Cloning and characterization of a cytolytic and mosquitocidal δ- endotoxin from Bacillus thuringiensis subsp. jegathesan

Abstract

A cytolytic toxin gene encoding a 30.1-kDa Cyt2Bb1 toxin protein from B. thuringiensis subsp.jegathesan was cloned employing a limited-growth PCR screening method with forward and reverse oligonucleotide primers designed from N-terminal amino acid sequences of native and trypsin-cleaved protein, respectively. The expressed protein showed little cross-reactivity to the antibody raised against the Cyt1Aa protein. Unlike Cyt1Aa and Cyt2Aa expression, there was little or no visible crystal inclusion formation under microscopic observation. The amino acid sequence alignment indicated 31 and 66% identity to Cyt1Aa and Cyt2Aa, respectively. The sequence alignment for five known cytolytic proteins indicated three highly conserved regions, two in the loop regions between α-helices and β-sheets and one in the loop region between β-sheets 5 and 6. β-Blocks 4 to 7 are also conserved, not only structurally but also among the amino acids in the hydrophobic faces. Mosquitocidal activity assays indicated that the Cyt2Bb toxin had less toxicity than Cyt1Aa and had about 600-times-lower toxicity than the wild- type whole toxin crystal. However, both the Cyt2Bb and the Cyt1Aa toxin showed comparable levels of hemolytic activity.