J Mol Biol 181: 517-31 (1985)
Abstract
The structure of single-stranded RNA from the bacteriophage MS2 has
been examined by electron microscopy in the presence of the polyamine
spermidine. The molecules are found in two alternate conformations. The
first of these can be characterized as a cruciform structure composed of
three large loops approximately 500 to 700 nucleotides in size. The
interior of the molecule has extensive base-paired regions which connect
distant regions of the molecule; the farthest being 2500 nucleotides
apart. In the second conformation, the molecules appear rod-like. Two of
the large loops disappear, and these regions form, instead, extensive
long-range helices. Computer modeling has been employed to explore the
base-pairing potential of the sequence of bacteriophage MS2 RNA.
Double-stranded regions identified by electron microscopy are shown to
occur in local G + C-rich stretches of the RNA. Detailed models have
been calculated for two regions of long-range contact. One of these
includes the ribosome-binding site for the viral coat protein gene. The
results are discussed in the context of the known role of RNA structure
in the regulation of viral gene expression.
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