Abstract
We have examined the localization and targeting of the RNA polymerase II (pol II) transcription elongation factor TFIIS in amphibian oocyte nuclei by immunofluorescence. Using a novel antibody against Xenopus TFIIS the major sites of immunostaining were found to be Cajal bodies, nuclear organelles that also contain pol II. Small granular structures attached to lampbrush chromosomes were also specifically stained but the transcriptionally active loops were not. Similar localization patterns were found for the newly synthesized myc-tagged TFIIS produced after injection of synthetic transcripts into the cytoplasm. The basis of the rapid and preferential targeting of TFIIS to Cajal bodies was investigated by examining the effects of deletion and site-specific mutations. Multiple regions of TFIIS contributed to efficient targeting including the domain required for its binding to pol II. The localization of TFIIS in Cajal bodies, and in particular the apparent involvement of pol II binding in achieving it, offer further support for a model in which Cajal bodies function in the preassembly of the transcriptional machinery. Although our findings are therefore consistent with TFIIS playing a role in early events of the transcription cycle, they also suggest that this elongation factor is not generally required during transcription in oocytes.