PgmNr Y3119: The role of the Mediator complex in Ty1 retrotransposition in S. cerevisiae.

Authors:
Alicia Salinero 1 ; M. Joan Curcio 1,2 ; Randall Morse 1,2


Institutes
1) University at Albany School of Public Health, Albany, NY; 2) Wadsworth Center, NYSDOH, Albany, NY.


Keyword: Transcription

Abstract:

Retrotransposons are mobile genetic elements that replicate via an RNA intermediary. These elements have been known to drive evolutionary rearrangements and contribute to genomic instability. In Saccharomyces cerevisiae, the most abundant and active retroelement is Ty1. This 6 kb element encodes Gag, protease, integrase, and reverse transcriptase, and relies on numerous additional host factors to successfully complete all stages of its mobility cycle. One such set of host factors is the 25 subunit Mediator transcriptional co-activator complex. The Mediator complex is organized into the core head, middle, and tail domains, with a transiently associated kinase domain. The tail domain functions mainly as a target of DNA binding transcriptional activator proteins, while the head and middle recruit the RNA polymerase II machinery to the transcription start site. With the exception of the kinase domain, deletion of non-essential subunits from all Mediator domains has a drastic impact on Ty1 mobility.  Disruption of the Mediator tail module in med2Δ, med3Δ, and med15Δ yeast decreases Ty1 activity to undetectable levels, while disruption of the head module (med 18Δ and med20Δ), and middle module (med1Δ, med5Δ, and med31Δ) increase Ty1 mobility substantially. However, these changes in overall Ty1 mobility do not correspond with significant differences in Ty1 transcription or Gag translation. It is only at the level of viral like particle assembly and reverse transcription of Ty1 cDNA that Mediator subunit deletions exhibit any observable alteration to Ty1 mobility. While Mediator affects Ty1 mobility post-translationally, the process depends on the presence of the Ty1 promoter, as determined by measuring the activity of Ty1 driven by a TEF1 promoter. Recent work from the Garfinkel lab has identified a truncated form of the Gag protein (p22) that disrupts VLP formation and prevents reverse transcription of Ty1 cDNA.  Translation of p22 relies on increased levels of an internal transcript known as Ty1i. Mediator tail mutant strains show an increase in this Ty1i RNA. Thus, Mediator plays a substantial role in Ty1 mobility by controlling levels of Ty1i mRNA.  Further research is underway to determine the mechanism by which this occurs.  

This work was supported by NSF grants MCB0949722 and MCB1516839 to RHM and NIH grant GM52072 to MJC.



Yeast Database Genetic Index
1. gene symbol: MED2; systematic name: YDL005C
2. gene symbol: PGD1; systematic name: YGL025C
3. gene symbol: GAL11; systematic name: YOL051W
4. gene symbol: SRB5; systematic name: YGR104C
5. gene symbol: SRB2; systematic name: YHR041C
6. gene symbol: MED1; systematic name: YPR070W
7. gene symbol: NUT1; systematic name: YGL151W
8. gene symbol: SOH1; systematic name: YGL127C
9. gene symbol: TY1