The dgt gene of Escherichia coli facilitates thymine utilization in thymine-requiring strains

The Escherichia coli dGTP triphosphohydrolase (dGTPase) encoded by the dgt gene catalyses the hydrolysis of dGTP to deoxyguanosine and triphosphate. The recent discovery of a mutator effect associated with deletion of dgt indicated participation of the triphosphohydrolase in preventing mutagenesis. Here, we have investigated the possible involvement of dgt in facilitating thymine utilization through its ability to provide intracellular deoxyguanosine, which is readily converted by the DeoD phosphorylase to deoxyribose-1-phosphate, the critical intermediate that enables uptake and utilization of thymine. Indeed, we observed that the minimal amount of thymine required for growth of thymine-requiring (thyA) strains decreased with increased expression level of the dgt gene. As expected, this dgt-mediated effect was dependent on the DeoD purine nucleoside phosphorylase. We also observed that thyA strains experience growth difficulties upon nutritional shift-up and that the dgt gene facilitates adaptation to the new growth conditions. Blockage of the alternative yjjG (dUMP phosphatase) pathway for deoxyribose-1-phosphate generation greatly exacerbated the severity of thymine starvation in enriched media, and under these conditions the dgt pathway becomes crucial in protecting the cells against thymineless death. Overall, our results suggest that the dgt-dependent pathway for deoxyribose-1-phosphate generation may operate under various cell conditions to provide deoxyribosyl donors.     

Direction of bacteriophage lambda DNA replication in a thymine requiring Escherichia coli K-12 strain. Effect of thymidine concentration

The direction of replication was established for the first round of bacteriophage lambda DNA replication in thymine requiring E. coli K-12 cells exposed to different concentrations of thymidine. It was found that a dramatic decrease in the proportion of bidirectionally replicating molecules followed a decrease in the concentration of thymidine. Moreover, the rightward mode of replication appears to be exclusively favored in unidirectionally replicating molecules found at low concentrations of thymidine.     

Deg phenotype of Escherichia coli lon mutants.

Deg. one of the Escherichia coli systems for degrading abnormal polypeptides (e.g., nonsense fragments), is also involved in the degradation of some classes of missense proteins. Both missense proteins of beta-galactosidase and temperature-sensitive phage products appear to be degraded by the Deg system. Mutations in the Deg system are indistinguishable from mutations classically called lon or capR; all map near proC, all are mucoid, defective in protein degradation, sensitive to radiomimetic agents, and defective in P1 lysogenization. All are able to propagate temperature-sensitive phage better than lon+ parental strains. Mutations that suppress the radiation sensitivity of these strains (sul) also suppress the P1 lysogenization defect, but do not affect mucoidy or the degradation defect.     

Escherichia coli thymine auxotrophs suppressible by carbon dioxide

Two independently-isolated thymine-requiring mutant strains of Escherichia coli were found to possess an unusual phenotype: in the presence of CO2, growth was independent of thymine. The two strains showed different profiles of temperature sensitivity. Glycine, serine and methionine were unable to relieve the thymine-dependence. Both strains were susceptible to trimethoprim under conditions where they were thymine-independent. The results are consistent with the occurrence of partial defects in thymidylate synthase.     

Identification of the Escherichia coli sohB gene, a multicopy suppressor of the HtrA (DegP) null phenotype.

We cloned and sequenced the sohB gene of Escherichia coli. The temperature-sensitive phenotype of bacteria that carry a Tn10 insertion in the htrA (degP) gene is relieved when the sohB gene is present in the cell on a multicopy plasmid (30 to 50 copies per cell). The htrA gene encodes a periplasmic protease required for bacterial viability only at high temperature, i.e., above 39 degrees C. The sohB gene maps to 28 min on the E. coli chromosome, precisely between the topA and btuR genes. The gene encodes a 39,000-Mr precursor protein which is processed to a 37,000-Mr mature form. Sequencing of a DNA fragment containing the gene revealed an open reading frame which could encode a protein of Mr 39,474 with a predicted signal sequence cleavage site between amino acids 22 and 23. Cleavage at this site would reduce the size of the processed protein to 37,474 Mr. The predicted protein encoded by the open reading frame has homology with the inner membrane enzyme protease IV of E. coli, which digests cleaved signal peptides. Therefore, it is possible that the sohB gene encodes a previously undiscovered periplasmic protease in E. coli that, when overexpressed, can partially compensate for the missing HtrA protein function.     

Biochemical characterization of the uncA phenotype of Escherichia coli.

The ¹H n.m.r. spectra of apo-, Cu(I) and Cu(II) azurins from $\text{Pseudomonas aeruginosa}$ have been measured. Three of the four histidines have been assigned. The effect of the copper(II) ion acting as an intrinsic paramagnetic perturbant leads to the proposal that one of the histidines is far from the metal and another is closer, but not bound, to the copper. The possibility that the remaining two histidines are ligands to the copper is considered. The relationship to the sequence is discussed.     

Adherent-invasive Escherichia coli phenotype displayed by intestinal pathogenic E. coli strains from cats, dogs, and swine

The adherent-invasive Escherichia coli (AIEC) pathotype, which has been associated with Crohn's disease, shows similar traits to human and animal extraintestinal pathogenic E. coli (ExPEC) with respect to their phylogenetic origin and virulence gene profiles. Here, we demonstrate that animal ExPEC strains generally do not share the AIEC phenotype. In contrast, this phenotype is very frequent among animal intestinal pathogenic E. coli (InPEC) strains, particularly of feline and canine origin, that genetically resemble ExPEC. These results strengthen the particular identity and disease specificity of the AIEC pathotype and the putative role animals might play in the transmission of AIEC-like strains to humans.     

The EnvC phenotype and its expression in various Escherichia coli K12 strains

Abstract The septation-deficient mutant of Escherichia coli K12, PM61 envC , has been reported to have many membrane-related anomalies. To distinguish between real pleiotropy and a multimutational phenotype, the envC region was co-transduced with a selectable marker into 4 strains and the phenotypes of the transductants examined. It was found that (1) lysophosphatidylethanolamine accumulation did not cause EnvC morphology, nor was it caused by the envC lesion; (2) a low phosphatidylglycerol/diphosphatidylglycerol ratio (PG/DPG), hypersensitivity to tetracycline and resistance to novobiocin were dissociable from envC by transduction, but hypersensitivity to deoxycholate was not; (3) septation deficiency was expressed to variable degrees in different host strains. Thus, PM61 bears several envelope-related mutations in the 81 min region. The transduction results also suggested that the envC site is slightly upstream from cysE .