A novel role of transcriptional repressors on the targets of the Jak/Stat pathway, in larval hematopoiesis. Aditi Vyas, Soichi Tanda. Dept. of Biological Sciences, Ohio University, Athens, OH.

   The Janus Kinase and Signal Transducer and Activator of Transcription (Jak/Stat) pathway is known to control larval hematopoiesis in Drosophila. A gain of function allele of the Drosophila Jak gene (hop^Tum-l) causes hyperactivation of the pathway, leading to a tenfold increase in the hemocyte count and increased differentiation to a specific hemocyte type called lamellocytes. Decreasing the negative regulator, phosphatase 61F in the hop^Tum-l background, causes a further increase in the pathway activity. The total cell count of these larvae is reduced by half and there is a four fold greater differentiation to lamellocytes, when compared to hop^Tum-l larvae. We hypothesize that there are two sets of Stat92E target genes; low threshold genes that are turned on at moderate levels of pathway (i.e. hop^Tum-l levels) and high threshold genes that are transcribed at higher levels of the pathway. This phenomenon could result from either of two possible mechanisms. One mechanism predicts that the Stat92E binding sites regulating the transcription of the two sets of genes have different binding affinities for Stat92E. The other mechanism assumes a greater role of transcriptional repressors in regulating the transcription of high threshold genes. To test if transcriptional repressors have a role in regulating Stat92E target genes, we screened loss of function mutants of co-repressors. We found that when larvae carried loss of function alleles of the co-repressor, C-terminal Binding Protein (CtBP) in the hop^Tum-l background, the hemocyte counts were similar to when we increased the pathway activity levels. We then screened the DNA-binding partners of CtBP, and identified Suppressor of Hairless [Su(H)] as a repressor regulating the transcription of the high threshold genes. In another approach, we used a modENCODE dataset for CtBP binding regions and scanned it for the presence of Stat92E and repressor binding sites, using the bioinformatics tool Target Explorer. We generated a list of 32 genes, that lie within 5kb of these identified regions and are now testing them for roles in hematopoiesis.