Generation of a restriction minus enteropathogenic Escherichia coli E2348/69 strain that is efficiently transformed with large, low copy plasmids.
BACKGROUND
Many microbes possess restriction-modification systems that protect them from parasitic DNA molecules. Unfortunately, the presence of a restriction-modification system in a given microbe also hampers genetic analysis. Although plasmids can be successfully conjugated into the enteropathogenic Escherichia coli strain E2348/69 and optimized protocols for competent cell preparation have been developed, we found that a large, low copy (approximately 15) bioluminescent reporter plasmid, pJW15, that we modified for use in EPEC, was exceedingly difficult to transform into E2348/69. We reasoned that a restriction-modification system could be responsible for the low transformation efficiency of E2348/69 and sought to identify and inactivate the responsible gene(s), with the goal of creating an easily transformable strain of EPEC that could complement existing protocols for genetic manipulation of this important pathogen.


RESULTS
Using bioinformatics, we identified genes in the unfinished enteropathogenic Escherichia coli (EPEC) strain E2348/69 genome whose predicted products bear homology to the HsdM methyltransferases, HsdS specificity subunits, and HsdR restriction endonucleases of type I restriction-modification systems. We constructed a strain carrying a deletion of the conserved enzymatic domain of the EPEC HsdR homologue, NH4, and showed that its transformation efficiency was up to four orders of magnitude higher than that of the parent strain. Further, the modification capacity of NH4 remained intact, since plasmids that were normally recalcitrant to transformation into E2348/69 could be transformed upon passage through NH4. NH4 was unaffected in virulence factor production, since bundle forming pilus (BFP) subunits and type III secreted (T3S) proteins were present at equivalent levels to those seen in E2348/69. Further, NH4 was indistinguishable from E2348/69 in tissue culture infection model assays of localized adherence and T3S.


CONCLUSIONS
We have shown that EPEC strain E2348/69 utilizes a type I restriction-modification system to limit entry of new DNA. This restriction-modification system does not appear to be involved in virulence determinant expression or infection phenotypes. The hsdR mutant strain should prove useful in genetic analysis of the important diarrheal pathogen EPEC.
Hobson
Neil
N
Department of Biological Sciences, University of Alberta, Edmonton, AB, T6G 2E9, Canada. nhobson@ualberta.ca
Price
Nancy L
NL
Ward
Jordan D
JD
Raivio
Tracy L
TL
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D015280:DNA Restriction-Modification Enzymes / Q000235:genetics / Q000378:metabolism; D015253:Deoxyribonucleases, Type I Site-Specific / Q000235:genetics / Q000378:metabolism; D054308:Enteropathogenic Escherichia coli / Q000235:genetics* / Q000378:metabolism; D004927:Escherichia coli Infections / Q000382:microbiology; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D018628:Gene Dosage; D016680:Genome, Bacterial; D006801:Humans; D009154:Mutation; D010957:Plasmids / Q000235:genetics*; D014161:Transduction, Genetic; D014169:Transformation, Bacterial; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism
D001426:Bacterial Proteins; D015280:DNA Restriction-Modification Enzymes; D029968:Escherichia coli Proteins; D037521:Virulence Factors; C109829:HsdM protein, Bacteria; D015253:Deoxyribonucleases, Type I Site-Specific; C109830:HsdR protein, E coli