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chapter 25

RNA and Protein Synthesis

Cham elongation, term ination

seq uence reached

Core enzym e

a

factor

Prom oter

Term ination

DNA

sequence

Initiation

Chain elongation

RNA

RNA

released

n

released

Release of core

Dissociation of

n

enzym e and of RNA

FIGURE 25-6

Transcription cycle of

E. coli

RNA polymerase showing dissociation of the

a

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

a

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

Rho.

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

antitermination.

The

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