SUMmOning Daxx-mediated repression

Bacteria encounter fluctuations in both their external and internal environments, and to manage these conditions, they employ various control mechanisms. In this issue of Molecular Cell, Hart et al. (2011) investigate how E. coli robustly controls nitrogen assimilation.     

Catabolite repression 1985

The present status of catabolite repression is summarized with respect to the involvement of cyclic AMP and other mediators. A model is presented which may account for the relationship between positive control of gene expression exerted by cAMP and its receptor, CAP, and negative control of catabolite repression mediated by specific metabolites.     

Catabolite repression of the lac operon. Separate repression of two enzymes

1. Catabolite repression of β-galactosidase and of thiogalactoside transacetylase was studied in several strains of Escherichia coli K 12, in an attempt to show whether a single site within the structural genes of the lac operon co-ordinately controls translational repression for the two enzymes. In all experiments the rate of synthesis of the enzymes was compared in glycerol–minimal medium and in glucose–minimal medium. 2. In a wild-type strain, glucose repressed the synthesis of the two enzymes equally. 3. The possibility that repression was co-ordinate was investigated by studies of mutant strains that carry deletions in the genes for β-galactosidase or galactoside permease or both. In all of the strains with deletions, the repression of thiogalactoside transacetylase persisted, and it is concluded that there is no part of the structural gene for β-galactosidase that is essential for catabolite repression of thiogalactoside transacetylase. 4. Subculture of one strain through several transfers in rich medium greatly increased its susceptibility to catabolite repression by glucose. It is concluded that unknown features of the genotype can markedly affect sensitivity to catabolite repression. 5. These results make it clear that one cannot draw valid conclusions about the effect of known genotypic differences on catabolite repression from a comparison of two separate strains; to study the effect of a particular genetic change in a lac operon it is necessary to construct a partially diploid strain so that catabolite repression suffered by one lac operon can be compared with that suffered by another. 6. Four such partial diploids were constructed. In all of them catabolite repression of β-galactosidase synthesized by one operon was equal in extent to catabolite repression of thiogalactoside transacetylase synthesized by the other. 7. Taken together, these results suggest that catabolite repression of β-galactosidase and thiogalactoside transacetylase is separate but equal.     

Catabolite repression mutants of yeast

The mechaṅism of catabolite repression in yeast is not well understood, although it has been established that cAMP does not play a role similar to that found in Escherichia coli. To identify the elements implicated in catabolite repression in yeast, a variety of mutants affected in this process have been isolated by different research groups. A systematic review of the results reported in the literature is presented. The conclusion that can be drawn is that the mechanism of catabolite repression is a complex one, with no single gene controlling all the genes subject to repression. The expression of a given gene or set of genes is controlled by several regulatory genes, but it is not yet known whether these genes act cooperatively or sequentially.     

Allosteric repression: An Analysis

A kinetic analysis of repression is presented based on the accepted sequence of molecular events and the assumption that the co-repressor-repressor interaction involves an allosteric transition. This leads to an expression which relates the two experimentally accessible variables—enzyme and signal—when the system is operating in vivo. A suitable plot allows the estimation of a coefficient, h_max, which is related to the number of protomers, n, of the oligomeric repressor protein. This parameter is similar to but distinct from a Hill coefficient for allosteric inhibitions. Two examples from the literature are analysed in terms of the model.