dELL is an essential RNA polymerase II elongation factor with a general role in development.

Several eukaryotic proteins increase RNA polymerase II (Pol II) transcription rates in vitro. The relative contributions of these factors to gene expression in vivo is unknown. The ELL family of proteins promote Pol II elongation in vitro, and the Drosophila ELL homolog (dELL) is associated with Pol II at sites of transcription in vivo. The purpose of this study was to test whether an ELL family protein is required for gene expression in vivo. We show that dELL is encoded by the Suppressor of Triplo-lethal locus [Su(Tpl)]. We have characterized seven distinct mutant alleles of Su(Tpl) and show that a dELL transgene rescues recessive lethality of Su(Tpl). Su(Tpl) mutations cause abnormal embryonic segmentation and dominantly modify expression of diverse genes during development. These data show that an ELL family elongation factor is essential, acts broadly in development, and is not functionally redundant to other elongation factors in vivo.