GENETIC DIVERSITY AMONG ERWINIA AMYLOVORA'S UBIQUITOUS PLASMID PEA29
Non-transmissible plasmid pEA29 is found in all naturally occurring strains of Erwinia amylovora and PCR-based assays for this plasmid are the most commonly used method for detection and identification of the pathogen. Little is known about the genetic diversity of pEA29 beyond the presence of short sequence repeats within the pEA29-PCR fragment used for detection. The number of repeats in this fragment can vary between strains resulting in variation in the length of PCR fragments and in restriction fragment length polymorphism. Other regions of pEA29 were not well characterized until recently when the complete sequence of plasmid pEA29 from E. amylovora strain Ea88 became available (GenBank accession no. AF264948). Although pEA29 is approximately 28.2 kb in strain Ea88, it can range in size from 27.7 to 35.0 kb due to the occurrence of deletions and insertions. Restriction patterns for pEA29 plasmids possessing deletions or insertions can vary widely. PstI and KpnI restriction patterns for pEA29 isolated from apple, pear, Indian hawthorn, and quince strains of streptomycin sensitive E. amylovora were homogenous and different from those for pEA29 isolated from Rubus (bramble) strains. All Rubus derivatives of pEA29 contained a point mutation that eliminated a PstI site and a 1,264-bp region that replaced 1,890 bp of sequence found in pEA29 in strain Ea88. The replacement resulted in the loss of a second PstI site and an increase in the length of a KpnI fragment. In a Rubus strain, pEA29 contained an insertion sequence, ISEam1, and in several apple strains the transposon Tn5393 carrying the streptomycin resistance genes strA and strB was detected inserted in various locations on the plasmid. The examination of Michigan apple isolates for Tn5393 revealed a 138-bp sequence that does not occur in pEA29 from Ea88. Derivatives of pEA29 are easily detected using restriction enzyme analysis, providing a valuable tool that could be used to monitor the spread of regional fire blight infections, track sources of new streptomycin resistant outbreaks, and observe the population dynamic of the pathogen within a fruit growing region.
McGhee, G.C., Foster, G.C. and Jones, A.L. (2002). GENETIC DIVERSITY AMONG ERWINIA AMYLOVORA'S UBIQUITOUS PLASMID PEA29. Acta Hortic. 590, 413-421
fire blight, plasmid diversity, restriction mapping