Nucleocytoplasmic transport of mRNA

Date of Completion

January 1996


Biology, Molecular|Biology, Cell




The movement of mRNA from the nucleus to the cytoplasm is one of the crucial checkpoints in the regulation of gene expression. However, the molecular mechanism of RNA transport is largely unknown. The aim of this thesis is to investigate some of the rules that govern this process.^ The first part of the thesis is focused on how the process of splicing affects mRNA export. The specific questions asked were whether the process of splicing promotes the export of mRNA from the nucleus, or whether removal of splice sites is actually required. The mouse polyoma virus was used as a model system to approach these issues. Results led to the proposal that the process of splicing in the polyoma system is not necessary for mRNA export, nor does it make export more efficient. Instead, removal of splicing machinery from mRNA may be required for export.^ The second part of the thesis was to learn how naturally intronless gene transcripts are transported from the nucleus to the cytoplasm and to determine whether these transcripts contain positive cis-acting transport elements. By using a variety of approaches, a novel cis-acting transport element, called HTE, contained within the mouse histone H2a gene coding region was identified. This element facilitates the cytoplasmic accumulation of intronless gene transcripts and of unspliced HIV-1-related mRNAs. HTE is the first example of cellular element that has such functions.^ The last part of the thesis was to determine the role of polyadenylation in mRNA nuclear export. By exploiting a cis-acting ribozyme that uncouples 3$\sp\prime$ end formation from the cellular polyadenylation machinery, evidence that polyadenylation is a prerequisite for mRNA nuclear export and that a long stretch of poly(A) near the 3$\sp\prime$ end of an mRNA is not sufficient for mRNA export was obtained. ^