Transcriptional studies of SV40 infected CV1 cell indicate that nuclear transcripts may appear in the cytoplasmic fraction depending on the method used to fractionate nuclei from cytoplasm. It therefore appears that late in SV40 infection unspliced 19S RNA is a nuclear species that may leak into the cytoplasm during fractionation. Deletion mutants of the late 19S intron-leader junction were analyzed for transcription. Deletions which have lost the 19S intron express unspliced 19S RNA in the cytoplasm. One such deletion which has lost the 19S leader-intron junction as well as the 19S intron-body junction was found to be viable. It appears that the presence of the 19S intron may prevent transport of nuclear transcripts and that the loss of this sequence either by splicing or deletion allows cytoplasmic transport.
