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The splicing factor UAP56/HEL/Sub2p is essential for mRNA export [1-4]. It has been proposed [1,2] that UAP56/HEL/Sub2p interacts with the pre-mRNA during splicing and recruits the export factor Aly/REF/Yra1 (reviewed in [5]) to the spliced mRNA. However, UAP56/HEL/Sub2p also participates in the transport of intronless mRNAs, and thus its role in export is not necessarily coupled to splicing [2-4]. Here, we characterize the HEL protein of Chironomus tentans and we analyze in situ the interaction of HEL with a natural export substrate, the Balbiani ring pre-messenger ribonucleoprotein (BR pre-mRNP, reviewed in [6]). Using immunoelectron microscopy, we show that HEL binds to the BR pre-mRNP cotranscriptionally and that incorporation of HEL into the pre-mRNP is independent of the location of introns along the BR pre-mRNA. We also show that HEL accompanies the BR mRNP to the nuclear pore and is …