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Tamarin: Principles of Genetics, Seventh Edition
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III. Molecular Genetics
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14. Gene Expression: Control in Prokaryotes and Phages
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The McGraw Hill Companies, 2001
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Trp Operon (Attentuator-Controlled System)
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permit stem-loop 2 3 to form, which precludes the formation of the terminator stem-loop (3 4). In this con guration, transcription is not terminated, so that eventually, the whole operon is transcribed and translated, raising the level of tryptophan in the cell.The stem-loop 2 3 structure is referred to as the preemptor stem. Note that the preemptor stem is not a rho-independent transcription terminator and thus, without the rho protein present, will not terminate transcription (see chapter 10).
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the cell is starved for other amino acids. Presumably, it makes no sense to manufacture tryptophan when other amino acids are in short supply. Hence, the cell can carefully bring up the levels of the various amino acids in the most ef cient manner.
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The tryptophan operon in bacilli such as Bacillus subtilis is also controlled by attenuation, but secondary structure in the mRNA transcript is induced by binding not the ribosome, but a trp RNA-binding attenuation protein (TRAP). This protein attaches to the nascent messenger RNA only after the protein binds tryptophan molecules; the result is a terminator stem that forms in the messenger RNA. In the absence of excess tryptophan,TRAP does not bind to the messenger RNA, a preemptor (also called an antiterminator) stem, not the terminator stem, forms, and transcription continues ( g. 14.17a). Recently, the
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A nal con guration is possible, as shown on the far right in gure 14.16. Here, no ribosome interferes with stem formation and, presumably, stem-loops 1 2 and 3 4 (terminator) form. This con guration also terminates transcription because of the terminator stem. It is believed that this con guration occurs if the ribosome is stalled on the 5 side of the trp codons, which happens when
Figure 14.17 The trp operon control by attenuation in Bacillus subtilis. (a) The top of the gure shows the leader region of the DNA with the two parts of the antitermination stem (A, B) and the termination stem (C, D). The triplets (GAG and TAG in DNA, or GAG and UAG in the messenger RNA) that the trp RNA-binding attenuation protein (TRAP) binds to are circled. The label trpEDCFBA refers to the structural genes of the trp operon. Nucleotides 108 133 (C, D) form the terminator stem, and nucleotides 60 111 (A, B) form the antiterminator stem. The arrows below the boxed letters A D indicate the inverted repeat sequences forming the stems. Without TRAP, the antiterminator stem forms; with TRAP, the terminator stem forms as TRAP is bound by nucleotides 36 91 of the messenger RNA. Part (b) is a close-up of the mRNA (ball-and-stick model) wrapped around TRAP (ribbon diagram with subunits in different colors) bound by tryptophan molecules (spheres). (From Alfred A. Antson, et al.,
Structure of the trp RNA-binding attenuation protein, TRAP, bound to RNA in Nature, Vol.40, September 16, 1999, g. 1 p. 234 and g. 2a p. 237. Reprinted by permission of
Macmillan Ltd.)
Tamarin: Principles of Genetics, Seventh Edition
III. Molecular Genetics
14. Gene Expression: Control in Prokaryotes and Phages
The McGraw Hill Companies, 2001
Fourteen
Gene Expression: Control in Prokaryotes and Phages
structure of the protein was worked out; it has eleven symmetrical loops, each of which can bind a tryptophan molecule. When TRAP is bound to tryptophan molecules, it can attach to triplets in the messenger RNA transcript, triplets of GAG or UAG. The TRAP wraps the mRNA around itself, forming an elegant pinwheel ( g. 14.17b).
Redundant Controls
Some amino acid operons are controlled only by attenuation, such as the his operon in E. coli, in which the leader peptide gene contains seven histidine codons in a row, or the trp operon in B. subtilis. Redundant control (repression and attenuation) of tryptophan biosynthesis in E. coli allows the cell to test both the tryptophan levels (tryptophan is the corepressor) and the tryptophanyltRNA levels (in the attenuator control system). The attenuator system also allows the cell to regulate tryptophan synthesis on the basis of the shortage of other amino acids. For example, when there is a shortage of both tryptophan and arginine, operator control allows transcription to begin, but attenuator control terminates transcription because stem-loops 1 2 and 3 4 form ( g. 14.16).
erator sites. The CI repressor acts to favor lysogeny: it represses the lytic cycle. The Cro repressor favors the lytic cycle and represses lysogeny. The operator sites, when bound by either CI or Cro, can either enhance or repress transcription. Other control mechanisms are also involved in determining aspects of the life cycle, including antitermination and multiple promoters for the same genes.
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