Promoters of crystal protein genes do not control crystal formation inside exosporium of Bacillus thuringiensis ssp. finitimus strain YBT-020.

Most Bacillus thuringiensis parasporal crystals separate from spores after sporulation. A special phenomenon called spore-crystal association (SCA) occurs in a few subspecies (e.g. ssp. finitimus) where enclosed crystals are associated with spores. In this study, the involvement of crystal protein gene promoters in SCA was investigated. Two crystal protein genes, cry26Aa and cry28Aa, were isolated from subspecies finitimus strain YBT-020, and each or both were then transferred to acrystalliferous B. thuringiensis strain BMB171 and the plasmid-cured derivative of strain YBT-020. SCA was not observed with any recombinant strain, implying that the crystal protein genes are not sufficient to cause SCA. When the typical crystal protein gene cry1Ca was introduced into strain YBT-020, free bipyramidal crystals formed in addition to SCA. Recombinant genes containing the promoter of cry26Aa or cry28Aa fused with the coding sequence (CDS) of cry1Ca were introduced into strain YBT-020, and the typical cry1Ca phenotype was observed. Another two fusion genes consisting of the promoter of cry1Ca and the CDS of cry26Aa or cry28Aa were also transferred to strain YBT-020. Only enclosed crystals formed. These results indicate that the promoters of the crystal protein genes are not the key factor determining the crystal location in strain YBT-020.