Derepression of uridine diphosphate-glucose pyrophosphorylase (galU) in capR(lon), capS, and capT mutants and studies on the galU repressor

Studies of intracellular thymidine nucleotides. Relationship between the synthesis of deoxyribonucleic acid and the thymidine triphosphate pool in Escherichia coli K12

The two types of mutant strains which show resistance to T-even phage infection have been isolated and been shown to have either a higher or lower ratio of dTDP-sugar to dTTP than that of the parent strains. The one with a higher ratio of dTDP-sugar to dTTP than the parents has a large dTDP-sugar pool and small dTTP pool, and a high level of dTDPG pyrophosphorylase activity. The other one, with a lower ratio of dTDP-sugar to dTTP than the parents, has a small dTDP-sugar pool and large dTTP pool, and a low or deficient level of this enzyme activity. They form an entirely mucoid colony in the synthetic agar plate. Mutant cells (Ter-6 and Ter-21) which have deficient dTDPG pyrophosphorylase activity show 2 -- 3 times higher activity of UDPG pyrophosphoyrlase than that of parent cells. The dTDPG pyrophosphorylase-deficient mutants (Ter-15 and Ter-21) have a 3 -- 4 times higher concentration of dTTP and a faster rate of DNA synthesis and cell division than those of parent strains in growth with external thymine. The dTDPG pyrophosphorylase constitutive mutant (Ter-4) has a 0.5 -- 0.33 smaller dTTP pool and a slower rate of DNA synthesis and cell division than those of parent cells grown in the same medium. In the Ter-15 and Ter-21 mutants, the intracellular dTTP-dependent DNA synthesis rapidly disappeared in thymine suboptimal concentration, but the Ter-4 mutant maintained its dTTP-dependent DNA synthesis over a 20 muM concentration of external thymine. In high concentration (100 muM) of external thymidine, the thymidine effects on the intracellular dTTP concentration do not significantly appear in these enzyme-deficient mutants (Ter-15 and Ter-21). Also, the concentration of intracellular dTTP in the cell growth with external thymidine is 2.5 times greater than that with external thymine in these enzyme-deficient mutants (Ter-15 and Ter-21).

Regulation of galactose operon at the gal operator-promoter region in Escherichia coli K-12

The capR (lon) product controls expression of the gal operon independently of the galR repressor. Previously, mutations of the gal operon have been isolated that are semiconstitutive and alter response to the capR and/or capT product. Such mutants imply the existence of a distinct site in the operon that responds to capR (capT) control. This mutation could be either in a site near the operator-distal end of the galE gene, which signals rho factor termination of transcription in vitro or in a site in the operator-promoter region. Bacteriophage U3 was used to isolate galE mutations in HC2142 (a mutant exhibiting reduced response to capR control). P1 transduction was used to cross these mutants with a set of galE gene deletion. Analysis of the resulting Gal+ recombinants indicates that the regulatory site is in the operator-promoter region. Hence, it is unlikely that capR functions in control as an anti-rho factor at the operator-distal end of the galE gene, but more likely as previously suggested, at a second operator distinct from one responding to galR repressor control. Upon induction with D-fucose, a promoter mutant (UV211) isolated previously expressed 20 to 30% of the galactose enzymes that the wild type exhibited in the presence of the inducer D-fucose. The effects of various mutations in cya, capR, and galR on galactokinase synthesis in this mutant were determined. Galactokinase was derepressed by capR as well as galR, but the presence or absence of the cya gene product was unimportant.

Multiple regulation of the galactose operon-genetic evidence for a distinct site in the galactose operon that responds to capR gene regulation in Escherichia coli K-12

Previous results demonstrated that the capR (lon) locus, which is not linked to the gal operon, independently controls the synthesis of the gal operon enzymes and gal mRNA, i.e., galO(+)capR9 strains are derepressed 4- to 6-fold as compared to galO(+)capR(+) strains. A mutation has been isolated and localized in the galactose operator region that defines a new and distinct site of control. Mutation in this site, designated galO(capR+), causes a 4-fold increase in the galactose enzymes, galactokinase (EC 2.7.1.6) and UDP-galactose-4-epimerase (EC 5.1.3.2), in a capR(+) background. These mutants exhibit a reduced response to regulation by the unlinked regulator gene capR (lon). However, the galO(capR+) mutants are still subject to control by the galR repressor, since they can be further derepressed by growth in the presence of D-fucose. They also synthesize more galactokinase when grown in glycerol as compared to glucose. Thus there are now at least three, and probably four, sites for control of mRNA synthesis in the operator-promoter regions of the gal operon, making it one of the most complex control systems to date for a single operon in bacteria. The complexity is sufficient to accommodate models for differentiation in higher organisms that require more than one "switch" to control a single group of genes.