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8.4 Differences in Gene Expression Between Prokaryotes and Eukaryotes 297

essential concepts In Eukaryotes, the Nuclear Membrane
Prevents the Coupling of Transcription
• Translation is the process by which ribosomes synthesize and Translation
proteins according to the instructions in mRNAs.
Ribosomes have specific binding sites for tRNAs (the A, P, In E. coli and other prokaryotes, transcription takes place
and E sites) and supply peptidyl transferase, the ribozyme in an open intracellular space undivided by a nuclear mem-
that forms peptide bonds between amino acids. brane. Translation occurs in the same open space and is
• Transfer RNAs (tRNAs) are the adapters that link mRNA sometimes coupled directly with transcription (Table 8.1).
codons to amino acids at the ribosome. Aminoacyl-tRNA This coupling is possible because transcription extends
synthetases connect the correct amino acids to their mRNAs in the same 5′-to-3′ direction as the ribosome
corresponding tRNAs. moves along the mRNA. As a result, ribosomes can begin
• Each tRNA has an anticodon complementary to the to translate a partial mRNA that the RNA polymerase is
mRNA codon specifying the particular amino acid. still in the process of transcribing from the DNA.
Because of wobble, a tRNA may recognize more than The coupling of transcription and translation has sig-
one codon. nificant consequences for the regulation of gene expression
• Translation initiation begins when a charged tRNA fMet in prokaryotes. For example, in an important regulatory
(or tRNA Met ) binds the start codon, AUG, at the mechanism called attenuation, which we describe in Chap-
ribosomal P site. ter 16, the rate of translation of some mRNAs directly de-
• During elongation, the amino acid connected to the tRNA termines the rate at which the corresponding genes are
at the P site forms a peptide bond with the amino acid transcribed into these mRNAs.
connected to the tRNA at the A site. The ribosome then
moves in the 5′-to-3′ direction along the mRNA to the Such coupling cannot occur in eukaryotes because the
next codon. nuclear envelope physically separates the sites of transcrip-
• Termination occurs when the ribosome encounters an tion and RNA processing in the nucleus from the site of
in-frame stop codon in the mRNA. translation in the cytoplasm. As a result, translation in eu-
• Posttranslational processing enzymes may cleave a karyotes can affect the rate at which genes are transcribed
only in more indirect ways.
polypeptide or add chemical constituents to it.
Distant Enhancer Sequences and
8.4 Differences in Gene Interactions with Chromatin Influence

Expression Between Prokaryotes Eukaryotic Promoters

and Eukaryotes In eukaryotes, the promoters recognized by RNA poly-
merase to initiate transcription are affected by two situa-
tions not seen in prokaryotes (Table 8.1). First, as previously
learning objectives seen in Fig. 8.11, the stability of RNA polymerase’s inter-
action with the promoter is often affected by enhancer se-
1. Explain how the nuclear membrane affects gene quences located far from the promoter. In prokaryotes, the
expression in eukaryotes. DNA sequences that regulate transcription are all found
2. Discuss the function of enhancer sequences in much closer to the promoter. Second, eukaryotic chromo-
eukaryotic transcription. somes are tightly wound around histone proteins in a DNA/
3. Describe the differences in translation initiation protein complex called chromatin. To be recognized by
between prokaryotes and eukaryotes. RNA polymerase, the promoter of a eukaryotic gene must
4. List the steps in mRNA formation that occur in first be unwound from chromatin. Interestingly, clearing
eukaryotes but not in prokaryotes. the histones from the promoter is an important function of
enhancers. (Histones and chromatin and their roles in tran-
scription will be discussed in Chapters 12 and 17.)
The processes of transcription and translation in eukaryotes
and prokaryotes are similar in many ways but also are af- Prokaryotes and Eukaryotes Initiate
fected by certain differences, including (1) the presence of a Translation Differently
nuclear membrane in eukaryotes, (2) eukaryotic-specific
complexities in the mechanisms by which RNA polymerase In prokaryotes, translation begins at a ribosome binding
recognizes promoters to start transcription, (3) variations in site on the mRNA, which is defined by a short, characteris-
the way in which translation is initiated, and (4) the need for tic sequence of nucleotides called a Shine-Dalgarno box
additional transcript processing in eukaryotes. adjacent to an initiating AUG codon (review Fig. 8.25a).