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RNA is transported through a membrane as a ribonucleoprotein particle.
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All eukaryotic RNAs that function in the cytoplasm must be exported from the nucleus.
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tRNAs and the RNA component of a ribonuclease are imported into mitochondria.
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mRNAs can travel long distances between plant cells.
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A bacterium consists of only a single
compartment, so all the RNAs function in the same environment in which
they are synthesized. This is most striking in the case of mRNA, where
translation occurs simultaneously with transcription
(see The life cycle of bacterial messenger RNA ).
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Figure 5.28
RNAs are transported through membranes in a variety of systems.
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RNA is transported through membranes in the variety of instances summarized in Figure 5.28.
It poses a significant thermodynamic problem to transport a highly
negative RNA through a hydrophobic membrane, and the solution is to
transport the RNA packaged with proteins.
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In eukaryotic cells, RNAs are transcribed
in the nucleus, but translation occurs in the cytoplasm. Each type of
RNA must be transported into the cytoplasm to assemble the apparatus
for translation. The rRNA assembles with ribosomal proteins into
immature ribosome subunits that are the substrates for the transport
system. tRNA is transported by a specific protein system
(see Transport receptors carry cargo proteins through the pore).
mRNA is transported as a ribonucleoprotein, which forms on the RNA transcript
in the nucleus (see RNA splicing and processing).
These processes are common to all eukaryotic cells. Many mRNAs are
translated in the cytosol, but some are localized within the cell, by
means of attachment to a cytoskeletal element. One situation in which
localization occurs is when it is important for a protein product to be
produced near to the site of its incorporation into some macromolecular
structure (1921).
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Some RNAs are made in the nucleus,
exported to the cytosol, and then imported into mitochondria. The
mitochondria of some organisms do not code for all of the tRNAs that
are required for protein synthesis (see Organelle genomes are circular DNAs that code for organelle proteins).
In these cases, the additional tRNAs must be imported from the cytosol.
The enzyme ribonuclease P, which contains both RNA and protein
subunits, is coded by nuclear genes, but is found in mitochondria as
well as the nucleus (1915). This means that the RNA must be imported into the mitochondria.
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We know of some situations in which mRNA
is even transported between cells. During development of the oocyte in
Drosophila, certain mRNAs are transported into the egg from the nurse
cells that surround it. The nurse cells have specialized junctions with
the oocyte that allow passage of material needed for early development.
This material includes certain mRNAs. Once in the egg, these mRNAs take
up specific locations. Some simply diffuse from the anterior end where
they enter, but others are transported the full length of the egg to
the posterior end by a motor attached to microtubules
(see How are mRNAs and proteins transported and localized?).
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The most striking case of transport of
mRNA has been found in plants. Movement of individual nucleic acids
over long distances was first discovered in plants, where viral
movement proteins help propagate the viral infection by transporting an
RNA virus genome through the plasmodesmata (connections between cells)
(see 1919; 1920). Plants also have a defense system,
that causes cells to silence an infecting virus, and this too may
involve the spread of components including RNA over long distance
between cells (see 1918). Now it has turned out that similar
systems may transport mRNAs between plant cells. Although the existence
of the systems has been known for some time, it is only recently that
their functional importance has been demonstrated (1916).
This was shown by grafting wild-type tomato plants onto plants that had the dominant mutation Me
(which causes a change in the shape of the leaf). mRNA from the mutant
stock was transported into the leaves of the wild-type graft, where it
changed their shape.
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Last Revised on 7-23-2001
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© Jones and Bartlett Publishers (2007)
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