Regulation of the Nucleocytoplasmic Transport of HTLV-1 RNA.

King, Jason Anthony (1997). Regulation of the Nucleocytoplasmic Transport of HTLV-1 RNA. PhD thesis The Open University.



Human retroviruses exert a high level of control over their own gene expression as well as that of their host cell. In addition to the structural genes (gag, pol and env), expressed by all retroviruses, human retroviruses also encode additional regulatory genes. One regulatory gene promotes transcription of the viral genome and modulates the expression of specific cellular genes. The second gene (expressed by HIV and HTLV viruses) regulates viral expression posttranscriptionally by overcoming the effect of cis-acting negative elements present within the incompletely spliced mRNAs.

In the human T cell leukaemia virus type 1 (HTLV-I) retrovirus, the regulatory gene is known as rex. The binding of Rex protein to a sequence at the 3' end of viral mRNA, known as the Rex response element (RxRE) leads to removal of posttranscriptional repression. A sequence within the U5 region of the 5' long terminal repeat (LTR) has previously been described as exerting posttranscriptional repression. This element was termed a cis-repressive sequence (CRS).

This thesis describes the discovery of a novel cis-repressive sequence present within the 3' LTR of HTLV-I, termed the "downstream negative element" or ONE. Its function is investigated and compared to that of the known CRS. Each repressive element is shown to partially block the nucleocytoplasmic transport of LTR transcribed mRNAs. Presence of both elements is seen to completely block transport. Deletion of both elements completely removes Rex dependency. Also described is the discovery of a cis-positive element within the murine leukaemia virus extended packaging sequence (psi+). The MLV psi+ is used to enhance the packaging efficiency of pseudotyped retroviruses. Presence of the MLV psi+ is shown to partially remove Rex dependency from CRS-containing mRNAs. The positive element is shown to be present within the 3' terminal 312 bp of the MLV psi+ and to remove R ex-dependency by activating the nucleocytoplasmic transport of psi 312-containing mRNAs. This element was termed a constitutive transport element or "CTE", after showing the same mechanism of action as a small element from the Mason-Pfizer monkey virus (MPMV).

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