Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 51
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Note that NtrB and NtrC, discussed above, function as sensor and response regulator proteins, respectively,in the two-component regulatory system that controls transcription of glnA. Similar two-component histidyl-aspartylphosphorelay regulatory systems are also found in plants.KEY CONCEPTS OF SECTION 4.3Control of Gene Expression in ProkaryotesGene expression in both prokaryotes and eukaryotes isregulated primarily by mechanisms that control the initiation of transcription.■Binding of the subunit in an RNA polymerase to apromoter region is the first step in the initiation of transcription in E. coli.■The nucleotide sequence of a promoter determines itsstrength, that is, how frequently different RNA polymerasemolecules can bind and initiate transcription per minute.■Repressors are proteins that bind to operator sequences,which overlap or lie adjacent to promoters.
Binding of arepressor to an operator inhibits transcription initiation.■The DNA-binding activity of most bacterial repressorsis modulated by small effector molecules (inducers). Thisallows bacterial cells to regulate transcription of specificgenes in response to changes in the concentration of various nutrients in the environment.■The lac operon and some other bacterial genes also areregulated by activator proteins that bind next to promoters and increase the rate of transcription initiation by RNApolymerase.■The major sigma factor in E. coli is 70, but several otherless abundant sigma factors are also found, each recognizing different consensus promoter sequences.■Transcription initiation by all E.
coli RNA polymerases,except those containing 54, can be regulated by repressors and activators that bind near the transcription startsite (see Figure 4-16).■Genes transcribed by 54–RNA polymerase are regulatedby activators that bind to enhancers located ≈100 base■1194.4 • The Three Roles of RNA in Translationpairs upstream from the start site. When the activator and54–RNA polymerase interact, the DNA between theirbinding sites forms a loop (see Figure 4-17).aa 1 aa 2aa 3Growingaa 4polypeptidechainIn two-component regulatory systems, one protein actsas a sensor, monitoring the level of nutrients or other components in the environment. Under appropriate conditions,the -phosphate of an ATP is transferred first to a histidine in the sensor protein and then to an aspartic acid ina second protein, the response regulator.
The phosphorylated response regulator then binds to DNA regulatory sequences, thereby stimulating or repressing transcription ofspecific genes (see Figure 4-18).■H CRibosomeAlthough DNA stores the information for protein synthesisand mRNA conveys the instructions encoded in DNA, mostbiological activities are carried out by proteins. As we sawin Chapter 3, the linear order of amino acids in each proteindetermines its three-dimensional structure and activity. Forthis reason, assembly of amino acids in their correct order,as encoded in DNA, is critical to production of functionalproteins and hence the proper functioning of cells andorganisms.Translation is the whole process by which the nucleotidesequence of an mRNA is used to order and to join the aminoacids in a polypeptide chain (see Figure 4-1, step 3).
In eukaryotic cells, protein synthesis occurs in the cytoplasm,where three types of RNA molecules come together to perform different but cooperative functions (Figure 4-19):1. Messenger RNA (mRNA) carries the genetic informationtranscribed from DNA in the form of a series of threenucleotide sequences, called codons, each of which specifiesa particular amino acid.2. Transfer RNA (tRNA) is the key to deciphering thecodons in mRNA.
Each type of amino acid has its ownsubset of tRNAs, which bind the amino acid and carry itto the growing end of a polypeptide chain if the next codonin the mRNA calls for it. The correct tRNA with its attachedamino acid is selected at each step because each specifictRNA molecule contains a three-nucleotide sequence, ananticodon, that can base-pair with its complementary codonin the mRNA.3. Ribosomal RNA (rRNA) associates with a set ofproteins to form ribosomes. These complex structures,which physically move along an mRNA molecule, catalyzethe assembly of amino acids into polypeptide chains. Theyalso bind tRNAs and various accessory proteins necessaryfor protein synthesis.
Ribosomes are composed of a largeand a small subunit, each of which contains its own rRNAmolecule or molecules.H HN C R6mRNACCOC O H C OOOtRNA4leaving54.4 The Three Roles of RNAin TranslationR5aa7-tRNA7arriving HH2N CR7C OC AGCU U U A G CG G G A A A U C G G U C3Codon Codon Codon Codon Codon Codon Codonaa1aa2aa3aa4aa5aa6aa7Movement of ribosome▲ FIGURE 4-19 The three roles of RNA in proteinsynthesis. Messenger RNA (mRNA) is translated into proteinby the joint action of transfer RNA (tRNA) and the ribosome,which is composed of numerous proteins and two majorribosomal RNA (rRNA) molecules (not shown).
Note the basepairing between tRNA anticodons and complementary codonsin the mRNA. Formation of a peptide bond between the aminogroup N on the incoming aa-tRNA and the carboxyl-terminal Con the growing protein chain (purple) is catalyzed by one of therRNAs. aa amino acid; R side group. [Adapted fromA. J. F. Griffiths et al., 1999, Modern Genetic Analysis, W. H. Freemanand Company.]These three types of RNA participate in translation in allcells.
Indeed, development of three functionally distinctRNAs was probably the molecular key to the origin of life.How the structure of each RNA relates to its specific task isdescribed in this section; how the three types work together,along with required protein factors, to synthesize proteins isdetailed in the following section. Since translation is essentialfor protein synthesis, the two processes commonly are referred to interchangeably. However, the polypeptide chainsresulting from translation undergo post-translational foldingand often other changes (e.g., chemical modifications, association with other chains) that are required for production ofmature, functional proteins (Chapter 3).Messenger RNA Carries Information from DNAin a Three-Letter Genetic CodeAs noted above, the genetic code used by cells is a tripletcode, with every three-nucleotide sequence, or codon, being“read” from a specified starting point in the mRNA.
Of the64 possible codons in the genetic code, 61 specify individualamino acids and three are stop codons. Table 4-1 shows thatmost amino acids are encoded by more than one codon.Only two—methionine and tryptophan—have a single120CHAPTER 4 • Basic Molecular Genetic MechanismsTABLE 4-1 The Genetic Code (RNA to Amino Acids)*FirstPosition(5 end)ThirdPosition(3 end)Second PositionUCAGPhePheSerSerTyrTyrCysCysUCLeuLeuSerSerStopStopStopTrpAGLeuLeuProProHisHisArgArgUCLeuLeu (Met)*ProProGlnGlnArgArgAGIleIleThrThrAsnAsnSerSerUCIleMet (start)ThrThrLysLysArgArgAGValValAlaAlaAspAspGlyGlyUCValVal (Met)*AlaAlaGluGluGlyGlyAGUCAG*AUG is the most common initiator codon; GUG usually codes for valine, and CUG for leucine, but, rarely,these codons can also code for methionine to initiate a protein chain.codon; at the other extreme, leucine, serine, and arginine areeach specified by six different codons.
The different codonsfor a given amino acid are said to be synonymous. The codeitself is termed degenerate, meaning that more than onecodon can specify the same amino acid.Synthesis of all polypeptide chains in prokaryotic and eukaryotic cells begins with the amino acid methionine. In mostmRNAs, the start (initiator) codon specifying this aminoterminal methionine is AUG. In a few bacterial mRNAs,GUG is used as the initiator codon, and CUG occasionallyis used as an initiator codon for methionine in eukaryotes.The three codons UAA, UGA, and UAG do not specifyamino acids but constitute stop (termination) codons thatmark the carboxyl terminus of polypeptide chains in almostall cells. The sequence of codons that runs from a specificstart codon to a stop codon is called a reading frame.
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