DNADependent RNA Polymerase Initiates Transcription at a Distinct Site the Promoter

DNA-dependent RNA polymerase is the enzyme responsible for the polymerization of ribonucleotides into a sequence complementary to the template strand of the gene (see Figures 37-2 and 37-3). The enzyme attaches at a specific site—the promoter—on the template strand. This is followed by initiation of RNA synthesis at the starting point, and the process continues until a termination sequence is reached (Figure 37-3). A transcription unit is defined as that region of DNA that includes the signals for transcription initiation, elongation, and termination. The RNA product, which is synthesized in the 5' to 3' direction, is the primary transcript. In prokaryotes, this can represent the product of several contiguous genes; in mammalian cells, it usually represents the product of a single gene. The 5' terminals of the primary RNA transcript and the mature cytoplasmic RNA are identical. Thus, the starting point of transcription corresponds to the 5' nucleotide of the mRNA This is designated position +1, as is the corresponding nucleotide in the DNA. The

RNA transcript

RNA transcript

Figure 37-2. RNA polymerase (RNAP) catalyzes the polymerization of ribonucleotides into an RNA sequence that is complementary to the template strand of the gene. The RNA transcript has the same polarity (5' to 3') as the coding strand but contains U rather than T. Ecoli RNAP consists of a core complex of two a subunits and two p subunits (P and p'). The holoen-zyme contains the o subunit bound to the a2pp' core assembly. The m subunit is not shown. The transcription "bubble" is an approximately 20-bp area of melted DNA, and the entire complex covers 30-75 bp, depending on the conformation of RNAP.

Figure 37-2. RNA polymerase (RNAP) catalyzes the polymerization of ribonucleotides into an RNA sequence that is complementary to the template strand of the gene. The RNA transcript has the same polarity (5' to 3') as the coding strand but contains U rather than T. Ecoli RNAP consists of a core complex of two a subunits and two p subunits (P and p'). The holoen-zyme contains the o subunit bound to the a2pp' core assembly. The m subunit is not shown. The transcription "bubble" is an approximately 20-bp area of melted DNA, and the entire complex covers 30-75 bp, depending on the conformation of RNAP.

(1) Template binding

(1) Template binding

Figure 37-3. The transcription cycle in bacteria. Bacterial RNA transcription is described in four steps: (1) Template binding: RNA polymerase (RNAP) binds to DNA and locates a promoter (P) melts the two DNA strands to form a preinitiation complex (PIC). (2) Chain initiation: RNAP holoenzyme (core + one of multiple sigma factors) catalyzes the coupling of the first base (usually ATP or GTP) to a second ribonucleoside triphosphate to form a dinucleotide. (3) Chain elongation: Successive residues are added to the 3'-OH terminus of the nascent RNA molecule. (4) Chain termination and release: The completed RNA chain and RNAP are released from the template. The RNAP holoenzyme re-forms, finds a promoter, and the cycle is repeated.

Gene A Gene B Gene C

Gene D

Gene A Gene B Gene C

Gene D

Template strands

Template strands

Figure 37-1. This figure illustrates that genes can be transcribed off both strands of DNA. The arrowheads indicate the direction of transcription (polarity). Note that the template strand is always read in the 3' to 5' direction. The opposite strand is called the coding strand because it is identical (except for T for U changes) to the mRNA transcript (the primary transcript in eukaryotic cells) that encodes the protein product of the gene.

Figure 37-3. The transcription cycle in bacteria. Bacterial RNA transcription is described in four steps: (1) Template binding: RNA polymerase (RNAP) binds to DNA and locates a promoter (P) melts the two DNA strands to form a preinitiation complex (PIC). (2) Chain initiation: RNAP holoenzyme (core + one of multiple sigma factors) catalyzes the coupling of the first base (usually ATP or GTP) to a second ribonucleoside triphosphate to form a dinucleotide. (3) Chain elongation: Successive residues are added to the 3'-OH terminus of the nascent RNA molecule. (4) Chain termination and release: The completed RNA chain and RNAP are released from the template. The RNAP holoenzyme re-forms, finds a promoter, and the cycle is repeated.

numbers increase as the sequence proceeds downstream. This convention makes it easy to locate particular regions, such as intron and exon boundaries. The nucleotide in the promoter adjacent to the transcription initiation site is designated -1, and these negative numbers increase as the sequence proceeds upstream, away from the initiation site. This provides a conventional way of defining the location of regulatory elements in the promoter.

The primary transcripts generated by RNA polymerase II—one of three distinct nuclear DNA-depen-dent RNA polymerases in eukaryotes—are promptly capped by 7-methylguanosine triphosphate caps (Figure 35-10) that persist and eventually appear on the 5' end of mature cytoplasmic mRNA. These caps are necessary for the subsequent processing of the primary transcript to mRNA, for the translation of the mRNA, and for protection of the mRNA against exonucleolytic attack.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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