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The RuvABC Resolvase Is Indispensable for Recombinational Repair in sbcB15 Mutants of Escherichia coli. Davor Zahradka, 2002.The RuvABC proteins of Escherichia coli play an important role in the processing of Holliday junctions during homologous recombination and recombinational repair . Mutations in the ruv genes have a moderate effect on recombination and repair in wild-type strains but confer pronounced recombination deficiency and extreme sensitivity to DNA-damaging agents in a recBC sbcBC background . Genetic analysis presented in this work revealed that the  ruvABC mutation causes an identical DNA repair defect in UV-irradiated recBC sbcBC, sbcBC, and sbcB strains, indicating that the sbcB mutation alone is responsible for the extreme UV sensitivity of recBC sbcBC ruv derivatives . In experiments with gamma irradiation and in conjugational crosses, however, sbcBC
ruvABC and sbcB
ruvABC mutants displayed higher recombination proficiency than the recBC sbcBC
ruvABC strain . The frequency of conjugational recombination observed with the sbcB
ruvABC strain was quite similar to that of the
ruvABC single mutant, indicating that the sbcB mutation does not increase the requirement for RuvABC in a recombinational process starting from preexisting DNA ends . The differences between the results obtained in three experimental systems used suggest that in UV-irradiated cells, the RuvABC complex might act in an early stage of recombinational repair . The results of this work are discussed in the context of recent recombination models which propose the participation of RuvABC proteins in the processing of Holliday junctions made from stalled replication forks . We suggest that the mutant SbcB protein stabilizes these junctions and makes their processing highly dependent on RuvABC resolvase .
Longitudinal Study of Escherichia coli O157 in a Cattle Finishing Unit. Elina Lahti, 2003.In a longitudinal study in a Finnish cattle finishing unit we investigated excretion and sources of Escherichia coli O157 in bulls from postweaning until slaughter . Three groups of 31 to 42 calves were sampled in a calf transporter before they entered the farm and four to seven times at approximately monthly intervals at the farm . All calves sampled in the livestock transporter were negative for E . coli O157 on arrival, whereas positive animals were detected 1 day later . During the fattening period the E . coli O157 infection rate varied between 0 and 38.5% . The animals were also found to be shedding during the cold months . E . coli O157 was isolated from samples taken from water cups, floors, and feed passages . E . coli O157 was detected in 9.7 to 38.9% of the fecal samples taken at slaughter, while only two rumen samples and one carcass surface sample were found to be positive . E . coli O157 was isolated from barn surface samples more often when the enrichment time was 6 h than when the enrichment time was 24 h (P < 0.0001) . Fecal samples taken at the abattoir had lower counts (  0.4 MPN/g) than fecal samples at the farm (P < 0.05) . E . coli O157 was isolated more often from 10-g fecal samples than from 1-g fecal samples (P < 0.0001) . Most farm isolates belonged to one pulsed-field gel electrophoresis (PFGE) genotype (79.6%), and the rest belonged to closely related PFGE genotypes . In conclusion, this study indicated that the finishing unit rather than introduction of new cattle was the source of E . coli O157 at the farm and that E . coli O157 seemed to persist well on barn surfaces .
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