section 25.9

Ribosomes

575

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.

25.9 Ribosomes

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

E. coli

ribosome has

been analyzed in great detail and serves as a model for all

ribosomes.

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,.

..,S21

(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

E. coli

ribosomal proteins are known. A

summary of the structure of the

E. coli

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.