|M.Sc Thesis||Department of Biology|
|Supervisor:||Prof. Kassir Yona|
|Full Thesis text - in Hebrew|
Sporulation of the yeast Saccharomyces cerevisiae is a process of cellular differentiation that begins when MATa/MATα diploid cells are starved for nitrogen, in the absence of a fermentable carbon source. Initiation and progression through the meiotic cycle is regulated by a transcriptional cascade governed by the master regulator: Ime1. The IME1 gene encodes a transcription factor that is required for the expression of meiosis-specific genes. Like many of the genes it regulates, IME1 itself exhibits a transient pattern of expression.
The aim of this research was to identify the repressor responsible for the reduction in IME1 transcription at late meiotic times. The first candidate was Sok2, which represses the transcription of IME1 in the presence of glucose. By comparing IME1-mRNA relative levels in wt and sok2Δ strains, we eliminated Sok2 as a possible candidate. In the absence of Sok2, IME1 still exhibited a transient pattern of expression, though its expression was slightly predated and its relative level was reduced. Those changes in IME1 expression led to slightly reduced levels of EMG (represented by IME2), but did not impair LMG (DIT1) expression or sporulation efficiency. We also revealed a possible negative role of Sok2 in the translational regulation of IME1.
We next examined whether the histone deacetylase: Rpd3 is required for IME1 transient expression. In cells deleted for RPD3, Ime1 accumulated 24 hours after transfer to sporulation medium, whereas in wt cells, Ime1 was hardly traceable at late meiotic times. In the absence of RPD3, the transcription of IME1 was also non-transient. Moreover, Rpd3 exhibited a complex effect on the transcription of IME1: In early meiotic times Rpd3 was a positive regulator whereas in late meiotic times it was converted into a negative regulator. Our results indicate that the effect of Rpd3 on IME1 expression is modulated through the IREu element present in IME1 promoter.
In accord with previous findings, we showed that Rpd3 functions as a negative regulator of IME2 (a representative of EMG) and as a positive regulator of NDT80 (a representative of pre-middle genes), CLB5 (a representative of MMG) and DIT1 (representative of LMG).
We suggest that Ime1 facilitates the dynamic transformation in Sin3/Rpd3 activity. In early meiotic times, Ime1 recruits the HDAC complex Sin3/Rpd3 and relieves Rpd3 repression. Sin3 and/or Rpd3 promotes transcriptional activation and/or enables transcriptional activation by a HAT complex. In late meiotic times, Ime1 is phosphorylated by Ime2 and repression by Rpd3 is re-established.