Biology Reference
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Fig. 1. Packaging signal sequences of MS2 phage (A), AMV (B), MMLV
(C) and HIV-1 (D).
structures of 20-30 nucleotides to hundreds of nucleotides distrib-
uted over several sites within the genome (Fig. 1). In addition to this
direct recognition of RNA and protein viruses that contain a multiple-
part genome also employ RNA-RNA interactions in order to facilitate
the accurate assembly of the new particles. An example of this can be
found in retro-viruses such as human immunodeficiency virus (HIV),
which package their genome as a dimer.
Specific packaging signals are a requirement for the packaging of
a particular genome; however, their simple existence does not always
guarantee packaging. One limiting factor is the total size of the RNA
containing the packaging signal when compared to the space available
inside the mature particle. The highest accuracy and efficiency of
packaging can only be achieved with a genomic RNA that “fits” into
the protein shell. This sets a natural size limit for the use of viral vec-
tors as a means of introducing foreign RNA into cells. Another factor
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