Screening of Escherichia coli temperature-sensitive mutants by pretreatment with glucose starvation

Employing site-specific recombination for conditional genetic analysis in Sinorhizobium meliloti

The ability to remove a genetic function from an organism with good temporal resolution is crucial for characterizing essential genes or genes that act in complex developmental programs. The rhizobium-legume symbiosis involves an elaborate two-organism interaction requiring multiple levels of signal exchange. As an important step toward probing rhizobium genetic functions with temporal resolution, we present the development of a conditional gene deletion system in Sinorhizobium meliloti that employs Cre/loxP site-specific recombination. This system enables chemically inducible and irreversible gene deletion or gene upregulation. Recombinase-mediated excision events can be positively or negatively selected or monitored by a colorimetric assay. The system may be adaptable to various bacterial species, in which recombinase activity may be placed under the control of diverse user-defined promoters. This system also shows promise for uses in promoter trapping and biosensing applications.

Target-directed proteolysis in vivo

The experimental problems associated with in vivo studies of essential proteins or integral membrane proteins have triggered geneticists to generate novel approaches that have often led to insights of general relevance (Shuman and Silhavy, 2003). In order to extend the experimental portfolio, we developed target-directed proteolysis (TDP), an in vivo method allowing structural and functional characterization of target proteins in living cells. TDP is based on the activity of the highly sequence-specific NIa protease from tobacco etch virus. When its recognition site of seven residues is engineered into target proteins and NIa protease is expressed under tight promoter control, substrates can be conditionally processed while other cellular proteins remain unaffected. Applications include conditional inactivation as well as functional characterization of target proteins.

Temperature-sensitive growth mutants as live vaccines against experimental murine salmonellosis

Temperature-sensitive growth mutants of Salmonella enteritidis, NUB 209 and NUB 323, characterized as protein synthesis mutants, were unable to proliferate at 37 C and lost the parent's virulence. In mice, these mutants conferred high levels of protection as live vaccines. Although the vaccination effect of NUB 323 was not so good as that of NUB 209, NUB 323 was preferred as a safer live vaccine because this mutant was completely avirulent and no back mutation appeared.

Effect of elevated temperatures on protein synthesis in Escherichia coli

Upon a temperature shift from 30 C to 42 to 44 C, Escherichia coli experiences a transient slowdown in rate of optical density increase, uridine incorporation, and amino acid incorporation. The data show that the primary target for thermal inactivation is a molecule(s) required for the initiation of protein synthesis. The effect on ribonucleic acid synthesis is a secondary effect.

New rifampin-resistant mutant of Escherichia coli

A rifampin-resistant ribonucleic acid (RNA) polymerase mutant, rif(r)51, derived from a presumptive RNA synthesis mutant of Escherichia coli K-12, complements rif(r) RNA polymerase mutants isolated from other strains of E. coli K-12.

Studies of sulfate utilization by algae: 8. The ubiquity of sulfate reduction to thiosulfate

Cell-free extracts from several microorganisms, when prepared by methods originally devised for Chlorella pyrenoidosa (Emerson strain 3) and incubated anaerobically with ATP, Mg(2+), and 2, 3-dimercaptopropan-1-ol, are capable of reducing sulfate-(35)S to thiosulfate. These microorganisms include, in addition to C. pyrenoidosa (Emerson strain 3), several other strains of C. pyrenoidosa, Chlorella protothecoides, Chlorella vulgaris, Anacystis sp., Chlamydomonas reinhardi, Escherichia coli, Salmonella typhimurium, and baker's yeast. Three of these organisms, E. coli, S. typhimurium, and baker's yeast, were previously reported by others to reduce sulfate to sulfite. Moreover, three mutant strains of S. typhimurium (Ba-25, Ce-363, and Bc-482) previously reported by other workers to be unable to reduce sulfate to sulfite also cannot form thiosulfate, and one mutant strain (Cd-68) reportedly able to form sulfite can also form thiosulfate. Taken together, this suggests that thiosulfate-forming activity may be a common feature of sulfate-reducing systems, and it may be present in enzymatic systems previously thought to be forming sulfite. Reasonably conclusive identification of thiosulfate is provided by ion exchange chromatography and by paper electrophoresis; the ambiguities associated with other analytical methods are discussed.

Metabolic events occurring during recovery from prolonged glucose starvation in Escherichia coli

The effects of starving Escherichia coli for glucose and required amino acids were determined. The disappearance of the majority of the ribosomal population, particularly of the ribonucleic acid (RNA) moiety, was noted. The events during recovery are detailed, with emphasis placed on the timing, requirements, and control of deoxyribonucleic acid, RNA, and protein synthesis and on ribosome reassembly. Finally, the applicability of the system to the study of temperature-sensitive mutants is documented and discussed.