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Journal of Bacteriology, April 1999, p. 2548-2554, Vol. 181, No. 8
División de Microbiología,
Centro de Biología Molecular y Celular, Campus de San Juan,
Universidad Miguel Hernández, 03550 San Juan de Alicante, Spain
Received 8 September 1998/Accepted 15 February 1999
Different strains of the Escherichia coli reference
collection (ECOR) differ widely in chromosomal size. To analyze the
nature of the differential gene pool carried by different strains, we have followed an approach in which random amplified polymorphic DNA
(RAPD) was used to generate several PCR fragments. Those present in
some but not all the strains were screened by hybridization to assess
their distribution throughout the ECOR collection. Thirteen fragments
with various degrees of occurrence were sequenced. Three of them
corresponded to RAPD markers of widespread distribution. Of these, two
were housekeeping genes shown by hybridization to be present in all the
E. coli strains and in Salmonella enterica LT2;
the third fragment contained a paralogous copy of dnaK with widespread, but not global, distribution. The other 10 RAPD markers were found in only a few strains. However, hybridization results demonstrated that four of them were actually present in a large selection of the ECOR collection (between 42 and 97% of the strains); three of these fragments contained open reading frames associated with
phages or plasmids known in E. coli K-12. The remaining six fragments were present in only between one and four strains; of these,
four fragments showed no similarity to any sequence in the databases,
and the other two had low but significant similarity to a protein
involved in the Klebsiella capsule synthesis and to RNA
helicases of archaeal genomes, respectively. Their percent GC,
dinucleotide content, and codon adaptation index suggested an exogenous
origin by horizontal transfer. These results can be interpreted as
reflecting the presence of a large pool of strain-specific genes, whose
origin could be outside the species boundaries.
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Accessory DNA in the Genomes of Representatives of
the Escherichia coli Reference Collection
*
Corresponding author. Mailing address: División
de Microbiología, Centro de Biología Molecular y
Celular, Campus de San Juan, Universidad Miguel Hernández, 03550 San Juan de Alicante, Spain. Phone: 34 96 5919451. Fax: 34 96 5919457. E-mail: FRVALERA{at}UMH.ES.
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