THE EXTENDED PACKAGING SEQUENCE OF MOMLV CONTAINS A CONSTITUTIVE MESSENGER-RNA NUCLEAR EXPORT FUNCTION

The present report shows that incorporation of defined sequences from the Moloney murine leukaemia virus (MoMLV) into Rex dependent expressi on vectors based on the human T-cell leukaemia virus (HTLV-1) allows R ex independent gene expression, Deletion mutagenesis of the MoMLV deri ved sequences allowed this function to be localised to a 312 nt length sequence overlapping the MoMLV gag p15/p12 open reading frame. This ' extended packaging sequence' has been reported to markedly increase th e titre of in vitro packaged retroviral vectors. Using fluorescent in situ hybridisation combined with confocal microscopy we show that the 312 nt element can replace Rex mediated nuclear export and expression of transcripts containing HTLV-1 cis acting repressive elements. Our o bservations are consistent with the extended packaging sequence of MoM LV exerting a constitutive mRNA nuclear export function. (C) 1998 Fede ration of European Biochemical Societies.