series of start and stop codons for each polypeptide. The
mechanism for initiating synthesis of the first protein
molecule in a polycistronic mRNA is the same as that
in a monocistronic mRNA. However, if a second protein
in a polycistronic mRNA is to be made, protein synthesis
must reinitiate after termination of the first protein. This
is usually accomplished by the start codon of the second
protein being so near the preceding stop codon that reinitia-
tion occurs before the ribosome and the mRNA dissociate.
Otherwise, a second initiation signal is needed.
A ribosome is a multicomponent structure that serves
to bring together a single mRNA molecule and charged
tRNA molecules so that the base sequence of the mRNA
molecule is translated into an amino acid sequence. The ri-
bosome also contains several enzymatic activities needed
for protein synthesis. The protein composition of the
prokaryotic ribosome is well known, but less is known
about the composition of eukaryotic ribosomes. However,
both share the same general structure and organization.
The proper ties of the bacterial ribosomes are constant
over a wide range of species, and the
been analyzed in great detail and serves as a model for all
Chemical Composition of Prokaryotic Ribosomes
A prokaryotic ribosome consists of two subunits. The in-
tact particle is called a 70S ribosome because of its S
(sedimentation) value of 70 Svedberg units. The subunits,
which are unequal in size and composition, are termed 30S
and 50S subunits. The 70S ribosome is the form active in
protein synthesis. At low concentrations of Mg2+, ribo-
somes dissociate into ribosomal subunits. At even lower
concentrations, the subunits dissociate, releasing indi
vidual RNA and protein molecules.
The composition of each subunit is as follows:
30S subunit : one 16S rRNA molecule + 21 different proteins
50S subunit : one 5S rRNA molecule + one 23S rRNA molecule
+ 34 different proteins
The proteins of the 30S subunit are termed S1,S2,.
(in this context, S is for “small” subunit); one copy of each
is contained in the 30S subunit. The proteins of the 50S
subunit are denoted by L (for “large” subunit). Each 50S
subunit contains one copy of each protein molecule, except
for proteins L7 and L I2, of which there are four copies,
and L26, which is not considered to be a true component
of the 50S subunit. Most ribosomal proteins are very basic
proteins, containing up to 34% basic amino acids. This ba-
sicity probably accounts in part for their strong association
with the RNA, which is acidic. The amino acid sequences
of most of the
ribosomal proteins are known. A
summary of the structure of the
70S ribosome is
shown in Figure 25-10.
Ribosomes Are Ribozymes
Ever since the discovery of the role of ribosomes in protein
synthesis there has been uncertainty as to the functional
roles of the rRNAs and the many ribosomal proteins.
Conventional wisdom held that the rRNAs provided the
scafolding (structural role) that oriented the ribosomal pro-
teins so that one or more could carry out the enzymatic
23S R N A
5S R N A
F I G U R E 2 5 - 1 0
Dissociation of a prokaryotic ribosome. The configuration of two overlapping circles is used throughout this chapter, for
the sake of simplicity.