Transient repression

Magasanik [93] suggested that the term permanent repression be used to refer to bacteria maintained on one carbon source, and transient repression to refer to bacteria as they are switched from one carbon source to another. Repression is transient in that it disappears as the cells adapt to growing under the conditions imposed by the new carbon source. Recent experiments by Tyler and Magasanik [95] indicate that there is no basic difference between the two effects. 

Perlman and Pastan [20] also showed that cyclic AMP affected the production of galactosidase. In bacteria made permeable by EDTA, 10 Af cyclic AMP increased the production of galactosidase by amounts ranging from 30 to 100%, and this stimulation was not accompanied by an increase in either total RNA or protein synthesis. These workers found, as did Ullman and Monod, that cyclic AMP could overcome transient repression of galactosidase synthesis in normal strains, constitutive strains, and cryptic mutant strains. Furthermore, Perlman and... 

Pastan and Perlman [100] investigated the relationship between catabolite repression, cyclic AMP action, and the promotor locus. They studied a promotor mutant (L8) which is insensitive to transient repression on addition of glucose. In this strain, cyclic AMP had no effect. However, revertant strains of L8 have regained their sensitivity to transient repression, and cyclic AMP increased the production of galactosidase, in the presence of glucose, by over 60%. In a different promotor mutant (L29), repression in response to glucose was normal and cyclic AMP displayed its normal ability to overcome repression. 

Perlman, R. L., Crombrugghe, B. de, Pastan, I. Cyclic AMP regulates catabolite and transient repression in E. coli. Nature (Lond.) 223, 810-812 (1969). 

Tyler, B., Magasanik, B. Molecular basis of transient repression of j -galactosidase in Escherichia coli. J. Bact. 97, 550-556 (1969). 

Babczinski and Tanner,S3 have reported an additional isoenzyme of /3-D-fructofuranosidase which they considered to be a precursor of the main enzyme. This new enzyme is associated with crude, membrane fractions of the cells it has a molecular weight of 190,000, and appears transiently, as < 5% of the total /3-D-fructofuranosidase, when the yeast is actively synthesizing the external enzyme. From studies on the effects of D-glucose repression and of cycloheximide, Sentandreu and coworkers (see Ref. 154) also considered that such an enzyme, associated more with internal membranes than with plasmalemma, might be, at least in part, a precursor of the external /3-D-fructofuranosidase in the process of secretion. 

Despite this elaborate binding complex, repression is not absolute. Binding of the Lac repressor reduces the rate of transcription initiation by a factor of 10s. If the 02 and 03 sites are eliminated by deletion or mutation, the binding of repressor to CL alone reduces transcription by a factor of about 102. Even in the repressed state, each cell has a few molecules of /3-galactosidase and galactoside permease, presumably synthesized on the rare occasions when the repressor transiently dissociates from the operators. This basal level of transcription is essential to operon regulation. 

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