Drosophila ELL is associated with actively elongating RNA polymerase II ontranscriptionally active sites in vivo

Several factors have been biochemically characterized based on their abilit y to increase the overall rate of transcription elongation catalyzed by the multiprotein complex RNA polymerase II (Pol II). Among these, the ELL fami ly of elongation factors has been shown to increase the catalytic rate of t ranscription elongation in vitro by suppressing transient pausing. Several fundamental biological aspects of this class of elongation factors are not known. We have cloned the Drosophila homolog (dELL) in order to test whethe r ELL family proteins are actually associated with the elongating Pol II in vivo. Here we report that dELL is a nuclear protein, which, like its mamma lian homologs, can increase the catalytic rate of transcription elongation by Pol II in vitro. Interestingly, we find that dELL co-localizes extensive ly with the phosphorylated, actively elongating form of Pol II at transcrip tionally active sites on Drosophila polytene chromosomes. Furthermore, dELL is relocalized from a widespread distribution pattern on polytenes under n ormal conditions to very few transcriptionally active puff sites upon heat shock. This observation indicates a dynamic pattern of localization of dELL in cells, which is a predicted characteristic of a Pol II general elongati on factor. We also demonstrate that dELL physically interacts with Pol II. Our results strongly suggest that dELL functions with elongating RNA polyme rase II in vivo.