Bacillus thuringiensis serovar. finitimus strain YBT-020 is a typical strain with the spore-crystal association (SCA) phenotype. In our previous studies, plasmid curing experiment suggested that native plasmid pBMB28 of strain YBT-020 might contribute to the SCA phenotype. Thus, plasmid pBMB28 was cloned in order to isolate the genes related to SCA on pBMB28. Using shuttle vector pEMB0557, a shuttle genomic bacterial artificial chromosome (BAC) library of B. thuringiensis strain YBT-020 was constructed. The plasmid pBMB231 containing crystal protein gene cry28Aa, which was located on plasmid pBMB28, was screened out. By SDS-PAGE analysis and microscopic observation, we discovered the recombinant strain BMB231 that originated from the electrotransfer strain BMB171 with pBMB231 could produce Cry28Aa protein. With the chromosome walking strategy and terminal sequencing of pBMB231, four clones covering the full length of plasmid pBMB28 were screened out from this BAC library. With pulsed gel analysis of the four BAC clones and terminal sequencing, the size of the plasmid was calculated to be 140 kb. This study additionally revealed that we could clone a large plasmid from B. thuringiensis by genomic BAC library con-struction and overlaping fragment screening.
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