Construction of a gene encoding the insect bactericidal protein attacin. Studies on its expression in Escherichia coli.

Attacin, a bactericidal small protein is produced by the giant silk moth Hyalophora cecropia. This paper deals with our efforts to clone the attacin cDNA in a bacterial vector to express it in Escherichia coli and produce the protein in sufficient amount, for further studies. We chose two inducible expression vector/bacterial cell systems: pPL-lambda/N99cI+ cells which is able to be induced by nalidixic acid, and pET3d/BL21(DE3) cells carrying a T7 RNA polymerase gene which is IPTG-inducible. After cloning in the pPL-lambda system and under no addition of the inducer, isolated transformants carried this plasmid with at least 2 concurrent deletions that drastically affected attacin expression, even though attacin gene seems to be intact as deduced by its PCR amplification. It was concluded that basal attacin expression occurred in this system and bacterial growth was limited. Plasmid deletions may have emerged by selection pressure as a way to avoid bactericidal expression and allow bacteria survival. The second cloning attempt was done in pET3d vector/BL21 cells, that should not express the cloned sequence (they lack T7 RNA polymerase gene). Transformed BL21 cells gave 3 recombinant plasmids, 2 of them presented a C deletion that generated an early stop signal in the attacin coding region. The third clone, pET-ATT18, carrying an intact gene, was transferred to BL21(DE3)-IPTG inducible cells in order to be expressed. Attacin was undetectable in stained gels or by Western blot analysis. However, expression was visualized in grown cells after 30 min of IPTG induction and 5 min of [35S]-methionine labeling, as a 22.5 kDa protein band by using gel electrophoresis and fluorography. This low level of expression drastically affected bacterial growth. Considering that attacin has no lytic activity, these results suggest that this molecule should block bacterial growth directly at the cytoplasm by an unknown mechanism, since no signal peptide coding sequence was incorporated in this gene construction, precluding periplasmic or external destination of this protein.