Nascent pre-mRNA transcripts are associated with nuclear regions enriched in splicing factors.

We have used in situ hybridization and immunocytochemistry to compare the nuclear localization of a specific nascent pre-mRNA and the essential non-snRNP splicing factor SC-35. Nascent c-fos transcripts were detected in serum-induced mouse fibroblasts by in situ hybridization with genomic c-fos probes. Prior to serum induction no c-fos RNA is detected, but these transcripts localize to two dots in the interphase nucleus after induction. The time course of appearance of the dots correlates with the previously determined time course of transcriptional activation of the gene. Upon further analysis by confocal laser scanning microscopy, we have determined that the dots extend through the depth of the nucleus, forming paths. By using high-voltage electron microscopy, we have found that the c-fos path extends out and comes into direct contact with the nuclear envelope. We have also compared the localization of c-fos transcripts with the speckled nuclear regions that are enriched in snRNPs and the non-snRNP splicing factor SC-35. Direct observations of three-dimensional rotations have revealed a close association between the c-fos transcripts and the nuclear speckles. This study demonstrates a direct link between specific nascent RNA transcripts and nuclear speckles that are enriched in pre-mRNA splicing factors.