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J. Virol., 02 1996, 944-951, Vol 70, No. 2
G Miele, A Mouland, GP Harrison, E Cohen and AM Lever
The role of the RNA secondary structure in the 5' packaging signal region
of human immunodeficiency virus type 1 (HIV-1) in initiating translation of
gag mRNA has been investigated both in vitro and in the presence of
cellular cofactors in vivo. Heat denaturation of the structure and
mutagenic deletion both lead to an increase in levels of translated
products, indicating that the structure is a significant inhibitor of
translation. The proximity of the gag AUG to the packaging signal structure
suggested that it might function as an internal ribosome entry site.
However, in both a cell-free system and eukaryotic cells, translation will
initiate at a novel upstream initiation codon introduced within the 5'
noncoding region. This codon is utilized exclusively, resulting in gag
protein products with an extra 11 amino acids at the amino terminus, which,
when expressed in T lymphocytes, are confined intracellularly, probably
because of the lack of an N- terminal glycine myristoylation signal.
Deletion of the secondary structure abolishes gag production even in the
presence of tat and rev in trans. Using dicistronic constructs containing
the HIV-1 5' leader cloned between two heterologous open reading frames, we
were unable to detect any significant expression of the second open reading
frame that would have been supportive of an internal ribosome entry site
mechanism. Using mutant proviruses either lacking the entire packaging
signal structure region or containing the introduced upstream initiation
codon in long-term replication studies, we were unable to detect reverse
transcriptase activity in culture supernatants. The 5' packaging signal
structure of HIV-1 does not serve as an internal ribosome entry site. The
translation of gag is consistent with ribosomal scanning. However, the
packaging signal structure causes significant translational inhibition.
Copyright © 1996, American Society for Microbiology
The human immunodeficiency virus type 1 5' packaging signal structure affects translation but does not function as an internal ribosome entry site structure
University of Cambridge Department of Medicine, Addenbrooke's Hospital, United Kingdom.
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