The aerobactin iron uptake system of plasmid ColV-K30 in Escherichia coli
Some strains of Escherichia coli possess an iron uptake system, first found on a ColV plasmid, which utilises the hydroxamate siderophore aerobactin and which significantly enhances the virulence of host bacteria in experimental infections of mice. The aerobactin system of plasmid ColV-K30, cloned as the multicopy recombinant plasmid pABN1, was localised to a 7.6 kb segment of DNA by transposon mapping. In the maxicell and minicell expression systems, the cluster of genes was found to specify five polypeptides, four of which are involved in aerobactin biosynthesis, the fifth being the outer membrane receptor for ferric-aerobactin. The linear order of the genes specifying these polypeptides was determined, the gene for the receptor being at the 3' end of the cluster. The 5' and 3' transcription initiation and termination sites were located by S1 nuclease transcriptional mapping, and further transcription studies revealed a probable internal promoter within the cluster of genes, a minor unregulated promoter for the receptor gene, in addition to the major regulated promoter(s) at the 5' end of the system. Transcription of the system on ColV-K30 and subcloned plasmids, and production of the siderophore by ColV-K30, as measured by b-galactosidase production by ColV: :Mu(Ap lac) aerobactin-deficient strains, were found to be under the control of the level of freely available iron in the growth medium. The aerobactin system on pABN1, like that on ColV-K30, was shown to enhance the virulence of host bacteria in experimental infections of mice, and was found to be widespread among E. coli strains isolated from a number of extraintestinal infections of man and animals. The system was found not to be exclusively associated with ColV plasmids among these strains, and was apparently located on the chromosome of some isolates.