chapter 25
RNA and Protein Synthesis
Cham elongation, term ination
seq uence reached
Core enzym e
Prom oter
Term ination
Chain elongation
Release of core
Dissociation of
enzym e and of RNA
Transcription cycle of
E. coli
RNA polymerase showing dissociation of the
subunit shortly after chain elongation
begins, dissociation of the core enzyme during termination, and re-formation of the holoenzyme from the core enzyme
and the
subunit. A previously joined core enzyme and a subunit will rarely become rejoined; instead, reassociation
occurs at random.
1. Cessation of RNA elongation,
2. Release of newly formed RNA, and
3. Release of the RNA polymerase from the DNA.
There are two kinds of termination events in prokary-
otes: those that are self-terminating (dependent on the
DNA base sequence only) and those that require the pres-
ence of a termination protein called
Both types of
events occur at specific but distinct base sequences. Rho,
an oligomeric protein, does not bind to the core poly-
merase or to the holoenzyme and binds only very weakly
to DNA. The action of Rho is poorly understood. Some
microorganisms regulate transcription of certain genes by
inhibiting Rho thereby allowing transcription to continue
into adjacent genes, a process called
transcription cycle of
E. coli
RNA polymerase is shown
in Figure 25-6.
Various drugs inhibit chain elongation. Cordycepin is
converted to a 5'-triphosphate form and then acts as a sub-
strate analogue, blocking chain elongation.
Lifetime of Prokaryotic mRNA
All mRNA molecules are subject to attack by RNases, and
this degradation is an essential aspect of the regulation of
gene expression. Proteins are not made when they are not
needed, and the rate of protein synthesis is determined by
a balance between the rates of RNA synthesis and RNA
degradation. The half-life of a typical prokaryotic mRNA
molecule is only a few minutes, so constant production
of a bacterial protein requires continued transcription. In
contrast, eukaryotic mRNA molecules have a lifetime of
hours to days. Presumably, the reason for the difference
is that bacteria must adapt to rapidly changing environ-
ments, whereas eukaryotic cells receive a constant supply
of nutrients that maintain a uniform environment.
25.5 Transcription in Eukaryotes
The chemistry of transcription in eukaryotes is the same
as in prokaryotes. However, the promoter structure and the
mechanism for initiation are strikingly different.
Eukaryotic RNA Polymerases
Eukaryotic cells contain three classes of RNA poly-
merases, denoted I, II, and III, which are distinguished
by their requirements for particular ions and by their sen-
sitivity to various toxins. All are found in the nucleus.
Minor RNA polymerases are found in mitochondria and
chloroplasts. Polymerase I molecules are located in the
nucleolus and are responsible for synthesis of 5.8S, 18S,
and 28S rRNA molecules. Polymerase II synthesizes all
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