mRNA composition and control of bacterial gene expression.
The expression of any given bacterial protein is predicted to depend on (i) the transcriptional regulation of the promoter and the translational regulation of its mRNA and (ii) the synthesis and translation of total (bulk) mRNA. This is because total mRNA acts as a competitor to the specific mRNA for the binding of initiation-ready free ribosomes. To characterize the effects of mRNA competition on gene expression, the specific activity of beta-galactosidase expressed from three different promoter-lacZ fusions (P(spc)-lacZ, P(RNAI)-lacZ, and P(RNAII)-lacZ) was measured (i) in a relA(+) background during exponential growth at different rates and (ii) in relA(+) and DeltarelA derivatives of Escherichia coli B/r after induction of a mild stringent or a relaxed response to raise or lower, respectively, the level of ppGpp. Expression from all three promoters was stimulated during slow exponential growth or at elevated levels of ppGpp and was reduced during fast exponential growth or at lower levels of ppGpp. From these observations and from other considerations, we propose (i) that the concentration of free, initiation-ready ribosomes is approximately constant and independent of the growth rate and (ii) that bulk mRNA made during slow growth and at elevated levels of ppGpp is less efficiently translated than bulk mRNA made during fast growth and at reduced levels of ppGpp. These features lead to an indirect enhancement in the expression of LacZ (or of any other protein) during growth in media of poor nutritional quality and at increased levels of ppGpp.
Liang
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Program in Molecular and Cell Biology, University of Texas at Dallas, Richardson, Texas 75083-0688, USA.
Xu
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YC
Dennis
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Bremer
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D001426:Bacterial Proteins / Q000096:biosynthesis; D001667:Binding, Competitive; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics* / Q000254:growth & development; D015964:Gene Expression Regulation, Bacterial; D006159:Guanosine Tetraphosphate / Q000378:metabolism; D006207:Half-Life; D007763:Lac Operon; D011401:Promoter Regions, Genetic; D014176:Protein Biosynthesis; D012329:RNA, Bacterial / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism*; D012269:Ribosomal Proteins / Q000096:biosynthesis
D001426:Bacterial Proteins; D012329:RNA, Bacterial; D012333:RNA, Messenger; D012269:Ribosomal Proteins; D006159:Guanosine Tetraphosphate; D012321:DNA-Directed RNA Polymerases