Harpers

342 / CHAPTER 37Table 37–1. Classes of eukaryotic RNA.RNA Types Abundance StabilityRibosomal 28S, 18S, 5.8S, 5S 80% of total Very stable(rRNA)Messenger ~10 5 different 2–5% of total Unstable to(mRNA) species verystableTransfer ~60 different ~15% of total Very stable(tRNA) speciesSmall nuclear ~30 different ≤ 1% of total Very stable(snRNA) speciesDNA-Dependent RNA PolymeraseInitiates Transcription at a DistinctSite, the PromoterDNA-dependent RNA polymerase is the enzyme responsiblefor the polymerization of ribonucleotides intoa sequence complementary to the template strand ofthe gene (see Figures 37–2 and 37–3). The enzyme attachesat a specific site—the promoter—on the templatestrand. This is followed by initiation of RNA synthesisat the starting point, and the process continuesuntil a termination sequence is reached (Figure 37–3).A transcription unit is defined as that region of DNAthat includes the signals for transcription initiation,elongation, and termination. The RNA product, whichis synthesized in the 5′ to 3′ direction, is the primarytranscript. In prokaryotes, this can represent the productof several contiguous genes; in mammalian cells, itusually represents the product of a single gene. The 5′terminals of the primary RNA transcript and the maturecytoplasmic RNA are identical. Thus, the startingpoint of transcription corresponds to the 5 nucleotideof the mRNA. This is designated position +1,as is the corresponding nucleotide in the DNA. TheGene A Gene B Gene C5′ 3′3′ 5′Template strandsGene DFigure 37–1. This figure illustrates that genes can betranscribed off both strands of DNA. The arrowheads indicatethe direction of transcription (polarity). Note thatthe template strand is always read in the 3′ to 5′ direction.The opposite strand is called the coding strand becauseit is identical (except for T for U changes) to themRNA transcript (the primary transcript in eukaryoticcells) that encodes the protein product of the gene.5′ P-P-P3′5′RNA transcriptβ3′ OHαβ′σRNAP complexαTranscriptionFigure 37–2. RNA polymerase (RNAP) catalyzes thepolymerization of ribonucleotides into an RNA sequencethat is complementary to the template strandof the gene. The RNA transcript has the same polarity(5′ to 3′) as the coding strand but contains U ratherthan T. E coli RNAP consists of a core complex of twoα subunits and two β subunits (β and β′). The holoenzymecontains the σ subunit bound to the α 2 ββ′ coreassembly. The ω subunit is not shown. The transcription“bubble” is an approximately 20-bp area of meltedDNA, and the entire complex covers 30–75 bp, dependingon the conformation of RNAP.pppApNRNAPpp(5) Chain terminationand RNAP releasepppApN(1) Template bindingpppApNATP + NTP(2) Chain initiationpppApN(4) Chain elongationNTPsNTPs(3) PromoterclearanceFigure 37–3. The transcription cycle in bacteria. BacterialRNA transcription is described in four steps:(1) Template binding: RNA polymerase (RNAP) bindsto DNA and locates a promoter (P) melts the two DNAstrands to form a preinitiation complex (PIC). (2) Chaininitiation: RNAP holoenzyme (core + one of multiplesigma factors) catalyzes the coupling of the first base(usually ATP or GTP) to a second ribonucleosidetriphosphate to form a dinucleotide. (3) Chain elongation:Successive residues are added to the 3′-OH terminusof the nascent RNA molecule. (4) Chain terminationand release: The completed RNA chain and RNAPare released from the template. The RNAP holoenzymere-forms, finds a promoter, and the cycle is repeated.5′3′