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Temporal Expression of Pertussis Toxin and Ptl Secretion Proteins by Bordetella pertussis. Amy A. Rambow-Larsen, 2004.Pertussis toxin is an AB5 toxin comprised of protein subunits S1 through S5 . The individual subunits are secreted by a Sec-dependent mechanism into the periplasm, where the toxin is assembled . The Ptl type IV secretion system mediates secretion of assembled toxin past the outer membrane . In this study, we examined the time course of protein expression, toxin assembly, and secretion as a function of the bacterial growth cycle . Logarithmic growth was observed after a 1-h lag phase . Secreted toxin was first observed at 3 h . Secretion continued throughout the logarithmic growth phase and decreased as the culture entered the stationary phase after about 24 h . On a per cell basis, toxin secretion occurred at a constant rate of 3 molecules/min/cell from 2 to 18 h . More of toxin subunits S1, S2, and S3 were produced than were secreted, resulting in periplasmic accumulation . Periplasmic S1, S2, and S3 were found to be soluble in the periplasm, as well as membrane associated . About one-half of the periplasmic S1, S2 and S3 subunits were incorporated into holotoxin . Secretion component PtlF was present at a low level at time zero, and the level increased between 2 and 24 h from 30 to 1,000 molecules per cell; however, the initial level of PtlF, 30 molecules per cell, supported maximal secretion . The accumulation of both periplasmic toxin and secretion components suggests that translation rates exceed the rate of secretion and that secretion, not toxin and Ptl complex assembly, is rate limiting . High Frequency of Mutator Strains among Human Uropathogenic Escherichia coli Isolates. Erick Denamur, 2002.By using a panel of 603 commensal and pathogenic Escherichia coli and Shigella isolates, we showed that mutation rates of strains vary considerably among different ecotypes . Uropathogenic strains had the highest frequency of mutators, while strains from patients with bacteremia had the lowest mutation rates . No correlation between the mutation rates and antibiotic resistance was observed among the studied strains . Evolutionary Relationships among Strains of Mycobacterium tuberculosis with Few Copies of IS6110. Jeremy W. Dale, 2003.Molecular typing of Mycobacterium tuberculosis by using IS6110 shows low discrimination when there are fewer than five copies of the insertion sequence . Using a collection of such isolates from a study of the epidemiology of tuberculosis in London, we have shown a substantial degree of congruence between IS6110 patterns and both spoligotype and PGRS type . This indicates that the IS6110 types mainly represent distinct families of strains rather than arising through the convergent insertion of IS6110 into favored positions . This is supported by identification of the genomic sites of the insertion of IS6110 in these strains . The combined data enable identification of the putative evolutionary relationships of these strains, comprising three lineages broadly associated with patients born in South Asia (India and Pakistan), Africa, and Europe, respectively . These lineages appear to be quite distinct from M . tuberculosis isolates with multiple copies of IS6110 .
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