The location of the aerobactin gene complex on either the chromosome or plasmid was determined in eight aerobactin-positive clinical isolates of Escherichia coli by Southern hybridization analysis, using as probes the cloned aerobactin genes from the ColV-K30 plasmid. The aerobactin genes were in two cases detected on large plasmids, whereas in the other strains the aerobactin genes are most likely located on the chromosome. Restriction mapping revealed only slight variations in the structural genes and an at least 3.4-kilobase-long upstream region conserved in all three plasmid-coded systems. A 7.7-kilobase Hindlll fragment upstream and adjacent to the 16.3-kilobase HindlIl fragment carrying the complete aerobactin system was cloned from the ColV-K30 plasmid. Fine-structure restriction mapping identified the left insertion sequence in the upstream region as IS], in inverted orientation to the IS] element downstream from the aerobactin operon. The upstream and downstream sequences of IS] appear to have perfect homology, as indicated by S1 nuclease resistance of a 760-base-pair DNA duplex formed by both IS) elements.
* Corresponding author.Recently we reported a survey of a large number of E. coli clinical isolates for aerobactin synthesis (J. Z. Montgomerie, A. Bindereif, J. B. Neilands, G. Kalmanson, and L. B. Guze, submitted for publication). A high incidence of aerobactin-positive strains (39%) was found. In this paper we determined the genetic location, viz., whether plasmid coded or chromosomal, of the aerobactin biosynthesis and transport determinants in eight aerobactin-positive clinical isolates of E. coli and compare the particular organization of the aerobactin gene complex in these strains.
MATERIALS AND METHODSClinical isolates of E. coli. A collection of 54 clinical isolates of E. coli was obtained from J. Z. Montgomerie, Rancho Los Amigos Hospital, Downey, Calif. In addition, the survey included E. coli 0111, a clinical isolate from the Centers for Disease Control, Atlanta, Ga. Recombinant DNA. Restriction enzymes, SI nuclease, DNA polymerase I, and T4 DNA ligase were purchased from Bethesda Research Laboratories (Bethesda, Md.). The large fragment of DNA polymerase I was from New England Biolabs (Beverly, Mass.) The conditions recommended by the supplier were used for restriction enzyme digestions and ligations. The protocols given by Maniatis et al. (26) were followed for most standard recombinant DNA procedures.The cloning vector for the construction of pABN20 was pUC8, 2.7 kilobases (kb) (49). E. coli JM83 [ara A(lac-pro) thi rpsL 4)80dlacZzAM15I was the host for pUC8-derived recombinant plasmids. E. coli JM83 was screened for plasmids by a quick minilysate procedure, which was also used for large-scale isolations of small plasmids (20). For largescale isolations of large plasmids (pColV-K30 and total plasmid DNA from E. coli isolates) the Portnoy procedure was employed (38). Cells from 11 cultures were harvested after 2 to 3 days of growth in Tris-buffered medium (41) with sodium succinate (30 ...