nitrogen
and the presence of acetate leads to G1 arrest as well as to the expression and activity of Ime1.
IME1 encodes a transcriptional activator required for the transcription of early meiosis-specific
genes (EMG). EMG are involved with premeiotic DNA replication and meiotic recombination.
Ndt80 and Ime2, a transcriptional activator and a protein kinase, whose transcription depends on
Ime1, are required for the transcription of middle meiosis-specific genes (MMG). MMG are
involved with nuclear division and spore formation. The transcription of the late meiosis-specific
genes (LMG) depends on Ime1 and Ime2, and these genes are required for spore maturation. The
nutrient signal has a direct affect also on the activity of Ime2 and the transcription of LMG.
An arrow represents a positive role, a line a negative role. A scissors symbolizes the negative
feedback role of Ime2 in regulating degradation of Ime1.
Fig. 3. Schematic structure of IME1 5’ untranslated region.
The regulated transcription of IME1 is mediated by the combinatorial effect of distinct elements.
The MAT signal mediates repression activity of 2 elements UCS3 and UCS4. The carbon source
signal is transmitted to four elements. UCS1, UASru and IREu function as repression elements in
the presence of glucose. In addition, UASru IREu, as well as UASrm function as activation
elements in the absence of glucose and the presence of acetate as the sole carbon source. UCS1
functions as a negative element in the presence of nitrogen.
Filled boxs - elements required for transcriptional activation. Open boxs – elements whose
function is only to repress transcription. A positive role is marked with an arrow, a negative role
by a line. Larger effects are denoted by heavier lines while lesser effects are denoted by slender
lines.
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