STRUCTURE OF THE SHIGELLA-DYSENTERIAE HEME TRANSPORT LOCUS AND ITS PHYLOGENETIC DISTRIBUTION IN ENTERIC BACTERIA

The ability to transport and use haemin as an iron source is frequentl y observed in clinical isolates of Shigella spp. and pathogenic Escher ichia coil, We found that many of these haem-utilizing E. coli strains contain a gene that hybridizes at high stringency to the S. dysenteri ae type 1 haem receptor gene, shuA, These shuA-positive strains belong to multiple phylogenetic groups and include clinical isolates from en teric, urinary tract and systemic infections. The distribution of shuA in these strains suggests horizontal transfer of the haem transport l ocus. Some haemutilizing pathogenic E. coil strains did not hybridize with shuA, so at least one other haem transport system is present in t his group. We also characterized the chromosomal region containing shu A in S. dysenteriae. The shuA gene is present in a discrete locus, des ignated the haem transport locus, containing eight open reading frames . Several of the proteins encoded in this locus participate with ShuA in haem transport, as a Salmonella typhimurium strain containing the e ntire haem transport locus used haem much more efficiently than the sa me strain containing only shuA, The haem transport locus is not presen t in E. coli K-12 strains, but the sequences flanking the haem transpo rt locus in S. dysenteriae matched those at the 78.7 minute region of E. coli K-12, The junctions and flanking sequences in the shuA-positiv e pathogenic E. coil strains tested were nearly identical to those in S. dysenteriae, indicating that, in these strains, the haem transport locus has an organization similar to that in S. dysenteriae, and it is located in the same relative position on the chromosome.