Parasporins from a Caribbean Island: Evidence for a Globally Dispersed Bacillus thuringiensis Strain

Abstract

Parasporins represent a new functional class of Cry (crystal protein) toxins produced by the bacterium Bacillus thuringiensis (Bt). Unlike Cry toxins that demonstrate activity mainly against some insect cells, parasporins are characterized as being non-hemolytic, yet capable of preferentially killing some human cancer cells. Globally, six different parasporin types, PS1–PS6, based on protein sequence homology, have been identified in only four countries (Japan, Vietnam, India, and Canada). Herein we report the results of a screening study of 160 Bt isolates collected from the Caribbean island of Trinidad. One isolate (strain 64-1-94) was shown to kill human cancer cells and to contain one ps6 and two ps1 parasporin genes. The two ps1 genes were located approximately 6 kb apart from each other, sharing a similar spatial arrangement, and high sequence homology, with two plasmid-located ps1 genes, ps1Aa6 and ps1Ad1, recently isolated from a Japanese strain. Evidence is also presented that a parasporin gene reported previously for a Canadian strain, ps1Aa2, is most likely derived from a recombination event between these same two genes found in the Trinidadian and Japanese strains. Notably, all three strains share a ps6 parasporin gene, presumably located on a separate plasmid. These data suggest that the global population of ps1 genes may be have originated from a single pair of parasporin genes. Given the large geographical distance between the collection sites, which are located on both continental land masses and islands at sea, ps1 genes are able to retain a remarkable level of homology not easily explained.