RESUMO
Positive selection vectors carry genes that upon expression produce proteins that cause host cell deaths. Insertion of foreign DNA fragments within the ORF of the gene disrupts the lethal effect of the expressed protein. This study described the cloning of Family I.4 Bacillus pumilus lipase gene whose expressed protein is toxic and lethal to Escherichia coli JM109 (DE3) cells. The determinant of toxicity was identified through Error-prone PCR to be the nature of amino acid residue resident at position 28 of the mature lipase protein. The presence of Thr/Ser28 within the mature lipases of B. pumilus and B. licheniformis resulted in lethality to E. coli cells. However, the Thr28Ala or Thr28Gly mutations relieved the lethal phenotype of mature Family I.4 Bacillus lipases. The toxic effect of the expressed mature B. pumilus lipase protein was exploited in the development of a positive selection cloning vector. The B. pumilus lipase gene was synthesised to contain 13 unique silent restriction sites within the ORF, and placed under the regulation of T7 promoter of the pET expression system. Insertional inactivation of the gene's toxic protein was achieved by cloning DNA fragments of different sizes within the designed multiple cloning sites. The toxic effect of the lipase protein was disrupted indicating the potential of the gene for application in suicidal positive selection cloning vectors. The results revealed that protein expression and engineering studies aimed at optimal production of mature Family I.4 Bacillus lipases in E. coli should take into consideration the nature of amino acid 28 resident.
