Plants respond to environmental stress by altering their gene expression. Under stress conditions some genes are activated and some genes are repressed. Even though a lot of work has been done to understand mechanisms of gene activation under abiotic stress very little information is available on how stress responsive genes are kept repressed under normal growth conditions. Recent work has revealed that plants use transcriptional repression as common mechanism of gene repression. Transcriptional repression is achieved by recruitment co-repressor complexes to the target genes. Recent studies have revealed that the co-repressor LUH complexes with SLK1 and SLK2 to silence Arabidopsis thaliana stress responsive genes. However, the transcription factors involved in the recruitment of this complex to its target genes are not known. In this study, we identified SLK2INT1, as a novel transcription factor that is involved in silencing of select Arabidopsis thaliana stress responsive genes by recruiting the LUH-SLK2 complex.

Dosage compensation is a mechanism that equalizes the expression of X-linked genes in those organisms in which males and females differ in the number of X chromosomes. In Drosophila melanogaster, dosage compensation is achieved by up-regulating the transcription of the single male X chromosome. This effect is mediated by a chromatin remodeling complex known as the Male Specific Lethal (MSL) complex or Dosage Compensation Complex (DCC). In female flies, dosage compensation is inhibited primarily because of the translational repression of the mRNA encoding one of the DCC subunits, MSL-2, by the female-specific RNA binding protein Sex-lethal (SXL). To inhibit translation, SXL binds to poly(U) stretches present in both the 5’ and 3’ UTRs of msl-2 mRNA. Sequences adjacent to those SXL-binding sites in the 3´UTR are also required for translation inhibition and are bound by co-repression. In this thesis work, we have designed an affinity chromatography assay to isolate the putative co-repressor(s), and have identified the protein Upstream of N-ras (UNR). Drosophila UNR (dUNR) is an ubiquitous, conserved protein that contains 5 cold shock domains (CSD) and a glutamine- (Q) rich amino- terminal extension. We show that dUNR is a necessary co-factor for SXL-mediated msl-2 repression. SXL recruits dUNR to the 3’ UTR of msl-2 mRNA, imparting a sex-specific function to this ubiquitous protein. Domain mapping experiments indicate that dUNR interacts with SXL and msl-2 mRNA through CSD1, and that the domains for translation inhibition and SXL interaction can be distinguished. Our data indicate that the Q-rich domain, together with CSDs 1 and 2, plays an important role in translational repression, and suggest that factors in addition to dUNR and SXL are required for repression of msl-2 mRNA. Using a combination of UNR immunoprecipitation and microarray analysis, we have identified the mRNAs that are bound to dUNR in male and female flies. Our results suggest that dUNR is not only a novel regulator of dosage compensation, but also a general post-transcriptional regulator of gene expression.