A Shiga-like toxin type II variant (
SLT-IIv) is produced by strains of Escherichia coli responsible for
edema disease of swine and is antigenically related to Shiga-like toxin type II (
SLT-II) of enterohemorrhagic E. coli. However,
SLT-IIv is only active against Vero cells, whereas
SLT-II is active against both Vero and HeLa cells. The structural genes for
SLT-IIv were cloned from E. coli S1191, and the nucleotide sequence was determined and compared with those of other members of the
Shiga toxin family. The A subunit genes for
SLT-IIv and
SLT-II were highly homologous (94%), whereas the B subunit genes were less homologous (79%). The
SLT-IIv genes were more distantly related (55 to 60% overall homology) to the genes for
Shiga toxin of Shigella dysenteriae type 1 and the nearly identical Shiga-like toxin type I (
SLT-I) of enterohemorrhagic E. coli. (These toxins are referred to together as
Shiga toxin/
SLT-I.) The A subunit of
SLT-IIv, like those of other members of this toxin family, had regions of homology with the
plant lectin ricin.
SLT-IIv did not bind to
galactose-alpha 1-4-galactose conjugated to
bovine serum albumin, which is an analog of the eucaryotic cell receptor for
Shiga toxin/
SLT-I and
SLT-II. These findings support the hypothesis that
SLT-IIv binds to a different cellular receptor than do other members of the
Shiga toxin family but has a similar mode of intracellular action. The organization of the
SLT-IIv operon was similar to that of other members of the
Shiga toxin family.
Iron did not suppress
SLT-IIv or
SLT-II production, in contrast with its effect on
Shiga toxin/
SLT-I. Therefore, the regulation of synthesis of
SLT-IIv and
SLT-II differs from that of
Shiga toxin/
SLT-I.