Role of polyadenylation in nucleocytoplasmic transport of mRNA

To examine the role of polyadenylation in the nuclear export of mRNA, we have replaced the poly(A) signal in a Rev-responsive human immunodeficiency virus type 1-based reporter gene with a cis-acting hammerhead ribozyme. Transcripts from this gene thus acquire a 3' terminus by cis-ribozyme cleavage rather than by polyadenylation. The nuclear and cytoplasmic distribution of transcripts was investigated using transient gene expression and quantitative RNase protection assays. In the absence of Rev, a basal level of polyadenylated unspliced mRNA transcribed from a poly(A) signal-containing control reporter gene was detected in the cytoplasm of transfected COS7 cells. However, cytoplasmic ribozyme-cleaved unspliced RNA was only barely detectable. The nuclear/cytoplasmic (n/c) ratio of polyadenylated RNAs was 3.8, while the n/c ratio for ribozyme cis-cleaved RNAs was 33. The cytoplasmic localization of the polyadenylated unspliced mRNA was enhanced about 10-fold in the presence of Rev and the Rev-responsive element. In marked contrast to this, ribozyme cleaved RNA accumulated almost exclusively (n/c ratio of 28) in the nucleus in the presence of Rev. Actinomycin D time course analysis suggested that the low levels of the cytoplasmic ribozyme-cleaved RNAs in both the presence and absence of Rev were due to serve export deficiency of ribozyme-cleaved RNA. Finally, by inserting a 90-nucleotide poly(A) stretch directly upstream of the ribozyme cassette, we have demonstrated that a long stretch of poly(A) near the 3' end of a ribozyme-cleaved transcript is not sufficient for directing mRNA export. Taken together, these results suggest that polyadenylation is required for the nucleocytoplasmic transport of mRNA and that Rev interaction with the Rev-responsive element cannot bypass this requirement.