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chapter 25
RNA and Protein Synthesis
FIGURE 25-15
Polysomes, (a) Electron micrograph of an
E. coli
polysome. (Courtesy of Barbara Hamkalo.) (b) Diagram showing
relative movement of the 70S ribosome and the mRNA and growth of the protein chain.
with some viruses; this molecule is cleaved at several
sites to yield different active proteins and hence is
called a
polyprotein.
Some cellular enzymatic
systems are also formed by cleavage of polyproteins.
Coupled Transcription and Translation
After about 25 amino acids have been joined together in
a polypeptide chain in prokaryotes, the AUG initiation
site of the mRNA molecule becomes exposed and a sec-
ond initiation complex then forms. The overall configu-
ration is of two 70S ribosomes moving along the mRNA
at the same rate. When the second ribosome has moved
a distance similar to that traversed by the first, a third ri-
bosome is able to attach. This process—movement and
reinitiation—continues until the mRNA is covered with
ribosomes separated by about 80 nucleotides. This large
translation unit is called a
polyribosome
or
polysome
(Figure 25-15). Polysomes greatly increase the overall rate
of protein synthesis, since
1 0
ribosomes traversing a sin-
gle segment of mRNA clearly can make 10 times as many
polypeptides per unit of time as can a single ribosome. In
prokaryotes, transcription and translation are coupled; this
can not occur in eukaryotes since transcription occurs in
the nucleus and translation in the cytoplasm.
Endoplasmic Reticulum
In most eukaryotic cells two major classes of ribosomes
exist: attached ribosomes and free ribosomes. The attached
ribosomes are bound to an extensive cytoplasmic network
of lipoprotein membranes called the endoplasmic reticu-
lum. The
rough endoplasmic reticulum
consists of bound
ribosomes; the
smooth endoplasmic reticulum
is devoid
of ribosomes. There is no structural difference between a
free and an attached ribosome, and attachment to the mem-
brane occurs after synthesis of particular proteins begins.
Most endoplasmic reticulum membranes enclose large,
irregularly shaped, discrete regions of the cell called
cis-
ternae.
In this sense, the membrane system has an inside
and an outside, and ribosomes are bound only to the out-
side. Cells responsible for secreting large amounts of a
particular protein (e.g., hormone-secreting cells) have an
extensive endoplasmic reticulum. Most proteins destined
to be secreted by the cell or to be stored in intracellu-
lar vesicles such as lysosomes (which contain degrada-
tive enzymes) and peroxisomes (which contain enzymes
for eliminating hydrogen peroxide) are synthesized by at-
tached ribosomes. These proteins are primarily found in
the cisternae of the endoplasmic reticulum. In contrast,
most proteins destined to be free in the cytoplasm are made
on free ribosomes.
The formation of the rough endoplasmic reticulum and
the secretion of newly synthesized proteins through mem-
brane is explained by the
signal hypothesis
(Figure 25-16).
The basic idea is that the signal for attachment of the
ribosome to the membrane is a sequence of very hydropho-
bic amino acids near the amino terminus of the grow-
ing polypeptide chain. When protein synthesis begins, the
ribosome is free. Then the amino terminal hydrophobic
amino acids interact with lipophilic membrane compo-
nents and somehow direct the association of the large ribo-
somal subunit to a ribosomal receptor protein on the mem-
brane surface. As protein synthesis continues, the protein
moves through the membrane to the cisternal side of the
endoplasmic reticulum. A specific protease termed the
sig-
nal peptidase
cleaves the amino terminal signal sequence.
Intracellular compartmentation of the newly synthe-
sized proteins is a complex process, and disorders in
this process lead to severe abnormalities (see below).
